Antenna tuner

Antenna tuner front view, with partially exposed interior

An antenna tuner (and any of the names in the list below) is a device that is inserted between a radio transmitter and its antenna; when properly adjusted (tuned) it improves power transfer by matching the impedance of the radio to the impedance of the end of the feedline connecting the antenna to the transmitter.

Various alternate names are used for this device: antenna matching unit, impedance matching unit, matchbox, matching network, transmatch, antenna match, antenna tuning unit (ATU), antenna coupler, feedline coupler. English language technical jargon makes no distinction between the terms.

Antenna tuners are particularly important for use with transmitters. Transmitters are typically designed to feed power into a reactance-free, resistive load of a specific value: Radio transmitters built after the 1950s are almost all designed for 50 Ω (Ohm) cabling. However the impedance of any antenna normally varies, depending on frequency and other factors, and consequently changes the impedance appearing at the other end of the feedline, connected to the transmitter. In addition to reducing the power radiated by the antenna, an impedance mismatch can distort the signal, and in high power transmitters may overheat the amplifier.

To avoid possible damage resulting from applying power into a mismatched load, ATUs are a standard part of almost all radio transmitting systems. If the transmitter detects a load impedance that departs from the designed load, safety circuits in modern transmitters automatically cut back the power output to protect the equipment from the consequences of the impedance mismatch.

The system ATU may be a circuit incorporated into the transmitter itself, or a separate piece of equipment connected anywhere between the transmitter and the antenna, or a combination of several of these. In transmitting systems with an antenna distant from the transmitter and connected to it by a long transmission line (feedline), in addition to an ATU where the feedline connects to the transmitter there may be a second matching network (or ATU) near the antenna, or incorporated into the design of the antenna, to match the transmission line's impedance to the antenna's.

Overview

Antenna tuners are particularly important for use with transmitters. Transmitters are designed to feed power into a resistive load of a specific value: 50 Ω (Ohms), by modern convention. If the impedance seen by the transmitter departs from this design value due to improper tuning of the combined feedline and antenna, overheating of the transmitter final stage, distortion, or loss of output power may occur.

Use with transmitters

Antenna tuners are used almost universally with solid-state transmitters. Without an ATU, in addition to reducing the power radiated by the antenna, the reflected (or "backlash") current can overheat transformer cores and cause signal distortion. In high-power transmitters it may overheat the transmitter's output amplifier. When reflected power is detected, self-protection circuits in modern transmitters automatically reduce power to safe levels, and hence reduce the power of the signal leaving the antenna even more than the loss from some of the power being reflected away from the antenna.

Automatic power reduction by safety circuits typically causes most of the power loss (see below).

Because of this, ATUs are a standard part of almost all radio transmitting systems. They may be a circuit incorporated into the transmitter itself, or a separate piece of equipment connected between the transmitter and the antenna. In transmitting systems with an antenna separated from the transmitter and connected to it by a transmission line (feedline), there may be another matching network (or ATU) at the antenna that matches the transmission line's impedance to the antenna.

Narrow-band transmitters like cell phones and walkie-talkies have an ATU circuit inside, permanently set to work with the installed antenna. In multi-frequency communication stations like amateur radio stations, and high power transmitters like radio broadcasting stations, the ATU is adjustable to accommodate changes in the transmitting system or its environment. Instruments such as SWR meters, antenna analyzers, or impedance bridges are used to measure the degree of match or mismatch. Testing to ensure the transmitter is correctly matched to the feedline from the antenna is needed after any change that might perturb the system.

High power transmitters like radio broadcasting stations have a matching unit that is adjustable, to accommodate changes in the transmit frequency, the transmitting unit, the antenna, or the antenna's environment. Adjusting the ATU to match the transmitter to the antenna is an important procedure which is done after any work on the transmitter or antenna occurs, or any drastic change in the weather affecting the antenna, such as hoar frost or dust storms.

The effect of this adjustment is typically measured using an instrument called an SWR meter, which indicates the aggregate mismatch between a reference impedance (typically the same as the transmitter: 50 + j 0 Ω) and the complex impedance at the point of insertion of the SWR meter. Other instruments such as antenna analyzers, or impedance bridges, provide more detailed information, such as the separate mismatches of the resistive and reactive parts of the impedance on the input and output sides of the ATU.

What an "antenna" tuner actually tunes

Despite its name, an "antenna" tuner does not actually tune the antenna: Actual 'tuning' of an antenna involves adjusting its length, or attaching extra segments to add capacitance or inductance to the path of currents through it, to eliminate reactance at the antenna feedpoint for the 'tuned' frequency. Instead, an antenna "tuning" unit matches the complex resistive + reactive impedance presented at the end of the feedline (sometimes very far from the antenna feedpoint) to the reactance-free, purely resistive (real) impedance required at the transmitter output connection, and in the same step, raising or lowering the resistance to the level required (by convention, usually 50  Ω).

If both the tuner and the feedline were ideal – lossless, or resistance-free – then tuning at the transmitter end would indeed produce a perfect match at every point in the transmitter-feedline-antenna system. However, in practical systems lossy feedlines limit the ability of the antenna tuner to remotely change the antenna's resonant frequency.

The feedline power loss will be low if the line length between the transmitter and the antenna is short, or if it has very low DC resistance per meter of length, or if it is built to carry power primarily as high voltage and low current (high impedance) (as opposed to low impedance: low voltage and high current). When feedline power loss is very low, a tuner at the transmitter end of the line can indeed produce a worthwhile degree of (imperfect) matching and tuning throughout the whole antenna and feedline network. However that is not the case when lossy and low-impedance feedline is used – like common 50 Ω coaxial cable. For low-impedance line, maximum power transfer occurs only if matching is done at the antenna, in conjunction with a matched transmitter and feedline, producing a match at both ends of the line and every point in between.

In any case, regardless of where it is placed, an ATU does not alter the gain, efficiency, or directivity of the antenna, nor does it change the internal complex impedances within the parts of the antenna itself, nor the impedance presented at the antenna's feedpoint.

Efficiency and SWR

If there is still a high standing wave ratio (SWR) beyond the ATU, in a significantly long segment of feedline, any loss in that part of the feedline is typically increased by the transmitted waves reflecting back and forth between the tuner and the antenna, causing resistive losses in the wires and possibly the insulation of the transmission line. Even with a matching unit at both ends of the feedline – the near ATU matching the transmitter to the feedline and the remote ATU matching the feedline to the antenna – losses in the circuitry of the two ATUs will slightly reduce power delivered to the antenna.

1. The most efficient use of a transmitter's power is to use a resonant antenna, fed with a matched-impedance feedline to a matched-impedance transmitter; there are still small losses in any feedline even when all impedances match, but matching minimizes loss.
2. It is almost equally efficient to feed a remote antenna tuner attached directly to the antenna, via a feedline matched to the transmitter and the ATU feed; the only extra losses are in the tuner circuitry, which can be kept small if the tuner is adjusted for a "good" match (see below) and the degree of mismatch carefully tested at or near the antenna (not at the transmitter).
3. It is usually inefficient to operate an antenna far from one of its resonant frequencies and attempt to compensate with an ATU next to the transmitter, far from the antenna; the entire feedline from the ATU to the antenna is still mismatched, which will aggravate normal loss in the feedline, particularly if it is low-impedance line, like standard 50 Ω coax.
4. The least efficient way to transmit is to feed a non-resonant antenna through a mis-matched, lossy feedline, with no impedance matching anywhere.

Use in receivers

ATUs are not widely used in shortwave receivers, and almost never used in mediumwave or longwave receivers. They are, however, helpful for receivers operating in the upper shortwave (upper HF), and needed for VHF and higher.

In a receiver, if the complex impedance of the antenna is not a conjugate match for the complex input impedance at the antenna end of the transmission line, then some of the incoming signal power will be reflected back out to the antenna and will not reach the receiver. However this is only important for frequencies at and above the middle HF band. In radio receivers working below roughly 10~20 MHz, atmospheric radio noise dominates the signal-to-noise ratio (SNR) of the incoming radio signal, and the power of the atmospheric noise that arrives with the signal is far greater than the inherent thermal noise generated within the receiver's own circuitry. Therefore, the receiver can amplify the weak signal to compensate for any inefficiency caused by impedance mismatch without perceptibly increasing noise in the output.

Atmospheric noise as a function of frequency in the LF, MF, and HF radio spectrum according to CCIR 322. The vertical axis is in decibels above the thermal noise floor. It can be seen that as frequency drops atmospheric noise dominates other sources.

At higher frequencies, however, receivers encounter very little atmospheric noise and noise added by the receiver's own front end amplifier dominates the SNR. At frequencies above about 10~20 MHz the internal circuit noise is the factor limiting sensitivity of the receiver for weak signals, and so as the frequency rises it becomes increasingly important that the receiving antenna's complex impedance be conjugately matched to the input impedance at the antenna end of the transmission line, and hence transfer the maximum available power from a weak signal into the first amplifier, to provide the amplifier with a signal significantly louder than its own internally-generated noise.

So impedance-matching circuits or impedance-matched antennas are incorporated in some receivers for the upper HF band, such as 'deluxe' CB radio receivers, and for most VHF and higher frequency receivers, such as FM broadcast receivers, and scanners for aircraft and public safety radio.

Broad band matching methods

Strictly speaking, transformers, autotransformers, and baluns are not complete antenna matching units: Even though they do transform the magnitude of impedances, when well made, they are not themselves able to bridge mismatched phases, and so are unable to produce a full conjugate match. None the less, transformers of these types are frequently incorporated into antenna feed systems to convert between balanced and unbalanced cabling, or seamlessly join different cabling impedances, providing an impedance match in the special case of reactance-free antenna feed systems. They are also sometimes used to augment the operation of the narrow band antenna tuner designs (discussed in following sections) since they can widen the range of impedances that an antenna tuner can match.

Transformers and baluns are usually designed with coil windings that have the minimum inductance needed to function, to ensure that the reactance they inadvertently contribute has only a small effect on the resonant frequency of either the antenna or narrow band transmitter circuits. This results in a trade-off, since at lower frequencies the coupling between the two sides of a transformer may not be strong enough, and at higher frequencies the stray inductance may be too high to ignore. Although these high and low frequency problems constrain the useful bandwidth of the devices, they never the less are typically extremely broadbanded compared to any other method of impedance matching.

Ferrite transformers

Solid-state power amplifiers operating from 1–30 MHz typically use one or more wideband transformers wound on ferrite cores. MOSFETs and bipolar junction transistors typically used in modern radio frequency amplifiers are designed to operate into a low impedance, so the transformer primary typically has a single turn, while the 50 Ω secondary will have 2 to 4 turns. This design of feedline system has the advantage of reducing the retuning required when the operating frequency is changed.

A 300-to-75-ohm TV line balun, showing a coaxial connector (balun inside) on the left with twin-lead trailing off to the right

A similar design can match an antenna to a transmission line: For example, many TV antennas have a 300 Ω impedance but feed the signal to the TV through a 75 Ω coaxial line. A small ferrite core transformer makes the broad band impedance transformation. This transformer does not need, nor is it capable of adjustment. For receive-only use in a TV the small SWR variation with frequency is not a significant problem.

Also note that many ferrite transformers perform a balanced-to-unbalanced transformation in addition to the impedance change. When the balanced to unbalanced function is present these transformers are called a balun (otherwise an unun). The most common baluns have either a 1:1 or a 1:4 impedance transformation.

Autotransformers

There are several designs for impedance matching using an autotransformer, which is a simple, single-coil transformer with different connection points or taps spaced along the coil windings. They are distinguished mainly by their impedance transform ratio, and whether the input and output sides share a common ground, or are matched from a cable that is grounded on one side (unbalanced) to an ungrounded (usually balanced) cable. When autotransformers connect balanced and unbalanced lines they are called baluns, just as two-winding transformers are.

1:1, 1:4 and 1:9 autotransformer

The circuit pictured at the right has three identical windings wrapped in the same direction around either an "air" core (for very high frequencies) or ferrite core (for middle frequencies) or a powdered-iron core (for very low frequencies). The three equal windings shown are wired for a common ground shared by two unbalanced lines (so this design is an unun), and can be used as 1:1, 1:4, or 1:9 impedance match, depending on the tap chosen.

For example, if the right-hand side is connected to a resistive load of 10 Ω, the user can attach a source at any of the three ungrounded terminals on the left side of the autotransformer to get a different impedance. Notice that on the left side, the line with more windings between the line's tap-point and the ground tap measures greater impedance for the same 10 Ω load on the right.

Narrow band vs. broad band matching methods

The "narrow-band" tuned circuit methods described in the following sections all cover a very much smaller span of frequencies, by comparison with the broadband transformer methods described above.

Antenna matching methods that use transformers tend to cover a wide range of frequencies. A single, typical, commercially available balun can cover frequencies from 3.5–30.0 MHz, or nearly the entire shortwave band. Matching to an antenna using a cut segment of transmission line (described below) is perhaps the most efficient of all matching schemes in terms of electrical power, but typically can only cover a range about 3.5–3.7 MHz wide in the HF band – a very small range indeed, compared to the 27 MHz bandwidth of a well-made broadband balun.

Antenna coupling or feedline matching circuits are also narrowband for any single setting, but can be re-tuned more conveniently. However they are perhaps the least efficient in terms of power-loss (aside from having no impedance matching at all!).

Transmission line antenna tuning methods

See also: Stub (electronics) and Distributed-element filter § Stub band-pass filters

There are two different impedance matching techniques using sections of feedline: Either the original feedline can have a deliberately mismatched section of line spliced into it (called section matching), or a short stub of line can branch off from the original line, with the stub's end either shorted or left unconnected (called stub matching). In both cases, the location of the section of extra line on the original feedline and its length require careful placement and adjustment, which will almost surely only work for one desired frequency.

Section matching

A special section of transmission line can be used to match the main line to the antenna, if that line section's characteristic impedance is different from that of the main line. The technique is essentially to fix a mismatch by creating an opposite mismatch: A line segment with the proper impedance and proper length, inserted at the proper distance from the antenna, can perform complicated matching effects with very high efficiency. The drawback is that matching with line segments only works for a very limited frequency range for which the segment's length and position are appropriate.

A simple example of this method is the quarter-wave impedance transformer formed by a section of mismatched transmission line. If a quarter-wavelength of 75 Ω coaxial cable is linked to a 50 Ω load, the SWR in the 75 Ω quarter wavelength of line can be calculated as 75 Ω/ 50 Ω  = 1.5 , when there is no reactance; the quarter-wavelength of line transforms the mismatched impedance to 112.5 Ω ( 75 Ω × 1.5 = 112.5 Ω ). Thus this inserted section matches a 112 Ω antenna to a 50 Ω main line.

The 1/ 6  wavelength coaxial transformer is a useful way to match 50 to 75 Ω using the same general method.

Stub matching

A second common method is the use of a stub: Either a shorted or open section of line is connected in parallel with the main feedline, forming a dead-end branch off the main line; with coax this is done using a 'T'-connector. A stub less than a quarter-wave long whose end is short-circuited acts as an inductor, that subtracts susceptance from the line; if its end is left open (unconnected), the stub acts as a capacitor and adds susceptance.

The stub is placed at one of the points along the line where the non-reactive part of the impedance happens to match the line below the stub; the length of the stub placed there is chosen so that its susceptance is equal-and-opposite to the susceptance at that point on the line. The combined effect of location and length removes the effects of the complex impedance or SWR from the antenna, and leaves the remaining conductance matched to the line beyond the connection point.

The J-pole antenna and the related Zepp antenna are both examples of an antenna with a built-in stub match.

More elaborate stub matching methods involve using two stubs, either in series or in parallel, to create an L-C tuning circuit, some of which are electrically equivalent to 'L' networks, described in the following sub-sections.

Basic network

Basic two-element L-network

The most basic form of lumped circuit matching is with the 'L'-network: It is the simplest circuit that will achieve the desired transformation, and always consists of exactly two reactive components. The 'L' circuit is important not only in that many automatic antenna tuners use it, but also because more complicated circuits can be analyzed as chains of 'L'-networks, as will be shown in later sections, in descriptions of larger tuning circuits.

For any one given load and frequency, one must use a circuit from one of the eight possible configurations shown below.

An automatic ATU for amateur transceiver. The columns of white components are relays that switch toroidal coils (red, right-most column) and wafer capacitors (black, central column) into and out of the matching circuit. The large black square at the lower left is the CPU that operates the circuitry.

Commercially available automatic antenna tuners most often are 'L'-networks, since they involve the fewest parts, and have a unique setting for the automatic self-adjustment circuitry to seek.

Further information: Electronic filter topology § Ladder topologies

This circuit is called an "ell" network, not because it contains an inductor (in fact some 'L'-networks consist of two capacitors), but instead because of the shape: In the schematic, the two components are at right angles to each other, in the shape of a Latin letter 'L' either rotated (┌─) or flipped and rotated (─┐). The basic circuit required when pairs of lumped capacitors and / or inductors are used is shown in the chart of schematics below.

The 'T' ("tee") network and the 'π' ("pie" / "pee") network also have their parts laid out in a shape similar to the Latin and Greek letters they are named after. The 'T' network is electrically equivalent to two back-to-back 'L' networks: ─┐┌─; the 'π' network is equivalent to two nose-to-nose 'L' networks: ┌─ ─┐.

L-network math

This basic network is able to act as an impedance transformer. If the output has an impedance consisting of resistive part R_load and reactive part X_load, which add to make a single complex number (j² = −1). The input is to be attached to a source which has an impedance of R_source resistance and X_source reactance, then

$jX_{\text{L}}={\sqrt {{\Big (}R_{\text{source}}+jX_{\text{source}}{\Big )}{\Big (}(R_{\text{source}}+jX_{\text{source}})-(R_{\text{load}}+jX_{\text{load}}){\Big )}}}$

and

$jX_{\text{C}}=(R_{\text{load}}+jX_{\text{load}}){\sqrt {\frac {(R_{\text{source}}+jX_{\text{source}})}{(R_{\text{load}}+jX_{\text{load}})-(R_{\text{source}}+jX_{\text{source}})}}}$.

In this example circuit, X_L and X_C can be swapped. All the ATU circuits below create this network, which exists between systems with different impedances.

For instance, if the source has a resistive impedance of 50 Ω and the load has a resistive impedance of 1000 Ω :

$jX_{\text{L}}={\sqrt {(50)(50-1000)}}={\sqrt {(-47500)}}$

$X_{\text{L}}=217.94\,{\mathsf {\Omega }}$

$X_{\text{C}}=1000{\sqrt {\frac {50}{(1000-50)}}}=1000\,\times \,0.2294\,{\mathsf {\Omega }}=229.4\,{\mathsf {\Omega }}$

If the frequency is 28 MHz,

As,

$X_{\text{C}}={\frac {1}{2\pi fC}}$

then, $2\pi fX_{\text{C}}={\frac {1}{C}}$

So, ${\frac {1}{2\pi fX_{\text{C}}}}=C=24.78\,{\text{pF}}$

While as, $X_{\text{L}}=2\pi fL\,$

then, $L={\frac {X_{\text{L}}}{2\pi f}}=1.239\,{\text{μH}}$

'L'-network Theory and practice

Two networks in a circuit; both have the same impedance

A parallel network, consisting of a resistive element (1000 Ω) and a reactive element (−j 229.415 Ω), will have the same impedance and power factor as a series network consisting of resistive (50 Ω) and reactive elements (−j 217.94 Ω).

Three networks in a circuit, all with the same impedance

By adding another element in series (which has a reactive impedance of +j 217.94 Ω), the impedance is 50 Ω (resistive).

Types of 'L' networks and their uses

Schematics for all eight 'L'-network configurations

'L' network schematics

Corresponding Smith charts

All 8 possible 'L' networks and their uses. The value for shunt or parallel antenna resistance (R) and inductive (L) or capacitive (C) reactance refer to the antenna or its feedline, connected at the right. The radio to match is connected at the left, presumed 50 Ohms reactance-free.

There are eight different configurations of components for an 'L' network, which are shown in the two left columns of the diagrams at the right, marked with numbers 1–8 with corresponding colors. The right column is three versions of the same Smith chart, showing antenna resistance (R) increasing toward the right on the horizontal axis, with the conventional 50 Ohms at the center point. Antenna reactance varies along vertical direction, with increasing inductive reactance (L) going upward from the big circle's center-line, and capacitive (C) reactance increasing going downward. The horizontal line cutting through the middle of the large circle is reactance-free.

Note that Smith chart vertical scales are all highly distorted arcs with non-linear measure.

Network selection

If a load impedance is plotted on a Smith chart, it will fall into one of the four regions shown: Upper half-labrys (rounded axe head,  ◠), lower half-labrys (◡), left inner-circle (⟄), and right inner-circle (⟃). For a complex impedance falling anywhere in the chart, either two, or four different 'L' networks may be used, so the user may choose other criteria to decide which of the two or four networks to use. Each of the two inner circles (left, ⟄, and right, ⟃) allows two different choices, and each of the half-labryses (upper, ◠, and lower, ◡) allows four.

Each region is color coded as well as marked with corresponding numbers to indicate which networks can be used to match an impedance in that region. For example, an impedance that falls within the right inner circle (⟃, either green or yellow, labeled "R > 50") can be matched using networks 1 or 3.

Additional selection criteria

Networks 1–4, shown in the top two rows, use one inductor and one capacitor; the pair with a series inductor (1 and 2) are low-pass; the next two, with the capacitor in series (3 and 4) are high pass. Normally, low-pass would be preferred with a transmitter, in order to attenuate possible harmonics. The high-pass configuration shown in the second row, (3 and 4) may be chosen if the required component values are more convenient, or if the radio already contains an internal low-pass filter, or if attenuation of low frequencies is desirable.

In some cases it may be desirable that the network either pass through DC currents used for power feed to devices on the antennas, such as relay switches, or to block DC used for those devices from reaching the transmitter. Thus, the series (horizontal) component should be either an inductor (L) to pass DC, or a capacitor (C) to block DC. In addition, it may be useful for the phase shift across the network to be either advanced or delayed (see below).

Automatic and manual 'L' networks often use either network 1 or 2. Many commercial tuners include a simple SPDT switch that connects the vertical (shunt, C) component to either the left or right side of the horizontal (series, L) component, making both networks 1 and 2 available with the same transmatch. As shown by the green and red sections of the top Smith chart, these two networks can cover all possible loads. Likewise the yellow and blue parts of the middle Smith chart show that networks 3 and 4 are also capable of covering all loads.

Loads such as a small transmitting loop may be highly inductive. The impedance will fall well into the region of the Smith chart dominated by inductive reactance (orange shaded upper half-labrys, ◠, labelled "L dominant"). In addition to networks 1 and 4, they can use the low-loss all-capacitor networks 5 or 6. Short vertical antennas such as used for HF mobile, are dominated by capacitive reactance (purple shaded lower half-labrys, ◡, labelled "C dominant"), in addition to networks 2 and 3, they can be easily matched with inductor-only networks 7 or 8, which are electrically the same as connecting two taps on a single grounded coil.

Measuring instrument limitations

Commonly used SWR meters do not indicate complex impedance, so they are not very helpful for determining which of the 'L' network networks to use. Antenna analyzers, however, can separately show the resistive and reactive parts of the antenna impedance, and are suitable for selecting the orientation of an 'L' network. The most convenient of these analyzers are able to plot the complex impedance on a Smith chart display. When an instrument indicates the complex series impedance, but not the shunt (parallel) equivalent, formulas or a calculator can be used to make the conversion to the parallel values.

Q and phase shift

Unlike the more complicated networks, described below, the 'L' network does not allow independent choice of operating Q, nor phase shift. High Q implies less loss, but also narrow operating bandwidth. 'L' network Q is fixed at the geometric mean of the input and output impedances, hence it is greater when the impedances to be matched are greatly different.

Phase shift can be made to either lead or lag by choosing an alternate network, but like the Q, for 'L' networks its value is fixed by the impedance ratio, and odds are that none of the two or four possible networks will provide the right phase shift and the right impedance match at the same setting. Phase shift is only important if two or more loads are to be fed, such as used for directional arrays for AM broadcasting at high powers; luckily for many radio amateurs and small-scale broadcasters, single-antenna operation doesn't require controlled phase shift.

Three element tuners for unbalanced feedlines

This section only discusses circuit designs for unbalanced lines; the following sections discuss tuners for balanced lines.

In contrast to two-element 'L'-networks, the circuits described below all have three or more components, and hence have many more choices for inductance and capacitance that will produce an impedance match, unfortunately including some bad choices. The two main goals of a good match are:

1. to minimize loss in the matching circuit, and
2. to maximize bandwidth – e.g. the widest span of frequencies that are matched tolerably well.

To obtain good matches and avoid bad ones, with every antenna and matching circuit combination, the radio operator must experiment, test, and use judgement to choose among the many adjustments that match the same impedances (see the maximum capacitance rule below).

All of the three element designs also allow a somewhat independent choice of how much the phase is shifted by the matching unit. Since phase matching is an advanced topic, mainly of use for multi-tower broadcast arrays, it is omitted here for brevity. A good summary of phase change by matching networks is given in the NAB Engineering Handbook.

High-pass T-network

T-network transmatch

This configuration is currently popular because it is capable of matching a large impedance range with capacitors in commonly available sizes. However, it is a high-pass filter and will not attenuate spurious radiation above the cutoff frequency nearly as well as other designs (see the π-network section, below). Due to its low losses and simplicity, many home-built and commercial manually tuned ATUs use this circuit. The tuning coil is normally also adjustable (not shown).

The 'T' network shown here may be analyzed as a high-pass step-down 'L' network on the input side feeding into a high-pass step-up 'L' network on the output side (─┐┌─). The two side-by-side vertical (shunt) inductors in the conjoined circuit are combined into a single equivalent inductor.

Theory and practice

T-network match for a partly reactive load

An example of matching with the low pass 'T' network is shown here.

The load measures Z_load = 200 − j 75 Ω (Ohms) with 200 Ω representing the resistive part and −j 75 Ω the capacitively reactive part of the combined impedance Z_load. Conceptually, the −j 75 Ω can be cancelled by adding a series inductor with +j 75 Ω reactance. This leaves a pure resistance of 200 Ω to be matched to 50 Ω.

The resistance-matching is done with a circuit that mimics a 100 Ω Quarter wave impedance transformer, consisting of two inductors with +j 100 Ω reactance and a shunt capacitor with −j 100 Ω. The quarter wave-style transformer circuit uses equal and opposite reactances, each of which is the geometric mean of the two resistances to be matched: ${\sqrt {\ 50{\mathsf {\ \Omega }}\ \times \ 200{\mathsf {\ \Omega }}\;}}={\sqrt {\ 10\,000{\mathsf {\ \Omega }}^{2}\;}}=100{\mathsf {\ \Omega }}\;.$

The output inductor of the quarter wave network can be merged with the inductor used to cancel the reactance of the load, by replacing the pair with one inductor with the sum of the two inductances. The final network will have +j 100 Ω for the input inductor, −j 100 Ω for the capacitor and +j 175 Ω for the output inductor.

This quarter-wave-style solution will cause a phase shift of 90 degrees. If the output phase matters, then one of the many other possible solutions for the capacitance and two inductances can be used instead. This solution uses a low pass configuration. Swapping the inductors and capacitors, and appropriately adjusting their reactances, would give a high pass configuration.

Low-pass π network

The π-network

A π (pi) network can also be used; it is the electrical conjugate of a low pass 'T' network. This ATU has exceptionally good attenuation of harmonics, and was incorporated into the output stage of tube-based 'vintage' transmitters and many modern tube-based RF amplifiers. However, the standard π circuit is not popular for stand-alone multiband antenna tuners, since the variable capacitors needed for the 160 m and 80 / 75 m amateur bands are prohibitively large and expensive.

The π network shown here may be described mathematically as a low-pass step-up 'L' network on the input side feeding into a low-pass step-down 'L' network on the output side (┌─ ─┐). The two noze-to-noze inductors in the joined circuit are replaced with a single equivalent inductor.

Drake's modified π-network

Modified π-network circuit used in Drake tuners

A modified version of the π-network is more practical as it uses a fixed input capacitor (left), which can be several thousand picofarads, allowing the variable capacitors (the two on the right) to be smaller. A band switch (not shown) sets the inductor and the left-side input capacitor (shown as fixed components in the schematic). This circuit was used in tuners covering 1.8–30 MHz made before the popularity of the simpler 'T'‑network, above.

In all antenna tuner circuits each of the available adjustments affects both the reactive and resistive parts of the impedance match. The Drake π-network circuit is somewhat unusual in that regard: For a given setting of the band switch, the upper right, series capacitor mostly adjusts the reactive part of the impedance match, and the lower right, shunt capacitor mostly affects the resistive part of the impedance match. This makes it easier to estimate how to adjust the two variable capacitor settings, when the operator knows the type and location of the antenna's resonant frequency nearest to the radio's operating frequency.

It can also be viewed as two 'L' networks coupled front to back: A capacitor-inductor low pass step-up network on the left, feeding into a capacitor-capacitor step-up network on the right (┌─┌─).

SPC tuner

SPC transmatch schematic. Although not shown, the inductor is normally adjustable.

The Series Parallel Capacitor or SPC tuner uses a band-pass circuit that can act both as an antenna coupler and as a preselector. Because it is a band-pass circuit, the SPC tuner has much better harmonic suppression than the T-match above, but uses similar-cost tuning capacitors; its performance is better than the "Ultimate" circuit below. The SPC's harmonic suppression is only surpassed by the π-network tuners, described above.

The SPC circuit is equivalent to a back-to-back pair of 'L' networks: A high-pass capacitor-inductor step down network on the input side feeding into a capacitor-capacitor step up network on the output side (─┐┌─). The combination of the vertical (shunt) inductor and shunt capacitor parallel to it is a tank circuit that grounds out-of-tune signals. When tuned to exploit that action, the tank circuit makes the SPC a band-pass filter that eliminates harmonics as effectively as the π network, although requiring more careful adjustment for best results.

With the SPC tuner the losses will be somewhat higher than with the 'T'-network, since the grounded capacitor will shunt some reactive current to ground, which must be cancelled by additional current in the inductor. The trade-off is that the effective inductance of the coil is increased, thus allowing operation at lower frequencies than would otherwise be possible.

Ultimate Transmatch

Schematic diagram of the so called "Ultimate Transmatch"

Originally, the Ultimate transmatch was promoted as a way to make the components more manageable at the lowest frequencies of interest, and to also get some harmonic attenuation. A version of McCoy's Ultimate transmatch network is shown in the illustration to the right. The circuit is now considered obsolete; the design goals were better realized by the Series-Parallel Capacitor (SPC) network, shown above, using identical parts.

The 'Ultimate' circuit has the same general front-to-back topology (┌─┌─) as the Drake modified π, above, but with a high-pass 'L'-component (instead of a low-pass component) which is placed on the output side instead of input. Unfortunately, with the capacitor-capacitor 'L'-component placed on the input side, it can neither help match impedance, nor appreciably reduce harmonic output.

Balanced versions of unbalanced tuner circuits

Balanced (open line) transmission lines require a tuner that has two "hot" output terminals, rather than one "hot" terminal and one "cold" (grounded). Since all modern transmitters have unbalanced (co-axial) output – almost always 50 Ω – the most efficient system has the tuner provide a balun (balanced to unbalanced) transformation as well as providing an impedance match.

All of the unbalanced tuner circuits described in the preceding main section can be converted to an equivalent balanced circuit by a standard procedure.

Commercially available "inherantly balanced" tuners are made as balanced versions of L, T, and π circuits. Their drawback is that the components used for each of the two output channels must be carefully matched and attached pairs, so that adjusting them causes an identical tuning change to both "hot" sides of the circuit. Hence, most "inherently balanced" tuners are much more difficult to make, and more than twice as expensive as unbalanced tuners.

Balanced voltage taps on the coil of an unbalanced circuit

Even with a single-winding transformer, some unbalanced transmatch designs can be adapted to create balanced output without the need for two, independent windings: Most matching networks include a coil, and that coil can accept or produce balanced output if the antenna feed's tap-points are placed symmetrically above and below an electrically neutral point on the coil.

The effect is to force balanced voltages, instead of the desired balanced currents.

This technique was experimented with in early years of the 20th century, but appears to no longer be in use. This article does not include any such circuit designs, as yet.

Balanced-line matching with tuned-transformers

The following balanced networks (see diagram) all have been used for line matching. They are all based on tuned transformer circuits; none of the designs discussed in this section are balanced versions of the unbalanced circuits, mentioned above.

Six balanced tuner schematics

Fixed link with taps

The Fixed link with taps (top left on the diagram) is the most basic circuit. The $Q$ factor will be nearly constant and is set by the number of relative turns on the input link. The match is found by tuning the capacitor and selecting taps on the main coil, which may be done with a switch accessing various taps or by physically moving clips from turn to turn. If the turns on the main coil are changed to move to a higher or lower frequency, the link turns should also change.

Hairpin tuner

The Hairpin tuner (top right) is effectively the same electrical circuit as the fixed link with taps, above, but uses "hairpin" inductors (a tapped transmission line, short-circuited at the far end) instead of coiled inductors. Moving the tap points along the hairpin allows continuous adjustment of the impedance transformation, which is difficult on a solenoid coil.

It is useful for very short wavelengths from about 10 meters to 70 cm (frequencies about 30 MHz to 430 MHz) where a coiled inductor would have too few turns to allow fine adjustment. These tuners typically operate over at most a 2:1 frequency range.

Series cap with taps & for low-Z lines

The middle section of the illustration shows two alternate configurations of nearly the same circuit:

• Series cap with taps (middle, left) adds a series capacitor to the input side of the Fixed link with taps. The input capacitor allows fine adjustment with fewer taps on the main coil.
• An alternate connection (middle, right) for the series cap circuit is useful for low impedances only, but avoids the taps (For low-Z lines in the illustration).

Swinging link with taps

Swinging link with taps (bottom left). A swinging link inserted into the Fixed link with taps also allows fine adjustment with fewer coil taps. The swinging link is a form of variable transformer, that changes the coils' mutual inductance by swinging the input coil in and out of the gap between halves of the main coil. The variable inductance makes these tuners more flexible than the basic circuit, but at some cost in complexity, both in terms of construction and in terms of dealing with more possible adjustments.

Fixed link with differential capacitors

The Fixed link with differential capacitors circuit (bottom right) was the design used for the well-regarded Johnson Matchbox (JMB) tuners.

The four output capacitor-sections (C2) are a "ganged" double-differential capacitor: The axes of the four sections are mechanically connected and their plates aligned, so that as the top and bottom capacitor sections increase in value the two middle sections decrease in value, and vice versa (notice the arrow heads on C2 in the diagram are shown with both matching and contrary directions). This provides a smooth change of loading that is electrically equivalent to moving taps on the main coil. The Johnson Matchbox used a band switch (not shown) to change the number of turns on the main inductor for each of the five frequency bands available to hams in the 1950s.

The JMB design has been criticized since the two middle-section capacitors in C2 are not strictly necessary to obtain a match; however, the middle sections conveniently limit the disturbance of the adjustment for C1 caused by changes to C2.

Double-tuned link with differential capacitors

Alfred Annecke's c. 1970 enhanced version of the Johnson Matchbox antenna tuner

Later designs enhancing the limited range of the otherwise respected Johnson Matchbox (JMB), to accommodate the many more modern shortwave amateur bands, either add switched taps to the link (input) inductor, or may include a capacitor in series with the input coil winding. Both of these extra adjustments are shown in the schematic, right. As in the case of the input capacitor and swinging link, described above, these are both ways to allow fine-tuning without requiring changes to the JMB bandswitch (not shown) and its intricately soldered connections to the output-side coil which changes the number of turns used on the output coil.

Adjusting the number of taps on the input coil changes the Q of the network, widening or narrowing its matched frequency span. Using C1 to tune or de-tune the left side of the transformer to the setting for C2 on the right side, has approximately the same effect as moving the two sides of the transformer closer or further apart. Note that including the band switch (not shown), this circuit has five separate available controls, which makes it complicated to adjust.

Z match

Schematic of Z match antenna tuner

Another approach to creating a balanced output from a matching circuit is to incorporate a conventional two-winding transformer into the transmatch. The separate input and output windings isolate the ground on the input side from the output side (grounded or ungrounded). This is the approach taken with the Z-match design.

The Z match tuner response. Note the low frequency and high frequency peaks.

The Z-match is an ATU widely used for low-power amateur radio which is commonly used both as an unbalanced and as a balanced tuner. The Z match is a doubled version of a resonant transformer circuit, with three tuning capacitors.

Two of the capacitors with separate connections to the primary transformer coil are ganged, and effectively constitute two separate resonant transformer circuits, which simultaneously tune two distinct resonant frequencies. The double-resonance enables the single circuit across the coil to cover a wider frequency range without needing to switch the inductance: Every setting offers two different frequencies, in separate frequency bands, that are both impedance matched at once. Because the output side is a transformer secondary (optionally grounded) it can be used to feed either balanced or unbalanced transmission lines, without any modification to the circuit.

The Z-match design is limited in its power output by the core used for the output transformer. A powdered iron or ferrite core about 1.6 inches in diameter should handle 100 W. A tuner built for low-power use ("QRP" – typically 5 W or less) can use a smaller core.

Balanced match from an unbalanced tuner and a balun

Another approach to feeding balanced lines is to use an unbalanced tuner with a balun on either the input (transmitter) or output (antenna) side of the tuner. Most often using the popular high pass T circuit described above, with either a 1:1 current balun on the input side of the unbalanced tuner or a balun (typically 4:1) on the output side. It can be managed, but doing so both efficiently and safely is not easy.

Balun between the antenna and the ATU

Any balun placed on the output (antenna) side of a tuner must be built to withstand high voltage and current stresses, because of the wide range of impedances it must handle.

For a wide range of frequencies and impedances it may not be possible to build a robust balun that is adequately efficient. For a narrow range of frequencies, using transmission line stubs or sections for impedance transforms (as described above) may well be more feasible and will certainly be more efficient.

Balun between the transmitter and the ATU

The demands put on the balun are more modest if the balun is put on the input end of the tuner – between the tuner and the transmitter. Placed on that end it always operates into a constant 50 Ω impedance from the transmitter on one side, and has the matching network to protect it from wild swings in the feedline impedance on the other side: All to the good. Unfortunately, making the input from the transmitter balanced creates problems that must be remedied.

If an unbalanced tuner is fed with a balanced line from a balun instead of directly from the transmitter, then its normal antenna connection – the center wire of its output coaxial cable – provides the signal as usual to one side of the antenna. However the ground side of that same output connection now becomes the feed of an equal and opposite current to the other side of the antenna; the only unsatisfactory consequence is that the entire grounded portion of the tuner becomes "hot" with RF power, including the tuner's metal chassis, metal control knobs, and insulated knobs' metal set-screws, all touched by the operator.

The "hot ground" in the ATU

The "true" external ground voltage at the antenna and transmitter must lie halfway between the two "hot" feeds, one of which is the internal ground: Inside the ATU, the matching circuit's "false" ground level is equally different from the "true" ground level at either the antenna or the transmitter as the original "hot" wire is, but with opposite polarity. Either the usual "hot" output wire or the matching circuit "hot ground" will give you exactly the same shock if you touch it.

The tuner circuit must "float" above or below the exterior ground level in order for the ATU circuit ground (or common side) that formerly was attached to the output cable's ground wire to feed the second hot wire: The circuit's floating ground must provide a voltage difference adequate to drive current through an output terminal to make the second output "hot".

High voltages are normal in any efficient ("high Q") impedance matching circuit bridging a wide mismatch. Unless the incompatible grounds are carefully kept separate, the high voltages present between this interior floating ground (the "false" ground) and the exterior transmitter and antenna "true" grounds can lead to arcing, corona discharge, capacitively coupled ground currents, and electric shock.

Carefully keeping the incompatible grounds separate

To reduce power loss and protect the operator and the equipment, the tuner chassis must be double-layered: An outer chassis and an inner chassis. The outer chassis must enclose and separate the tuning circuit and its floating ground from the outside, while itself remaining at the level of the exterior "true" ground(s). With the protective outer chassis, the inner chassis can maintain its own incompatible floating ground level, safely isolated.

The inner chassis can be reduced to nothing more than a mounting platform inside the outer chassis, elevated on insulators to keep a safe distance between the "floating ground" and the outer chassis wired to the "true" electrical ground line(s). The inner tuning circuit's metal mounting chassis, and in particular the metal rods connected to adjustment knobs on the outer chassis must all be kept separate from the surface touched by the operator and from direct electrical contact with the transmitter's ground on its connection cable ("true" ground).

Isolating the controls is usually done by replacing at least part of the metal connecting rods between knobs on the outside surface and adjustable parts on the inside platform with an insulated rod, either made of a sturdy ceramic or a plastic that tolerates high temperatures. Further, the metal inner and outer parts must be spaced adequately far apart to prevent current leaking out via capacitive coupling when the interior voltages are high. Finally, all these arrangements must be secured with greater than usual care, to ensure that jostling, pressure, or heat expansion cannot create a contact between the inner and outer grounds.

Summary

Using an inherently unbalanced circuit for a balanced tuner puts difficult constraints on the tuner's construction and high demands on the builder's craftsmanship. The advantage of such a design is that its inner, inherently unbalanced matching circuit always requires only a single component where a balanced version of the same circuit often requires two. Hence it does not require identical pairs of components for the two "hot" ends of the circuit(s) in order to ensure balance to ground within the ATU, and its output is inherently balanced with respect to the exterior "true" ground, even though the interior circuit is unbalanced with respect to the interior "false" ground.

Antenna system losses

ATU placement

An ATU can be inserted anywhere along the line connecting the radio transmitter or receiver to the antenna. The antenna feedpoint is usually high in the air or far away, and a transmission line (feedline) must carry the signal between the transmitter and the antenna. The ATU can be placed anywhere along the feedline – at the transmitter output, at the antenna input, or anywhere in between – and if desired, two or more ATUs can be placed at different locations between the antenna and the transmitter (usually at the two ends of the feedline) and adjusted so that they co‑operatively create an impedance match throughout the antenna system.

Antenna matching is best done as close to the antenna as possible, to minimize loss, increase bandwidth, and reduce voltage and current peaks on the transmission line. Also, when the information being transmitted has frequency components whose wavelength is a significant fraction of the electrical length of the feed line, distortion of the transmitted information will occur if there are standing waves on the line. Analog TV and FM stereo broadcasts are affected in this way; for those modes, matching at or very near the antenna is mandatory.

When possible, an automatic or remotely-controlled tuner in a weather-proof case at or near the antenna is convenient and makes for an efficient system. With such a tuner, it is possible to match a wide variety of antennas over a broad range of frequencies (including concealed antennas).

High-impedance feedline

High impedance 450 Ω "window line",
a.k.a. ladder line and twin-lead

Low impedance 75 Ω type RG-59 coaxial cable:
(A) Outer plastic jacket, (B) Woven copper shield, (C) Inner dielectric insulator, (D) Copper core

When the ATU must be located near the radio for convenient adjustment, any significant SWR will increase the loss in the feedline. For that reason, when using an ATU at the transmitter, low-loss, high-impedance feedline is a great advantage (open-wire line, for example).

Through to the 1950s balanced transmission lines of at least 300 Ω were more-or-less standard for all shortwave transmitters and antennas, including amateurs' equipment. Most shortwave broadcasters continue to use high-impedance feedlines, even after automatic impedance matching has become commonly available. High impedance lines – such as most parallel-wire lines – carry power mostly as high voltage rather than high current, and current alone determines the power lost to line resistance. So for the same number of Watts delivered to the antenna, typically very little power is lost in high-impedance line even at severe SWR levels, when compared to losses for the same SWR in low-impedance line, like typical coaxial cable. For that reason, radio operators using high-impedance feedline can be more casual about where along the line they match up the impedances.

A short length of coaxial line with low loss is acceptable, but with longer coaxial lines the greater losses, aggravated by SWR, become very high. It is important to remember that when an ATU is placed near the transmitter and far from the antenna, even though the ATU matches the transmitter to the line there is no change in the line beyond the ATU. The backlash currents reflected from the antenna are retro-reflected by the ATU and so are invisible on the transmitter-side of the ATU. Individual waves are usually reflected between the antenna and the ATU several times; the result of the multiple reflections is compounded loss, higher voltage and / or higher currents on the line and in the ATU, and narrowed bandwidth. None of these bad effects can be remediated by an ATU sitting beside the transmitter.

Loss in antenna tuners

Every means of impedance match will introduce some power loss. This will vary from a few percent for a transformer with a ferrite core, to 50% or more for a complicated ATU that has been naïvely adjusted to a "bad" match, or is working near the limits of its tuning range.

Among the narrow-band tuner circuits, the L-network has the lowest loss, partly because it has the fewest components, but mainly because it can match at just one setting, and that setting is necessarily the lowest Q possible for a given impedance transformation. In effect, the 'L'-network does not have any bad match to choose: Its only available match is good.

The 'L'-network using only capacitors will have the lowest loss, but this network only works where the load impedance is very inductive, making it a good choice for a small loop antenna. Inductive impedance also occurs with straight-wire antennas used at frequencies above their first resonant frequency, where the antenna is too long – for example, a monopole longer than a quarter wave and shorter than half wave long at the operating frequency. One can deliberately adjust the size of an antenna so that it will be inductive on all its design frequencies (similar to a small loop) with the intention of using only capacitors to tune it, so as to have minimal tuning losses. Unfortunately, the typical problem encountered in the bottom HF bands is that antennas are too short for the frequency in use; their matching circuits require inductance.

With the high-pass 'T'-network, the loss in the tuner can vary from a few percent – if tuned for lowest loss – to over 50% if the tuner is adjusted to a "bad match" instead of a good one.

Maximum capacitance rule

As a rule of thumb, using the maximum possible capacitance for every tuner setting will involve the least loss, as compared to simply tuning for any match, without regard for the settings. In general, this is because increasing the capacitance produces less reactance; the frequent consequence is that less balancing reactance is needed from the inductor which means running current through fewer turns of wire on the inductor coil, and the loss in almost every ATU is mainly from resistance in the inductor wire (loss from dirty capacitor contacts comes in a distant second).

Sacrificing efficiency in exchange for harmonic suppression

If additional filtering is desired, the inductor in any of the three-element designs can be deliberately set to slightly larger values than the necessary minimum, raising the circuit Q and so provide a partial band pass effect. Either the high-pass 'T' or low-pass 'π' can be adjusted in this manner; the additional attenuation at harmonic frequencies can be increased significantly with only a small percentage of additional loss at the tuned frequency. The SPC tuner always blocks out-of-band signals, and gives an especially "sharp" band-pass effect when similarly adjusted for high Q.

At any match setting, the SPC tuner will always have much better harmonic rejection than the high-pass 'T', since the SPC design is a band-pass circuit, although the 'T' match's obtainable factor of 99% (20 dB) may be enough harmonic rejection, if the small additional loss is acceptable. The low-pass 'π' always has exceptional harmonic attenuation at any setting, including the lowest-loss. The only benefit of adjusting it for higher Q would be to partially block interference below the operating frequency.

Standing wave ratio

Cross-needle SWR meter on an antenna tuner

It is a common misconception that a high standing wave ratio (SWR) per se causes loss, or that an antenna must be resonant in order to transmit well; neither is true.

A well-adjusted ATU feeding an antenna through a low-loss line may have only a small percentage of additional loss compared with an intrinsically matched antenna, even with a high SWR (4:1, for example). An ATU sitting beside the transmitter just re-reflects energy reflected from the antenna ("backlash current") back yet again along the feedline to the antenna ("retro-reflection"). High losses arise from RF resistance in the feedline and antenna, and those multiple reflections due to high SWR cause feedline losses to be compounded.

Using low-loss, high-impedance feedline with an ATU results in very little loss, even with multiple reflections. However, if the feedline-antenna combination is "lossy", like coaxial line, then an identical high SWR may waste a considerable fraction of the transmitter's power output heating up the coax. On the other hand, parallel-wire, high impedance line typically has much lower loss, even when SWR is high. For that reason, radio operators using high-impedance line can be more casual about use of matching units and their placement on the feedline.

Without an ATU, the SWR from a mismatched antenna and feedline can present an improper load to the transmitter, causing distortion and loss of power or efficiency with heating and / or burning components in the output stage. Modern solid state transmitters are designed to automatically protect themselves by reducing power when confronted with backlash current. Consequently, all modern solid-state power stages are designed to only produce weak signals when the SWR rises above some cutoff level, often 1.5:1 .

Were it not for the problem created by that design conflict between circuit safety and delivered transmit power, even the losses from an SWR of 2:1 could otherwise be tolerated, since only 11 percent of transmitted power would be reflected and 89 percent sent through to the antenna (a loss of only 1/ 2  dB). So the main loss of power at high SWR is due to the output amplifier "backing off" its power when challenged by a high SWR.

Tube transmitters and amplifiers usually have an adjustable output network that can feed mismatched loads up to perhaps 3:1 SWR without trouble. In effect the 'π' network in the transmitter output stage acts as a built-in ATU. Further, tubes are electrically robust (despite being mechanically fragile) and can shrug off very high backlash current with impunity; so tube-based output stage amplifiers have no need to "back off" their output power.

Broadcast Applications

AM broadcast transmitters

Inside the coupling hut of a 250 KW, AM station with 6 antenna towers

One of the oldest applications for antenna tuners is in mediumwave and shortwave AM broadcast transmitters. AM band transmitters usually use vertical tower antennas, usually between  1 /5 and  5 /8 wavelengths tall. At the base of the tower (in the "coupling hut") an ATU is used to match the antenna to the either 50 Ω or 300 Ω transmission line from the transmitter. The most commonly used circuit is a low-pass 'T' network (two series inductors and a shunt capacitor between them, different from the 'T' network discussed above).

When multiple towers are used, the ATU network may also need to provide for a phase adjustment, to advance or delay the current to each tower, relative to the others; done properly, it can aim the combined signal in a desired direction.

High-power shortwave transmitters

High-power (50 kW and above) international shortwave broadcast stations change frequencies seasonally – even daily – to adapt to ionospheric propagation conditions, so their signals can best reach their intended audience. Frequent transmitting frequency changes require frequent adjustment of antenna matching, but modern broadcast transmitters typically include built-in automatic impedance-matching circuitry that can accommodate modest impedance changes, with similar circuitry increasingly common in amateur transmitters as well.

Modern internal ATU circuits typically can self-adjust to a new frequency or new output impedance within 15 seconds, for SWR up to 2:1 (at least). The matching networks in transmitters sometimes incorporate a balun or an external one can be installed at the transmitter in order to feed a balanced line.

The most commonly used shortwave antennas for international broadcasting are the HRS antenna (curtain array), which covers a 2:1 frequency range, and the log-periodic antenna, which can cover up to an 8:1 frequency range. Within the design range, the antenna SWR will vary, but these designs usually keep the SWR below 1.7 to 1 – easily within the range of SWR that can be tuned by built-in automatic antenna matching in many modern transmitters. So when feeding well-chosen antennas, a modern transmitter will be able to adjust itself as needed to match to the antenna at any frequency.

Automatic tuners

Automatic antenna tuning is used in flagship mobile phones; in transceivers for amateur radio; and in land mobile, marine, and tactical HF radio transceivers.

Two possible configurations of a transmitter comprising an antenna, a single-antenna-port antenna tuner (AT), a sensing unit (SU), a control unit (CU) and a transmission and signal processing unit (TSPU).

Several control schemes can be used, in a radio transceiver or radio transmitter, to automatically adjust an antenna tuner (AT). Each AT shown in the figure has a port, referred to as ″antenna port″, which is directly or indirectly coupled to an antenna, and another port, referred to as ″radio port″ (or as ″user port″), for transmitting and/or receiving radio signals through the AT and the antenna. Each AT shown in the figure is a single-antenna-port (SAP) AT, but a multiple-antenna-port (MAP) AT may be needed for MIMO radio transmission.

Several control schemes, which can be used to automatically adjust a SAP AT of a wireless transmitter, are based on one of the two configurations shown in the figure. In both configurations, the transmitter comprises: an antenna; the AT; a sensing unit (SU); a control unit (CU); and a transmission and signal processing unit (TSPU) which consists of all parts of the transmitter not shown elsewhere in the figure. The TX port of the TSPU delivers an excitation. The SU delivers, to the TSPU, one or more sensing unit output signals determined by one or more electrical variables (such as voltage, current, incident or forward voltage, etc) caused by the excitation, sensed at the radio port in the case of configuration (a) or at the antenna port in the case of configuration (b).

It is possible to define five types of antenna tuner control schemes. Type 0 designates the open-loop AT control schemes that do not use any SU, the adjustment being typically only based on the knowledge of an operating frequency. Type 1 and type 2 control schemes use configuration (a), type 2 using extremum-seeking control whereas type 1 doesn't. Type 3 and type 4 control schemes use configuration (b), type 4 using extremum-seeking control whereas type 3 doesn't. The control schemes may be compared as regards: their use of closed-loop control and/or open-loop control; the measurements used; their ability to mitigate the effects of the electromagnetic characteristics of the surroundings; their aim; their accuracy and speed; and their dependence on a model of the AT and CU.

See also

• American Radio Relay League
• Antenna tuning hut
• Coaxial cable
• Electrical length
• Electronic filter topology
• Impedance bridging
• Loading coil
• Preselector
• Smith chart
• Twin-lead

Further reading

• The Radio Communication Handbook (5th ed.). Bedford, UK: Radio Society of Great Britain. 1976. ISBN 0-900612-58-4.

• Rohde, Ulrich L. (1974). "Die Anpassung von kurzen Stabantennen für KW-Sender" [Matching of short rod-antennas for shortwave transmitters]. Funkschau (in German). No. 7.

• Rohde, Ulrich L. (13 September 1975). "Match any antenna over the 1.5 to 30 MHz range with only two adjustable elements". Electronic Design. Vol. 19.

• Wright, H.C. (1987). An Introduction to Antenna Theory. London, UK: Bernard Babani. BP198.

External links

• "American Radio Relay League website" (main page).
• How Antenna Tuners Work. lcfgroup – via YouTube.