Antenna tuner

Use in transmitters

Antenna tuners are used almost universally in transmitters. Without an ATU, in addition to reducing the power radiated by the antenna, the reflected current can overheat transformer cores and cause signal distortion. In high power transmitters it may overheat the transmitter, or cause self-protection circuits in the transmitter to automatically reduce power to safe levels but further reducing power of the signal leaving the antenna.

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. Transmitters with built-in antennas that only cover a narrow frequency band, such as cell phones and walkie-talkies, have an ATU circuit inside permanently set to work with the installed antenna.

High power transmitters like radio broadcasting stations have a matching unit that is adjustable to accommodate changes in the frequency, the transmitter, 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. This adjustment is made using an instrument called an SWR meter, which measures the amount of power reflected back toward the transmitter.

Use in receivers

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

In a receiver, if the impedance of the antenna, feedline, and receiver are mismatched 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 20 MHz, atmospheric radio noise dominates the signal to noise ratio (SNR) of the incoming radio signal, and the power of the radio signal is far above the level of inherent thermal radio noise in the receiver's circuits. Therefore the receiver can amplify the weak signal to compensate for any inefficiency caused by impedance mismatch without increasing noise in the output.

At higher frequencies, however, receivers encounter very little atmospheric noise and noise added by the receiver's own front end amplifier dominates the signal to noise ratio. At frequencies above 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 antenna be impedance-matched to the receiver's front end, to transfer the maximum power from a weak signal into the first amplifier to provide a stronger signal than its own internally-generated noise. So impedance-matching circuits are incorporated in some receivers for the upper HF band, such as CB radio, and for most VHF and higher frequency receivers, such as FM broadcast receivers, and scanners for aircraft and public safety radio.

What an "antenna tuner" actually tunes

Despite its name, an antenna ‘tuner ’ does not directly tune the antenna. It matches the transmitter to the complex impedance reflected back to the input end of the feedline.

If both tuner and transmission line were lossless, tuning at the transmitter end would indeed produce a perfect match at every point in the transmitter-feedline-antenna system.^[2] However, in practical systems feedline losses limit the ability of the antenna ‘tuner’ to match the antenna or change its resonant frequency. If the loss of power is low in the line carrying the transmitter's signal into the antenna, a tuner at the transmitter end can produce a worthwhile degree of matching and tuning for the antenna and feedline network as a whole.^[3][4] But with lossy, low-impedance feedlines like the commonly used 50 Ohm coaxial cable, maximum power transfer only occurs if matching is done at both ends of the line.

Efficiency and SWR

If there is still a high standing wave ratio (SWR) in the feedline beyond the ATU, any loss in the feedline is multiplied several times by the transmitted waves reflecting back and forth between the tuner and the antenna, heating the wire instead of sending out a signal. 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.

Hence, operating an antenna far from its design frequency and compensating with a transmatch between the transmitter and the feedline is not as efficient as using a resonant antenna with a matched-impedance feedline, nor as efficient as a matched feedline from the transmitter to a remote antenna tuner attached directly to the antenna.

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 ohm 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 similar design can match an antenna to a transmission line: For example, many TV antennas have a 300 Ohm impedance but feed the signal to the TV through a 75 ohm 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 major 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 single-wire transformer with different connection points or taps spaced along the windings. They are distinguished mainly by their impedance transform ratio (1:1, 1:4, 1:9, etc., the square of the winding 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. When two differently-grounded cables or circuits must be connected but the grounds kept independent, a full, two-winding transformer with the desired ratio is used instead.

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, or low frequencies). The three equal windings shown are wired for a common ground shared by two unbalanced lines (so this design is called an unun), and can be used as 1:1, 1:4, or 1:9 impedance match, depending on the tap chosen. (The same windings could be connected differently to make a balun instead.)

For example, if the right-hand side is connected to a resistive load of 10 ohms, 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 measures greater impedance for the same 10 ohm load on the right.

Transmission line antenna tuning methods

The insertion of a special section of transmission line, whose characteristic impedance differs from that of the main line, can be used to match the main line to the antenna. An inserted line with the proper impedance and connected at the proper location can perform complicated matching effects with very high efficiency, but spans a very limited frequency range.^[5]

The simplest example this method is the quarter-wave impedance transformer formed by a section of mismatched transmission line. If a quarter-wavelength of 75 ohm coaxial cable is linked to a 50 ohm load, the SWR in the 75 ohm quarter wavelength of line can be calculated as 75Ω / 50Ω = 1.5; the quarter-wavelength of line transforms the mismatched impedance to 112.5 ohms (75 ohms × 1.5 = 112.5 ohms). Thus this inserted section matches a 112 ohm antenna to a 50 ohm main line.

The ​¹⁄₆ wavelength coaxial transformer is a useful way to match 50 to 75 ohms using the same general method.^[6][7]

A second common method is the use of a stub: A shorted, or open section of line is connected in parallel with the main line. With coax this is done using a ‘T’-connector. The length of the stub and its location can be chosen so as to produce a matched line below the stub, regardless of the complex impedance or SWR of the antenna itself.^[8] The J-pole antenna is an example of an antenna with a built-in stub match.

Basic lumped circuit matching using the L network

Automatic ATU for amateur transceiver

The basic circuit required when lumped capacitances and inductors are used is shown below. This circuit is important in that many automatic antenna tuners use it, and also because more complicated circuits can be analyzed as groups of L-networks.

Basic network

This is called an “L” network not because it contains an inductor, (in fact some L-networks consist of two capacitors), but because the two components are at right angles to each other, having the shape of a rotated and sometimes reversed Roman letter ‘L’. The ‘T’ (“Tee”) network and the ‘π’ (“Pi”) network also have a shape similar to the Roman and Greek letters they are named after.

This basic network is able to act as an impedance transformer. If the output has an impedance consisting of resistance R_load and reactance j X_load, while the input is to be attached to a source which has an impedance of R_source resistance and j X_source reactance, then

and

.

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 :

If the frequency is 28 MHz,

As,

then,

So,

While as,

then,

Theory and practice

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).

Two networks in a circuit; both have 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).

Three networks in a circuit, all with the same impedance

Types of L networks and their use

The L-network can have eight different configurations, six of which are shown here. The two missing configurations are the same as the bottom row, but with the parallel element (wires vertical) on the right side of the series element (wires horizontal), instead of on the left, as shown.

In discussion of the diagrams that follows the in connector comes from the transmitter or "source", on the left; the out connector goes to the antenna or "load" on the right. The general rule (with some exceptions, described below) is that the horizontal element of an L-network goes in series with the side that has the lowest resistive impedance.^[9]

So for example, the three circuits in the left column and the two in the bottom row have the series (horizontal) element on the out side are generally used for stepping up from a low-impedance input (transmitter) to a high-impedance output (antenna), similar to the example analyzed in the section above. The top two circuits in the right column, with the series (horizontal) element on the in side, are generally useful for stepping down from a higher input to a lower output impedance.

The general rule only applies to loads that are mainly resistive, with very little reactance. In cases where the load is highly reactive – such as an antenna fed with a signals whose frequency is far away from any resonance – the opposite configuration may be required. If far from resonance, the bottom two step down (high-in to low-out) circuits would instead be used to connect for a step up (low-in to high-out that is mostly reactance).^[10]

The low- and high-pass versions of the four circuits shown in the top two rows use only one inductor and one capacitor. Normally, the low-pass would be preferred with a transmitter, in order to attenuate harmonics, but the high-pass configuration may be chosen if the components are more conveniently obtained, or if the radio already contains an internal low-pass filter, or if attenuation of low frequencies is desirable – for example when a local AM station broadcasting on a medium frequency may be overloading a high frequency receiver.

In the bottom row, the Low R, high C circuit is shown feeding a short vertical antenna, such as would be the case for a compact, mobile antenna or otherwise on frequencies below an antenna's lowest natural resonant frequency. Here the inherent capacitance of a short, random wire antenna is so high that the L-network is best realized with two inductors, instead of aggravating the problem by using a capacitor.

The Low R, high L circuit is shown feeding a small loop antenna. Below resonance this type of antenna has so much inductance, that more inductance from adding a coil would make the reactance even worse. Therefore, the L-network is composed of two capacitors.

An L-network is the simplest circuit that will achieve the desired transformation; for any one given antenna and frequency, once a circuit is selected from the eight possible configurations (of which six are shown above) only one set of component values will match the in impedance to the out impedance.

High-pass T network

T-network transmatch

This configuration, although capable of matching a large impedance range, is a high-pass filter and will not attenuate spurious radiation above the cutoff frequency as much as the other types. Due to its low losses and simplicity, many home built and commercial manually tuned ATUs use this circuit.

Low-pass π network

The π Network

A π (pi) network can also be used. This ATU has very good attenuation of harmonics, but for multiband tuners the standard π circuit is not popular, since the variable capacitors are inconveniently large for the lower Amateur bands.

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 which can be several thousand picofarads while allowing the two variable capacitors to be smaller. A band switch selects the input capacitor and inductor.^[11] This circuit was used in tuners covering 1.8 to 30 MHz made by the R. L. Drake Company.

SPC tuner

SPC transmatch

The Series Parallel Capacitor or SPC tuner can serve both as an antenna coupler and as a preselector. A simplified description of the SPC circuit follows: In the diagram, the upper capacitor on the right matches impedance to the antenna, and the single capacitor on the left matches impedance to the transmitter. The coil and the lower-right capacitor form a tank circuit that drains to ground out-of-tune signals. The coil is usually also adjustable (not shown), in order to widen or narrow the band-pass and to ensure that the ganged right-hand capacitors will be able to both match to the antenna and tune to the transceiver's operating frequency without compromising one or the other.

The functional description of the components is roughly correct, but too simple. In actual operation, the inductor and all of the capacitors interact to produce the overall result.^{[lower-alpha 1]}

Ultimate Transmatch

McCoy, Lew (1997) Figure 4-1(B) “Ultimate” transmatch circuit^[12]

Originally, the Ultimate Transmatch was promoted as a way to make the components more manageable at the lowest frequencies of interest and also to get some harmonic attenuation. A near-final version of McCoy's Ultimate Transmatch network is shown in the illustration to the right. It is now considered obsolete; the design goals were better realized with the Series-Parallel Capacitor (SPC) network, shown above, which was designed after the name Ultimate had already been used.

Fixed link with taps

The Fixed link with taps (top left on the diagram) is the most basic circuit. The 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) has the same circuit, but uses a “hairpin” inductor (a tapped transmission line, short-circuited at the far end).^[13] Moving the taps along the hairpin allows continuous adjustment of the impedance transformation, which is difficult with 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 the solenoid 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

The illustration shows two versions of essentially the same circuit: Series cap with taps and an alternate configuration For low-Z lines. 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 moves the input coil in and out of the space between turns in the main coil to change their mutual inductance. The variable inductance makes these tuners more flexible than the basic circuit, but at some cost in complexity.

Fixed link with differential capacitors

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

The four output capacitors sections (C2) are a 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. 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 to change the turns on the main and link inductors 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.

Z match

The Z match tuner response

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.^[14][15] The Z match has two tuning capacitors with separate connections to the primary transformer coil, producing two distinct resonant frequencies that enable it to cover a wide frequency range without switching the inductor. Because it uses a transformer on the output side, it can be used with either balanced or unbalanced transmission lines, without any modification to the tuner circuit.

The Z-match design is limited in its power output by the core used for the output transformer.

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.

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 is improperly tuned or working at the limits of its tuning range.^[18]

Among the narrow band tuner circuits, the L-network has the lowest loss, partly because it has the fewest components, but mainly because it necessarily operates at the lowest possible for a given impedance transformation. With the L-network, the loaded is not adjustable, but is fixed midway between the source and load impedances. Since most of the loss in practical tuners will be in the coil, changing from a low-pass to a high-pass circuit (or vice versa) may reduce the loss a little.

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 slightly above a resonant frequency, where the antenna is too long – for example, between a quarter and a half wave long at the operating frequency. However, problematic straight-wire antennas are typically too short for the frequency in use.

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 not properly adjusted. Using the maximum available capacitance will give less loss, than if one simply tunes for a match without regard for the settings.^[19] This is because using more capacitance means using fewer inductor turns, and the loss is mainly in the inductor.

With the SPC tuner the losses will be somewhat higher than with the T-network, since the added capacitance across the inductor will shunt some reactive current to ground which must be cancelled by additional current in the inductor.^[20] The trade-off is that the effective inductance of the coil is increased, thus allowing operation at lower frequencies than would otherwise be possible.

If additional filtering is desired, the inductor in any of the three-element designs can be deliberately set to large values, thus providing a partial band pass effect.^[21] Either the high-pass T or low-pass π can be adjusted in this manner; the SPC tuner provides a full band-pass effect when similarly adjusted. The additional attenuation at harmonic frequencies can be increased significantly with only a small percentage of additional loss at the tuned frequency.

When adjusted for minimum loss, the SPC tuner will always have better harmonic rejection than the high-pass T, since the SPC design is a band-pass circuit. Either type is capable of good harmonic rejection if a small additional loss is acceptable. The low-pass π has exceptional harmonic attenuation at any setting, including the lowest-loss.

ATU location

An ATU will be inserted somewhere along the line connecting the radio transmitter or receiver to the antenna.^[22] The antenna feedpoint is usually high in the air (for example, a dipole antenna) or far away (for example, an end-fed random wire antenna). A transmission line, or feedline, must carry the signal between the transmitter and the antenna. The ATU can be placed anywhere along the feedline: at the transmitter, at the antenna, or somewhere in between.

Antenna tuning is best done as close to the antenna as possible to minimize loss, increase bandwidth, and reduce voltage and current 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 the antenna is required.

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 range of antennas^[23] (including stealth antennas).^[24][25]

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). A short length of low-loss coaxial line is acceptable, but with longer lossy lines the additional loss due to SWR becomes very high.^[26]

It is very important to remember that when matching the transmitter to the line, as is done when the ATU is near the transmitter, there is no change in the SWR in the feedline. The backlash currents reflected from the antenna are retro-reflected by the ATU – usually several times between the two – and so are invisible on the transmitter-side of the ATU. The result of the multiple reflections is compounded loss, higher voltage or higher currents, and narrowed bandwidth, none of which can be corrected by an ATU sitting beside the transmitter.

Standing wave ratio

Cross-needle SWR meter on antenna tuner

It is a common misconception that a high standing wave ratio (SWR) per se causes loss.^[3] 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).^[27] 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”).^[3] 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’ then an identical high SWR may lose a considerable fraction of the transmitter's power output. 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 despite high SWR, very little power is lost in high-impedance line compared low-impedance line – typical coaxial cable, for example. For that reason, radio operators can be more casual about using tuners with high-impedance 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 of the output stage components. Modern solid state transmitters will automatically reduce power when challenged by a high SWR. Consequently, some solid-state power stages only produce weak signals if the SWR rises above 1.5 to 1. Were it not for that problem, even the losses from an SWR of 2:1 could be tolerated, since only 11 percent of transmitted power would be reflected and 89 percent sent out through to the antenna. So the main loss of power at high SWR is due to the transmitter ‘backing off’ its output power when confronted with backlash current.

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 built-in π-network of the transmitter output stage acts as an ATU. Further, since tubes are electrically robust (even though mechanically fragile), tube-based circuits can shrug off very high backlash current with impunity.

AM broadcast transmitters

ATU for a 250 KW, 6 tower AM Antenna

One of the oldest applications for antenna tuners is in AM and shortwave broadcasting transmitters. AM transmitters usually use a vertical antenna (tower) which can be from 0.20 to 0.68 wavelengths long. At the base of the tower an ATU is used to match the antenna to the 50 ohm transmission line from the transmitter. The most commonly used circuit is a T-network, using two series inductors with a shunt capacitor between them.

When multiple towers are used the ATU network may also provide for a phase adjustment so that the currents in each tower can be phased relative to the others to produce a signal in a desired direction. Stations are often required by law to prevent signals in directions that could produce interference with other stations. The transmitting station also benefits from more of the station’s signal power, paid for in its electrical bill, going into its assigned target area, on which its advertising revenue is based. Adjustment of the ATUs in a multitower array is a complicated, time consuming process, requiring considerable expertise.

High-power shortwave transmitters

For international shortwave broadcasting (50 kW and above) stations change frequencies to better reach their target audience on a seasonal or even a daily basis, which requires frequent adjustment of antenna matching and phasing circuitry. Modern shortwave transmitters typically include built-in impedance-matching circuitry for SWR up to 2:1 , and can adjust their output impedance within 15 seconds.

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. Balanced transmission lines of 300 ohms or more were more-or-less standard for all shortwave transmitters and antennas in the past, even by amateurs. Most shortwave broadcasters continue to use high-impedance feeds even after automatic impedance matching has become commonly available.

The most commonly used shortwave antennas for international broadcasting are the HRS antenna (curtain array), which covers a 2 to 1 frequency range, and the log-periodic antenna, which can cover up to an 8 to 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 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.