Icom Antenna Tuner

Standing waves are often associated with RF feeders, and they are generated when there is a mismatch between the feeder impedance and the load impedance. At th emismatch, power is reflected and the combined voltages and currents of the forward and reflected power form standing waves along the feeder.

Older valve or vacuum tube transmitters were more able to withstand the high voltages and current levels that can be caused by the high levels of reflected power. Nevertheless, they still benefitted from the lower loss levels that proper matching provided for any radio communication system.

It is found that for maximum power transfer, the impedance of the source and load must be the same. If this is not the case, then not all the available power can be transferred.

Having a basic understanding of standing waves helps understand the operation of antennas and the need for antenna tuners. It enables the best to be made of any antenna used for any for of radio communication.However the impedance of an antenna can vary very considerably dependent upon its characteristics and the frequency – often there are inductive and capacitive elements to the overall impedance of the antenna.

The antenna tuner or antenna tuning unit is a key element of many ham radio and professional transmitting and receiving HF radio communication stations.

It is worth placing a VSWR meter in the line to monitor the actual level of standing waves seen by the transmitter. Note: a separate meter may not be required if one is incorporated into the transmitter, unless it is more convenient to monitor the level separately.The ideal position for the location of the antenna tuner is at the point where the antenna is fed by the feeder. In this way, the antenna can be matched to the antenna and all the way through the system there are good matches, and the levels of SWR are low.

If there is a mismatch between the feeder and the antenna, then not all the power that is available is able to be transferred. As power cannot just disappear, the power that cannot be transferred has to go somewhere, so it is reflected back along the feeder, setting up voltage and current standing waves.

An antenna tuner or antenna tuning unit is a network of variable inductors and capacitors that can be altered to counterbalance the effects of the inductive and capacitive elements of the antenna with the aim of making the antenna appear as a resistive load of 50Ω.Every antenna, feeder and transmitter and receiver has what is termed a characteristic impedance. For most commercial, professional and ham radio applications the standard is 50 Ω.Their basic purpose is to ensure that the antenna system impedance matches that of the transmitter or receiver and thereby the optimum performance is achieved.

Modern semiconductor transmitters are very susceptible to damage caused by high levels of SWR and therefore they need to be presented with a good impedance match. The ATU / antenna tuner enables this to be achieved and as a result, it is an essential part of many radio communication stations used for broadcast, commercial, military and ham radio.For HF, antenna tuners enable the maximum amount of power to be transferred into the antenna, whether a low power transmitter is used of a high power one. Accordingly antenna tuners are widely used for all forms of two way radio communication as well as for broadcasting, monitoring and a variety of other applications for HF radio.

Unfortunately it is not always easy to locate the antenna tuner at the point where the antenna is fed. This may be some distance way from the transmitter, and could even be in a position that would not be accessible. Antenna tuners are essential items of equipment for any HF radio communications system. Although the principles hold good for VHF and above, the types of antennas and techniques used mean that antenna tuners are not normally required. When these standing waves reach the transmitter, the current and voltage peaks can cause the transmitter output devices to be damaged. To prevent this happening, many transmitters have protection circuitry that reduces the output of the transmitter to a level where the transmitter will not be damaged.There are several circuits that can be used for antenna tuners, each having its own attributes. They vary from simple L configurations with an inductor and a capacitor to other types with more components.

Under these circumstances, the antenna tuner can be located close to the transmitter, even when coaxial feeder is used to connect the tuner to the antenna.Provided the coaxial cable can operate with the higher voltage and current levels caused by the high SWR, then this is quite acceptable. The main issue is to prevent the transmitter seeing the high SWR as this might damage the output, or the protection circuitry will reduce the power output.

ICOM 91138, är designad för Icom GM800 GMDSS och Icom M802 SSB-radio för automatisk förbikoppling Tuning för att förbättra frekvensmottagningen. AT-141 matchar alla frekvenser på HF-marinbandet. Till exempel matchar tunern en 7 m lång trådantenn över 1,6-30 MHz.Full automatisk inställning, tryck bara på [TUNE] -knappen på sändtagaren, och Icom AT-141 stämmer omedelbart för en minimum SWR på alla frekvenser i HF marinbandet.För att minska inställningstiden lagrar Icom AT-141 automatiskt matchningsvillkoren för upp till 45 frekvenser. Att ställa in en memorerad frekvens tar ungefär 1 sekund. 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. 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 is nearly certain to work for only one desired frequency.

Strictly speaking, transformers, autotransformers, and baluns are not complete impedance matching units: Even though they do transform the magnitude of impedances, they are not themselves able to bridge mismatched phases, and so are unable to produce a full conjugate match. Nonetheless, 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.After WW II, surplus 50 Ω military cable became available at low cost, and amateur radio operators started using it. Coaxial cable is much less ”fussy” about where it is placed than unshielded cable, and if driven with balanced current will be unaffected by nearby metal fences, metal roofs or siding, or car chassis, and can be run through metal pipes or buried in soil. The previously common unshielded parallel wire feedline is vulnerable to its impedance being distorted if run near any such large piece of metal.Commonly used SWR meters do not indicate complex impedance, so they are not very helpful for determining which of the ‘L’ networks can be used for the needed match. 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 display the complex impedance on a Smith chart screen, and are able to switch back and forth between series and parallel representation.Because of this, feedline matching is 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 long transmission line (feedline), there may be another matching network (ATU) at the antenna that matches the transmission line’s impedance to the antenna.

The two columns of networks are called ”step down” (left) and ”step up” (middle). the sense of the metaphorical ”step” is always from the antenna to the radio; in all the diagrams in this article, that direction is right to left.

High losses arise from RF resistance in the feedline, the close by soil below the antenna, and the metal of the antenna. Multiple reflections due to high SWR cause all these losses to be compounded. However, the total of the losses for multiple passes greatly depends on the original, single-pass loss. Using a good ground system and low-loss, high-impedance feedline results in very little loss, even with multiple reflections, because even low radiation resistance in the antenna can out-compete line and ground resistances, if those are very low. However, if the combined feedline and ground-system is ”lossy”, like coaxial line, or a mere ground rod, then an identical high SWR may waste a considerable fraction of the transmitter’s power output heating up the coax and warming the soil. In comparison, parallel-wire, high impedance line typically has extremely low loss, even when SWR is high. For that reason, radio operators using high-impedance line with an extensive ground system can be more relaxed about use of matching units and their placement on the feedline.

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 wave fronts 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.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.For a ‘π’ match a different rule applies – a seemingly contrary minimum capacitance rule – but the reasoning for it is the same: Set either the left or right capacitor to its minimum value, the other capacitor to its maximum value, and the inductor set very low; antenna-side capacitor minimized if the antenna’s resistive impedance (not reactance) is low; transmitter side minimized if the antenna’s resistance is high. The minimized capacitor will have its maximum possible reactance, and that high reactance will obstruct the flow of current through it, and the bottomed-out capacitor will (almost) drop out of the matching network (almost become a broken circuit = approx. no connection). The remaining capacitor and the central inductor form (almost) an ‘L’ match, whose only match is optimal (lowest inductance / lowest loss). Since the search begins with the matching capacitor set at maximum (minimum reactance) and so is gradually reduced, the first matching inductance encountered will tend to be the lowest.

This configuration is currently popular because at shortwave frequencies 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 low pass ‘T’-network and ‘π’-network sections, 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).

Narrow-band transmitters like cell phones and walkie-talkies have a built-in matching circuit, permanently set to work with the installed antenna. In multi-frequency communication stations like amateur radio stations, and for multi-kilowatt transmitters like wide-area AM stations, the ATU is adjustable to accommodate changes in frequency, in the transmitting system, or to 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. When the ATU must be located near the radio for convenient adjustment, any significant SWR will increase the loss in the feedline, unless the antenna feedpoint itself is positioned at the radio and directly connects to the back of the tuner. For that reason, when using a remote antenna with an ATU sitting at the transmitter, low-loss, high-impedance feedline is a great advantage (open-wire line, for example). Antenna tuners are used almost universally with solid-state transmitters. Without a matching system, in addition to reducing the power radiated by the antenna, the reflected (or ”backlash”) current can cause signal distortion and overheat transformer cores. In high-power t
ransmitters it may overheat the transmitter’s output amplifier. When excessive 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.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.

At some point near the extreme possible settings for any one of the installed components, the possible match settings for the system will be curtailed, and the best match may be the first to drop out (require an impossibly high or low setting). If the configuration of component settings with the least loss isn’t feasible with the installed components, and the loss for the achievable setting is appreciably worse, then despite still being able to find a match for the impedance, the only match the ATU is able to provide is ”bad”. A simple clue that the matching network has reached or is near that point is when an available configuration has one of the setting knobs ”pegged out”, or nearly so, and adjusting away from the pegged setting only makes the match worse – that is, any alternative to the pegged match or near-match cannot be improved by a slight adjustment of the other components’ settings.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.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 are needed for VHF and higher.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). The diagrams, right, show two alternate configurations of nearly the same circuit: Series cap with taps (left) attaches the antenna in parallel with the transformer coil and capacitor C2, via taps, and Series cap for low-Z lines (right) attaches the antenna in series with the coil and capacitor C2. Using C1 to tune or de-tune the primary winding to the tuning of the secondary winding by C2 has approximately the same effect as moving the two windings closer or further apart, similar to the swinging link (described below). The SPC tuner is a band-pass circuit, so it always blocks out-of-band signals, both above and below, but it can be made to have an especially narrow pass-band when adjusted for similarly higher-than-necessary inductance. At any match setting, the SPC tuner will always have much better harmonic rejection than the high-pass ‘T’, although the ‘T’ match’s obtainable factor of 99% (20 dB) may be enough harmonic rejection, if the small additional loss is acceptable.

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 at least partially neutralized by even more current through the inductor to add contrary reactance. A trade-off is that the effective inductance of the coil-capacitor combination is higher than the coil alone, thus allowing operation at lower frequencies than would otherwise be possible.The radio operator needs to keep a sensible perspective on the limits to the relevance of matching network optimization: Matching losses are typically low, and if losses in the feedline beyond the tuner are high, achieving lowest losses in the matching network will be irrelevant – ‘only one drip in a bucket’. Cable losses beyond any tuner remain unimproved, regardless of whether the tuner settings are good or bad. The only cure for lossy cable is to place the tuner immediately next to the antenna feedpoint, and run any long cabling between the tuner and the transmitter:

The approach taken with the Z-match design is to incorporate a conventional two-winding transformer into the transmatch in order to deliver (optionally) balanced output from a matching circuit. The separate input and output windings isolate the ground on the input side from the output side (grounded or ungrounded), which permits the connection of either balanced or unbalanced loads on the output side, regardless of the input side connection, ensures that the output currents are balanced, and allows the output voltages to float with respect to ground.

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.The unrealistic case where either resistance is zero is not even of academic interest: Any antenna with zero total resistance is non-functional (see radiation resistance).

In all antenna tuner circuits each of the available adjustments affects both the reactive and resistive parts of the impedance match. Drake’s modified ‘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.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 : 1 , which is 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.

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: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.Automatic antenna tuning is used in flagship mobile phones; in transceivers for amateur radio; and in land mobile, marine, and tactical HF radio transceivers.However, at higher frequencies the ionosphere no longer traps radio waves inside the atmosphere and the noise radiates into space. In the high HF, VHF, and above, receivers encounter very little atmospheric noise, and the 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. So as the receive 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 the receiver end of the transmission line be matched to the receiver input connection; the combination of all this impedance matching effects the maximum transfer of power from a weak signal arriving at the antenna into the first amplifier, to try to provide the front end amplifier stage with a signal louder than the amplifier’s own internally-generated noise. 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. All of the matching networks in this section can be understood as composites of two ‘L’ networks. The descriptions for each network below break down the network into its component ‘L’ networks from the chart in the prior section; although that design information may be ’nice to know’, it is not ’need to know’, and that part of the line matching network description may be skipped.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.

The Hairpin tuner (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. In the high-pass ‘T’ circuit, setting either the left or right capacitor to its maximum causes it to have almost no reactance, and it almost vanishes from the circuit, leaving the remaining capacitor and inductor to approximate an ‘L’ network. The only impedance match possible for the (almost) ‘L’ network consisting of the two remaining components will be (approximately) optimal – that is, will have the lowest current possible through the inductor, hence lowest loss inside the network. 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 internal impedance pattern is low ─┬ high ┬─ low: Higher impedance in the center, hence higher voltage inside the network than at its connections on either side. The two side-by-side vertical (shunt) inductors in the conjoined circuit are combined into an equivalent single inductor.Many ferrite transformers are configured to 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.Although in most electronics it is typically a mistake to compare a series resistance to a parallel resistance, in this special case it works out to be correct. Also, although the shapes do resemble stair-steps, the sense of the step ”going up” and ”going down” is opposite that of the resistances, so probably more prone to cause confusion than to be helpful.

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.Like the high-pass ‘T’ network in the prior section, this low-pass network may also be analyzed as a step-down ‘L’ network on the input side feeding into a step-up ‘L’ network on the output side (─┬ ┬─). The impedance pattern is low ─┬ high ┬─ low, with higher impedance / higher voltage in the center of the network than at the connections on either side. The two side-by-side capacitors from the two ‘L’ networks are merged in the conjoined network into a single capacitor with the same total capacitance. The only real distinction between the high-pass network above, and this low-pass design, is that in this network both constituent ‘L’ networks are low-pass, whereas the network in the previous section uses back-to-back high-pass ‘L’ networks.

Ferrites are ceramics that are very effective conductors of magnetic fields, made from iron oxides (rust) and varying small amounts of nickel, zinc, tin, manganese, and various other metals. Different mixtures are blended for particular frequency ranges, normally one to several megahertz wide. Each mix becomes less effective at frequencies higher or lower than its intended range, and this in turn imposes further practical bandwidth limits on ferrite transformers.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.

As a rule of thumb only for a common high-pass ‘T’ match, using the maximum possible capacitance (and minimum possible inductance) 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 usual consequence of high capacitance (low reactance) is that less counter-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).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: Essentially all radio transmitters built after the 1950s are designed for 50 Ω (Ohm) output. 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 either the amplifier or the cores of transformers along the line.

Among the narrow-band tuner circuits, the ‘L’ network typically has low loss, or 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, any ‘L’ network gives its operator no option to choose a ”bad” match: The only ‘L’ network settings that produce a match are as good as it gets with the selected network.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, since ─┬ ┬─ ≅ ─┬┬─ ≅ ─┬─ ≅ ‘T’ ; the ‘π’ network is equivalent to two nose-to-nose ‘L’ networks, e.g. ┬─ ─┬ ≅ ┬─┬ ≅ ‘π’ . (See the individual ‘π’ and ‘T’ network descriptions below for more detail.) 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. Every matching network with three reactive components, given fixed settings for the first two components, almost always has two distinct settings (or no settings at all!) for the third component that each achieve a match. One setting typically results in higher loss than the other, and sometimes the difference is enough to be important; usually, but not necessarily, the setting that needs the highest inductance is the ”bad” match (highest loss), and that is what the ”maximum capacitance rule”, above, seeks to avoid. However, it is sometimes possible for a lower-inductance setting to wind up circulating more current through the coil, perhaps enough more to cause higher loss at the lower inductance. In that case, the above rule of thumb does not give good guidance. The previous sections only discuss networks designed for unbalanced lines; this section and all the following sections discuss tuners generally, or tuners for balanced lines. 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. 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. Since the early 1960s, perhaps earlier, almost all available coaxial cable is either 48~52 Ω or 70~75 Ω impedance. (Television cabling adopted 75 Ω cable, which is sometimes used by radio amateurs.) The only benefit of designing radios for 50 Ω cabling is standardization; it is merely convenient – not ideal – and like many standards, is only used for historical reasons.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.

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.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”.

To avoid possible damage resulting from applying power into a mismatched load and power drop due to self-protection circuits in the amplifier, matching networks are a standard part of almost all radio transmitting systems. The system transmatch may be a circuit incorporated into the transmitter itself, or a separate piece of equipment connected into the feedline 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 transmission line (feedline), in addition to an matching unit where the feedline connects to the transmitter, there may be a second matching network (ATU) near the antenna or incorporated into the design of the antenna, to bridge the transmission line impedance over to the antenna’s feedpoint impedance.

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.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 impedance pattern is high ┬─ low ─┬ high, hence lower impedance / higher current in the center, inside the network than at either side. The two noze-to-noze inductors in the joined circuit are replaced with a single inductor with the same total inductance.

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.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: at least 300 Ω = 300 volts pushing through every / 1 ampere of current flow ). 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 or 75 Ω coaxial cable (low impedance: low voltage and high current). 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. 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. 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 low-loss feedline to the antenna (”retro-reflection”), so the part of the reflected waves that survive the losses do eventually radiate out.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 (see schematics, right). As shown by the green and red sections of the top Smith chart, these two networks can together handle all possible loads. Likewise the yellow and blue parts of the middle Smith chart show that one of either network 3 or 4 can match any load. A modified version of the ‘π’ network is more practical as it uses a fixed input capacitor (left-most), 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. The four output capacitor-sections (C2a,b,c,d) 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 (C2a & C2d) increase in value the two middle sections (C2b & C2c) 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 1940s.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.

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.There are several simple rules of thumb for finding the optimum matchpoint and avoiding the ”bad” matchpoint. They are mainly intended for tuning using only an SWR meter and minimizing standing wave ratio, which gives no direct indication of how ”good” or ”bad” the found match may be. All are based on the fact that a three-element network can simulate an ‘L’ network, a
nd the match achieved by an ‘L’ network is presumably the lowest-loss for that network configuration (high-pass and low-pass ‘L’ networks may have different losses for matching the same antenna). However, note that losses in long cabling and the antenna’s ground system often overwhelm even ”bad match” tuner losses, in which case transmatch loss becomes irrelevant.

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 signal’s 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, raises or lowers the signal resistance to the level required by the transceiver (usually 50 Ω, by arbitrary convention).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 impedance change at the tuner output and the impedance change at the antenna feedpoint, compounding the normal resistive losses in the transmission line by making multiple passes through it. 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 – loss in the circuitry of the two ATUs will still slightly reduce power delivered to the antenna.

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.

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.

This configuration is popular for mediumwave transmitting systems, since it requires a shunt capacitor in commonly available sizes, whereas the high-pass form, if used at the same frequencies, would require exceptionally large capacitors in its series sections. Because it is a low-pass filter this network will effectively eliminate spurious harmonic radiation above its tuned frequency essentially equally as well as any other design, and AM broadcasters are liable to stricter surveillance and larger financial penalties for interference with other commercial stations’ signals, than are amateurs operating in the shortwaves are.For the ”measure it” part that creates or extends a table of optimum settings in the first place, an SWR meter won’t work, since it doesn’t directly show loss, and can’t indicate just the losses in the transmatch. However, with a few hookup changes, the matching network’s losses can easily be found with an antenna analyzer or impedance bridge. A low-tech approach to measure ATU loss is to power-off the transmitter, soon after transmitting, and place one’s hand directly on the coil (after first discharging the coil to the matchbox chassis). If it feels too hot to touch or too warm to comfortably hold, then the coil losses are high and the setting is ”bad”; if the coil feels cool or just mildly warm then there is no significant loss, either because of a ”good” match or because of low power transmission.

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.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 set at 1.5 : 1 . This output stage power cutback is the main reason for weak transmission at high SWR, not the lesser losses to increased heating of the antenna system parts due to high SWR.

Antenna matching is best done as close to the antenna feedpoint connection as possible, to increase bandwidth, and to minimize loss in the transmission line by reducing its voltage and current peaks. Ideally, a tuning circuit made from nearly quarter-wave stubs might be incorporated into the body of the antenna itself, producing at least an approximate match at the antenna feed. Also, when the information being transmitted has frequency components whose wavelength is a significant fraction of the electrical length of the feedline, 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.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. 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. 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 voltage on the antenna side if the antenna feed’s tap-points are placed symmetrically above and below the electrically neutral point on the coil (so the coil must be grounded somewhere near its middle).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.

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.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.

Swinging link with taps modifies the Fixed link with taps by mounting the primary winding on a movable (”swinging”) platform that can be brought closer to, or further from, the transformer. The swinging link is a form of variable transformer, that changes the coils’ mutual inductance by swinging the primary coil in and out of the gap between halves of the secondary coil. Like putting a series capacitor on the primary, mentioned above, meshing the primary more completely inside the secondary winding also allows fine adjustment with fewer coil taps. 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. Conventionally, the connected tap points on the coil are positioned symmetrically around the coil’s center.Because the radio has no reactance (and no susceptance) its series and parallel resistances are the same. So for these rules about orienting an ‘L’ network, the radio side is always 50 Ω, regardless of whether it is connected to the series or parallel side of the network. If the des
cription above, or a rule below, calls for using a series or parallel resistance on the radio side, they are both 50 Ω. However, on the antenna side, they are usually different: If the antenna’s impedance has any reactance in it (or equivalently, its admittance has any susceptance in it), then the parallel resistance will be different from the series resistance; for choosing the orientation it matters that the correct resistance value on the antenna side is considered. (The parallel form of resistance is always a larger number than the series form. The formulas in the follow-on section may be used to convert between them.)

Using C1 to tune or de-tune the primary side of the transformer to the settings for C2 + C3 on the secondary side has approximately the same effect as moving the two sides of the transformer closer or further apart, hence simulating a swinging link. Adjusting the number of taps on the primary coil adjusts the Q of the network, widening or narrowing its matched frequency span, and permits compensation for change in Q resulting from the bandswitch changing the number of connected secondary turns; it also gives purpose to the generally unused extra primary windings that were originally part of a separate relay-switched feed for older 600 Ω receivers, which were still in use during the 1940s.

When multiple towers are used, the matching 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, phasing can aim the combined signal in a desired direction.

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.At the antenna, if the end of the transmission line connected to the antenna is not a conjugate match to the antenna’s feedpoint impedance, a part of any intercepted signal will be trapped inside the antenna, eventually to be radiated back out. Similarly, at the receiver, if the complex impedance at the receiver end of the transmission line is not a match to the receiver’s input connection, then some of the incoming signal will be reflected back to the antenna, and not enter the receiver. These losses of signal power are 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 insignificantly small contribution by inherent thermal noise generated within the receiver’s own circuitry. Therefore, the receiver can freely amplify the weak signal to compensate for any antenna system inefficiencies caused by impedance mismatches without perceptibly increasing noise in the output.

For example, a single, exceptionally well-made, commercially available balun can cover frequencies from 3.5 to 29.7 MHz – a span over 26 MHz wide, or nearly the entire HF band. In contrast, matching a feedline to an antenna using a cut segment of transmission line (as described below) is perhaps the most efficient of all matching techniques, 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: The 26.2 MHz bandwidth of the example balun is a more than 7 times wider span of frequencies.

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.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).