Antenna tuner

An antenna tuner, a matchbox, transmatch, antenna tuning unit, antenna coupler, or feedline coupler is a device connected between a radio transmitter or receiver and its antenna to improve power transfer between them by matching the impedance of the radio RF port to the antenna's feedline. Antenna tuners are particularly important for use with transmitters. Transmitters feed power into a resistive load, very often 50 ohms, for which the transmitter is optimally designed for power output, efficiency, and low distortion. If the load seen by the transmitter departs from this design value due to improper tuning of the antenna/feedline combination the power output will change, distortion may occur and the transmitter may overheat.
ATUs are a standard part of almost all radio transmitters; they may be a circuit included inside the transmitter itself or a separate piece of equipment connected between the transmitter and the antenna. In transmitters in which the antenna is mounted separate from the transmitter and connected to it by a transmission line, there may be a second ATU at the antenna to match the impedance of the antenna to the transmission line. In low power transmitters with attached antennas, such as cell phones and walkie-talkies, the ATU is fixed to work with the antenna. In high power transmitters like radio stations, the ATU is adjustable to accommodate changes in the antenna or transmitter, and adjusting the ATU to match the transmitter to the antenna is an important procedure done after any changes to these components have been made. This adjustment is done with an instrument called a SWR meter.
In radio receivers ATUs are not so important, because in the low frequency part of the radio spectrum the signal to noise ratio is dominated by atmospheric noise. It does not matter if the impedance of the antenna and receiver are mismatched so some of the incoming power from the antenna is reflected and does not reach the receiver, because the signal can be amplified to make up for it. However in high frequency receivers the receiver's SNR is dominated by noise in the receiver's front end, so it is important that the receiving antenna is impedance-matched to the receiver to give maximum signal amplitude in the front end stages, to overcome noise.

Overview

An antenna's impedance is different at different frequencies. An antenna tuner matches a radio with a fixed impedance to the combination of the feedline and the antenna; useful when the impedance seen at the input end of the feedline is unknown, complex, or otherwise different from the transceiver. Coupling through an ATU allows the use of one antenna on a broad range of frequencies. However, despite its name, an antenna 'tuner ' actually matches the transmitter only 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 match at every point in the transmitter-feedline-antenna system. 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 very 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. With lossy feedlines maximum power transfer only occurs if matching is done at both ends of the line.
If there is still a high 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 reduce power delivered to the antenna. Therefore, 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.

Broad band matching methods

s, autotransformers, and baluns are sometimes incorporated into the design of narrow band antenna tuners and antenna cabling connections. They will all usually have little effect on the resonant frequency of either the antenna or the narrow band transmitter circuits, but can widen the range of impedances that the antenna tuner can match, and/or convert between balanced and unbalanced cabling where needed.

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 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 feedline system design 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 and feed the signal to the TV via 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.
It should be added that many ferrite based transformers perform a balanced to unbalanced transformation along with the impedance change. When the balanced to unbalanced function is present these transformers are called a balun. 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, and whether the input and output sides share a common ground, or are matched from a cable that is grounded on one side to an ungrounded 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.
The circuit pictured at the right has three identical windings wrapped in the same direction around either an "air" core or ferrite core. The three equal windings shown are wired for a common ground shared by two unbalanced lines, 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 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.

Narrow band design

The "narrow-band" methods described below cover a very much smaller span of frequencies, by comparison with the broadband 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 radio band. Matching to an antenna using a cut segment of transmission line 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 - a very small range indeed, compared to a 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.

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.
The simplest example this method is the quarter-wave impedance transformer formed by a spliced 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. Thus this inserted section matches a 112 Ohm antenna to a 50 Ohm main line.
The coaxial transformer is a useful way to match 50 to 75 Ohms using the same general method.
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. The J-pole antenna is an example of an antenna with a built-in stub match.

Basic lumped circuit matching using the L network

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 complex circuits can be analyzed as groups of L-networks.
This is called an L network not because it contains an inductor,, but because the two components are at right angles to each other, having the shape of a rotated and sometimes reversed English letter 'L'. The 'T' network and the π network also have a shape similar to the English 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,