There is a vast selection of antenna tuners or antenna tuning units, ATUs available on the ham radio market to purchase.
Presented with the selection available, it is not always easy to know which one to select and whether all the options the more expensive ones offer will actually be used and be worth while.
Also is it wise to make an antenna tuner rather than buying one as there are plenty of circuits available.
When selecting an antenna tuner / antenna tuning unit, ATU to buy there are several points that may be of interest:
- Frequency coverage: Antenna tuning units / antenna tuners are typically intended for HF band operation, but before buying one, it is worth checking that it covers the bands that are required. Normally they cover the HF bands, either 160 metres to 10 metres or 80 metres to 10 metres, but some also cover 6 metres, etc.
- Power capability: There are many different ATUs / antenna tuners available. The power rating is an important parameter, especially if high power operation is envisaged. High power ones will work equally well with low powers, but it is important to ensure that the power rating is not exceeded. Switches, coils, etc may overheat or arcing may occur if the power rating is exceeded. As some antenna tuners are intended for low power, QRP operation, these should not be used for power levels over a few watts.
It is also wise to be wary about overloading automatic tuners - they use reed relays within their circuits to provide the switching, and overloading these relays can result in damage - possibly the relay being welded closed. In view of this, it is wise to check the power handling capability of the antenna tuner before buying.
- Manual or Auto: In the early days of amateur radio, all antenna tuners were manual and often boosted late control knobs on the front panel for easy and accurate adjustment of the variable capacitors - often a large switch was also used as this matched the capacitor knobs and also the switches tended to be large to withstand the power.
Today many antenna tuners that are available to buy can link to the transceiver and automatically tune themselves to provide the best match at the press of a button. Within the tuner unit, they contain a number of fixed inductors and capacitors. A series of reed relays under microprocessor or microcontroller control, then switches in the right capacitors and inductors to obtain the best match. Whilst this is happening the transceiver is set to provide a low level of power to enable the tuner to detect the right match. All of this takes just a few seconds and it is much faster than trying to achieve this manually.
- Check that your transceiver is supported: When buying an automatic ATU / antenna tuner, check that your particular transceiver is supported. Most of the later ones from the main manufacturers, Kenwood, Icon, and Yaesu will be, but check. Also different options are required for the different manufacturers. This is typically a different interconnecting control lead, and there may be a link to be changed inside the antenna tuner.
- Tuning time: The tuning time is another specification that is given and is worth taking a look at when buying an antenna tuner. Often it may be as much as fifteen seconds, but normally it will be less. The specification for the particular tuner will be provided in all the sakes literature. Even at fifteen seconds, the actual tune time is much less than if it were done manually, so this is very good.
- Power for tuning: All automatic antenna tuners require some power for them to be able ti tune. They detect the level of SWR and then reduce this. Accordingly the transmitter needs to be activated. Normally this is done using a low level of power to prevent damage to the transmitter output, and to reduce interference. In general, the lower the better, but often it may be around ten watts or so, and providing a check when in normal transmit.
- Tuning memories: Automatic antenna tuners generally have memories so they can retain the settings for certain frequencies. Often they may have a few thousand memories, and some may have them segregated for different antennas. If multiple antennas may be used, it is worth checking how they work and whether the operation will meet your needs.
- Impedance match range: Despite some of the hype, not every ATU can match every antenna. They have an impedance range over which they can match. Before buying, check what this is and opt for the best one. It is normally not possible to know exactly what impedance range any antenna will present under all conditions, but make an educated guess. Select the one that gives the best range, etc.
Different antenna tuners will be able to provide a match over different ranges. Examples include the MFJ928 and MFJ939 which both provide a match over the range 6 to 1600Ω. Other antenna tuners will be able to match over different ranges so a judgement needs to be made. Antennas like resonant dipoles will be relatively close to 50Ω when operated on their intended band, but other antennas, especially end fed ones, or doublets, etc will have an impedance that might be much more variable.
- Balanced or unbalanced: Coaxial cable is an unbalanced feeder, and often ATUs / antenna tuners will match from unbalanced to unbalanced. If you are using a balanced antenna that uses twin / balanced feeder then you will need an ATU that supports this, or you can use an additional balun.
The balanced antennas are typically those that are fed with balanced feeder like twin, open wire, etc. Neither side of the feeder is earthed and therefore it is termed balanced. Antennas fed with coaxial cable are unbalanced as one side of the feeder acts as the earth.
Antennas that support unbalanced antennas will have a coaxial socket for the output to the antenna. Antenna tuners that support unbalanced operation will have two terminals to which the two wires from the open feeder are attached.
These days the use of balanced antennas is not as common as it used to be. Bringing balanced feeder into the home can cause the balance to be degraded and losses rise. Accordingly a more usual solution is to use a coaxial feeder on the output of the antenna tuner and then use a balun at the antenna or where the feeder transitions between balanced and unbalanced.
- Reliability: The reliability of the antenna tuner is of great importance when buying an antenna tuner, but it is one of the more difficult points to judge. Sadly without the experience of people who have used them it is a difficult point to judge. Most manual tuners are relatively reliable - the main issue is the switches. If they are treated well, not switched when power is passing through them (not good for the transmitter either), and the power is kept within limits, these antenna tuners can go on for very many years.
Automatic tuners can be a little more troublesome, but most are very reliable. They reduce the power of the transmitter and step through a variety of positions - this switching uses reed relays which have very long lifetimes. Again, excessive power should not be passed through them otherwise the reed relays can be damaged.
To gain a good view of the reliability, it is best to consult the equipment reviews and the comments of those who have used them. Also, don't forget that generally you get what you pay for.
Before buying an antenna tuner, it is best to have a good look to see what is available. Check out the reviews, and also speak to others to get a good view of the best antenna tuner to buy. A little time researching the various antenna tuners available, will ensure that the best option is chosen.