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AMATEUR RADIO BY GARRY CRATT, VK2YBX Antenna tuners: why they are useful If you browse through most catalogs of amateur equipment you will find a range of antenna tuners available for the amateur bands. Perhaps you may have had doubts about whether these devic­es are worthwhile. They are & this article explains why. Possibly the most commonly considered theory regarding the benefit of antenna tuners is that they improve antenna efficien­ cy and so assist in the effective radiation of signals by the antenna. In fact, nothing could be further from the truth. Anten­ na tuners do nothing to improve antenna efficiency but there certainly are other good reasons to use one. These days, all modern transmitters are designed to operate into a nominal resonant 50Ω load. This is all very well in theo­ry but in practice very few antennas present such an ideal impedance to the transmitter. In addition, solid state transmit­ters are designed so that their output power drops as the load SWR increases, to protect the final output stage from excessive dissipation which would occur when feeding a highly reactive load. So any antenna mismatch leads to increased SWR and there­fore a subsequent reduction in radiated power. For the VHF and UHF bands, the scale of resonant antennas is such that they can be made with quite manageable physical dimensions. They can also be made to provide relatively wide bandwidth, whilst maintaining a reasonable Q. We know that any piece of wire connected to a transmitter will radiate This heavy duty antenna tuner from Emtronics is based on the Pi network shown in Fig.3 but it also features monitoring of forward & reflected power via a twin needle meter to give SWR readings. It can handle HF band transmitters with output powers rated up to 1000 watts. 84  Silicon Chip signals to some degree, so it is logical that there will be a considerable advantage in using a device which assists in the matching of an HF antenna to the transmitter output stage, maximising the current flowing in the antenna, and thereby resulting in improved field strength. A related factor to be considered is the Q, or “quality” factor of the antenna. Generally, RF experience indicates that the higher the Q, the better. However, this is not necessarily the preferred situation with HF antennas. A high Q means a narrow bandwidth and readjustment of the antenna tuning unit may be necessary, even for small changes in frequency. For HF antennas, a low Q is preferred. As the Q of an antenna is determined by both the radiation and DC resistance, it may be preferable to select an antenna tuner where capacitive reactance is added to bring the antenna system to resonance, lowering the Q and generally giving broader bandwidth. Most long wire or vertical HF antennas are loaded against ground and need to be only one quarter wavelength long at the resonant frequency. For an antenna tuner to assist in matching this type of antenna, it is important to have a good low im­pedance ground, so that equal currents can flow in both the antenna and ground, hence producing an antenna radiation pattern which will be of some use. If an insufficient ground is provided, an imbalance will exist, and the resultant radiation pattern will have (in the instance of a vertical) a high angle of radiation. Measuring antenna current A simple antenna current indicator can be made using a 25mm ferrite toroid slipped over the antenna wire. Fig.1: this circuit arrangement can be used to monitor the current flowing in a wire to an antenna or in the ground return. The two capacitors are each 100pF disc ceramics while the diode is any germanium type such as OA91. The meter is a 1mA movement. This photo shows the interior of the Emtron EAT-1000A antenna tuner. Note the wide spaced variable capacitors and the large tapped inductor. Fig.2: the simplest configuration for an antenna tuner, used to match a low impedance (50Ω) transmitter to a high impedance line is either a parallel (a) or series (b) tuned circuit, resonant at the operating frequency. A pick up wire, comprising several turns around the toroid, feeds a diode and 100pF capacitor, wired to a 0-1mA meter which has a another 100pF capacitor across it. Fig.1 shows the circuit. By applying some RF energy from the transmitter and adjusting the antenna tuner, an increase in antenna current can be verified. The same circuit can be used in the ground lead, to verify current flowing. The circuit configuration of an antenna tuner needed to transfer maximum power from the transmitter to the antenna de­pends to a large degree on the impedance of the feed line. The simplest configuration, used to match a low impedance, say 50Ω, transmitter to a high impedance line is either a series or paral­ lel tuned circuit, resonant at the operating frequency. Fig.2 shows various configurations of series and parallel matching networks. A superior arrangement, based on the Pi network shown in Fig.3, allows 50Ω or so to be matched to an impedance of up to several thousand ohms. Both capacitors C1 and C2 are Fig.3: based on a Pi network, this antenna variable and, in a high power tuner allows 50Ω or so to be matched to an situation, must have widely impedance of up to several thousand ohms. spaced plates, due to the large Both capacitors C1 & C2 are variable & amount of energy normally are usually ganged together. The inductor involved at HF. The induc­tor should be made from large diameter wire or should be made from large dicopper tubing, to minimise insertion losses. ameter wire or copper tubing, to minimise insertion losses. A well designed Pi network antenna received signal performance when tuner should also in­clude some form a high impedance antenna, such as of gas discharge protection circuit, to a long wire, is used with a low imprevent possible damage to the trans- pedance re­ceiver. The antenna tuner mitter equipment from atmospheric reduces the SWR by improving the discharge. antenna impedance matching, resultSome lower power designs use a ing in maximum transfer of energy. tapped inductor to ensure a “match” This is most noticeable when using a across a wide range of impedances. receiver without an RF stage, where In any case, the network is used to the antenna input is fed via a bandpass correct a mismatch problem and some filter to the mixer. In receivers having reduction in system efficiency when a high amount of RF gain in the first compared to a correctly matched stage, the effect is not as noticeable. an­ten­na at the same frequency will In summary, an antenna tuner is be noticed. However, this can still no substitute for a proper­ly designed provide a major improvement over an resonant antenna, but in cases where unmatched antenna without a tuner! such an antenna cannot be used, they Antenna tuners are also capable of can offer improved performance over SC an unmatched antenna system. making a noticeable improvement in July 1993  85