Propagation and antennas

Coupling the transmitter to the antenna

Modern commercial transmitters and transceivers have output circuits which are designed to 'look into' an unbalanced load of 50 to 70Ω. For optimum transfer of power to the antenna, impedances throughout the system must be matched. For example, if the antenna feed-point impedance is 50Ω, this should be connected to the transmitter output socket by an unbalanced line, ie coaxial cable of 50Ω impedance. The output power for which the transmitter is designed is then transferred to the antenna from which it is all radiated.

In practice the feed-point impedance of an antenna can vary widely from its nominal value. Two typical reasons are:

1. Siting conditions: the proximity of the antenna to local objects such as buildings and trees, and with HF antennas in particular, height above ground. The lower an antenna for a given band, the lower the impedance.
2. For a given antenna, excursions within a band, ie from one end to the other, will cause a change. Some types, in particular multi-band antennas, will vary more than others and also from band to band.

Hence, as the mismatch increases, the reflected (reverse) power rises and the SWR increases. Thus SWR is a measure of the effectiveness of the whole system.

Matching the feeder impedance to the antenna itself may not be a straightforward task, particularly in the case of multi-band antennas which are themselves a compromise.

Fig 7.10. Preferred arrangement of transmitter-to-antenna circuit

The solution to this problem, which has become virtually standard practice, is to match the transmitter to the feeder plus antenna as shown in Fig 7.10 and, in more detail, in Fig 9.6 in Chapter 9, 'Electromagnetic compatibility'.

Matching is achieved by means of an antenna tuning unit (ATU). This does not tune the antenna; it matches it to the transmitter output and hence should really be called an 'antenna matching unit', but it has been known as an ATU by common usage for very many years. Other names now used are 'antenna system tuning unit' or 'matching network'.

Thus the transmitter-to-ATU connection is a very short matched transmission line at 50/70Ω. This has the advantage that it can include means of measuring the SWR and a low-pass or other filter if necessary. These two items would be designed for use at the line impedance and so the filter would have its designed attenuation. The ATU is basically a tuned circuit and so provides about 25dB attenuation of harmonics and other unwanted frequencies in the transmitter output. It has an unbalanced input socket and generally it can provide both balanced and unbalanced outputs. Note that the low pass filter should be removed when setting the ATU since the SWR meter will give an incorrect reading when it is connected. This is most often achieved using a bypass switch on the filter unit. When the filter is reconnected, the SWR reading should be monitored only for any change (indicating a possible fault) since the meter reading will actually be a measure of the SWR of the filter's input network, not the antenna itself.

A basic circuit of an SWR meter is shown in Fig 10.7 in Chapter 10, 'Measurements'. This is the simple reflectometer type originated in the USA as the 'Monimatch'. The SWR meter and the ATU exist in many home-made and commercial versions.

This arrangement has become more significant in recent years due to the use of transistors in the transmitter output stage. Such transmitters often incorporate automatic means of switch-off or power reduction if the SWR rises above about 2.5 to 1. The more sophisticated transmitters now include an ATU and SWR meter in their output circuit.

This aspect is not so critical if the output stage uses valves. Earlier transmitters were capable of matching almost any length of wire to most amateur bands.

A perfectly matched system will have an SWR of 1 to 1. A modern commercial transmitter may switch off or reduce power at about 2.5 to 1. The question therefore arises: what is an acceptable maximum SWR? There are many possible errors in SWR measurement, and so an SWR of 1 to 1 could be regarded with a certain amount of suspicion. A system which appears to have an SWR greater than about 5 to 1 should certainly be investigated, although the power loss then is only just less than 3dB. It is probably more important to reduce SWR to safeguard a solid-state transmitter output stage than for any other reason.

In practice the consequences of SWR are:

1. Greater loss in the feeder. How much greater depends on feeder type and frequency. In general it can be said to be inconsequential at HF (up to 30MHz), but may be significant at VHF (144MHz) and will certainly be so at UHF (432MHz and upwards).
2. Use of very high power with an excessive SWR may cause breakdown of the feeder or units 'inline', such as filters or switches. Breakdown can be caused by flashover (due to high voltage) or melting of conductors or dielectric (due to high current).

Note that a high SWR, of itself, does not cause a feeder to radiate, or produce TVI or other interference.

The impedance matching of the transmitter/antenna system discussed above applies to the HF bands. At VHF and UHF, the transmitter and antenna are normally designed for single-band operation rather than for several bands. A much tighter control of impedance is therefore possible. Antenna length is much shorter compared with the height above ground, and hence there is less doubt about feed-point impedances. An SWR meter can be incorporated to check the integrity of the antenna system.

To summarise, current amateur technology is to match as closely as possible to the ATU input and to accept the SWR presented by the antenna to the ATU output. It may be difficult to measure this. It depends on how the antenna is fed: single wire, coaxial cable or open-wire feeder for example. The likely antenna performance can be judged fairly accurately by visual inspection, ie is it in the clear and at a reasonable height compared with the longest wavelength used? If it is, then it is likely to have a good performance, propagation conditions permitting! If the antenna is low and hemmed in by buildings and trees, it is then likely to be less good, although antennas so placed often perform surprisingly well.

Photo 7.4. A wideband SWR and PEP power meter