From The Dungeon

From The Dungeon. 13 Jan 2007 Allan Copland GM1SXX

Antennas and Antenna Tuners.

Were it possible to vote a word or term out of existence or expunge it from the popular parlance, the term at the top of my list without question is the phrase 'Antenna Tuner'.

Why do I have such a strong dislike for what is after all an almost universally used term? Well I quite often hear radio amateurs on the air talking about 'resonating their antenna' or 'tuning their antenna' with the help of their 'antenna tuner'. Nothing they are saying here makes any actual sense. The last time I listened to this sort of waffle was on a local VHF rag-chew two nights ago.

You tune or resonate an aerial (not an antenna, oh and the plural of antenna is 'antennae', not antennas) by means of a pair of wire-cutters or a saw in most cases. It tends to be a bit of a physical process.... chop bits off or add bits on until it's resonant. Adding them back on if you get it wrong tends to be a bit of a hassle, so sensible folks start oversize and prune to resonance..

To be resonant, the size of an aerial (lets ditch antenna) must be related to the length of the wave to be transmitted or received. The common half wave dipole is exactly that... an electrical, not physical, half of the wavelength in use. It therefore logically follows that a dipole will only be resonant over a small frequency range, or harmonics of that range. Of course, most of us want more than single-frequency operation and to get a non-resonant aerial to work with our transceivers, the transceiver wants to 'see' a 50 (nominally 52) ohm non-reactive load.

We then almost universally place the 'Antenna Tuner' in the shack, close to the transceiver, where it can do the least good, rather than at the feed-point of the aerial where it can do most good. Operating convenience takes precedence over performance.

So, what's wrong with placing the 'ATU' in the shack? Well if there is a mismatch at the aerial end, the electrical demands made of the feeder increase, along with the losses. If you visit a broadcasting station or the site of a radio-navigation beacon, you'll find a small building or enclosure close to the transmitting aerial. That's their 'aerial matching unit' and it's placed there for a good reason. In a single word.... efficiency. Common types of aerial matching units use L, T and PI configurations of capacitors and inductors.

Transmission lines can have various impedance characteristics ... 52, 72, 93, 300, 450 and 600 ohm being the most common. Matching at the aerial end of the transmission line is every bit as important as matching at the rig end.

For some reason unknown to the author, 52 is sold as 50 and 72 is sold as 75R

Most amateur transceivers are designed to work with 50 ohm cables (actually 52R). Since the cable in use is very likely to be of a 52R characteristic impedance, you then need to match the far end of your 'transmission line' to the aerial. Should you be using a resonant dipole (typical impedance 72R) then there will only be a small and acceptable mismatch should you connect it directly. Should you be using a wire aerial of unknown properties, the impedance could be anywhere from 20 to several thousand ohms. You should use an impedance transformer, at the aerial to match the aerial impedance to that of the transmission line. This is the ideal place for the AMU to go and where in general only professional radio users place it.

The 'Antenna Tuner' or more correctly (Aerial Matching Unit) has the job of making a 'presentable' impedance match. It matches the impedance 'seen' at the cable to that of the radio. Tuning is what it does NOT do. It is simply an impedance transforming device. This of course assumes that your antenna is already a good match to the cable (transmission line) impedance.

Matching is a two part process. Match the feed-line to the aerial and match the radio to the feed-line. Since you will generally have more problems matching the aerial to the feedline, it makes the greatest sense to place your expensive AMU at the aerial end where it can do the greatest good.

It has no effect whatsoever on the resonant frequency of the aerial, so you can see the sillyness of calling it an ATU. Think impedance transformer, not tuner.

The job of the AMU , Aerial Matching Unit, not 'tuner' or 'ATU', is to transform , or match, impedances.

Of course, a far more sensible place to place such an aerial matching unit is of course at the aerial feed-point rather than in the shack. By doing that, you can correctly match the aerial to the line while at the transceiver end, you should already have a good match between your transmission line and transceiver. Do many radio amateurs ever do that? Well, only a few, who value the true purpose of an aerial matching unit. By contrast, broadcasters and operators of radio beacons almost always do things this way to maximise their transmitted power and reduce system losses.

The most efficient radiator is one where the aerial is matched to the line and line is matched to the tranceiver ... (preferably WITHOUT a load of ferrite).

Here's where it works best... The New Galloway Non-Directional (navigation) Beacon in the Scottish Borders with its AMU at the feed-point.

For anyone interested, New Galloway (NGY CW) transmits a hefty signal on 399Khz. It is a non-Directional aeronautical radio beacon (NDB). It uses a small radiating mast vertical aerial and puts out a BIG signal .