G4NSJ – Aerial Tuning Unit


Correctly termed, the AMU, is the aerial matching unit. It does NOT tune the aerial.

The term ATU has become extremely popular over the years. But this device is not an aerial tuning unit. It does not tune the aerial. It’s a matching device, matching one impedance to another. Hence, aerial matching unit or AMU. The output impedance of most amateur transmitters is fifty ohms. The impedance of an end-fed wire could be anything between twenty and several thousand ohms. If the impedance of your end-fed aerial is two-thousand ohms, it doesn’t change. The matching unit doesn’t miraculously change the aerial feed impedance to fifty ohms. Neither does it change the fifty ohm output impedance of the radio to two-thousand ohms. The AMU transforms the impedance from two-thousand ohms to fifty ohms. In other words: two-thousand ohms in, and fifty ohms out. A mains transformer transforms 240 volts to, say, 12volts. It doesn’t somehow alter the mains supply voltage. That remains at 240 volts no matter what you do. Remember, an AMU does NOT improve the performance of an aerial. A poor aerial is a poor aerial, no matter what you do at the feed point. Frightening stuff? Not really. What is frightening is the misconception, the myths, surrounding this impedance matching device.


A simple L-match is shown in the circuit below, left. This is the arrangement I use to match my 150 feet of end-fed wire. L is a roller coaster and C a wide-spaced variable capacitor, about 500pF. From 80 metres to 10 metres, I get a perfect match.


For 160 metres, I have to switch the capacitor in series with the aerial and short out the coil, as in the diagram above, right. This is because my wire is too long for top band, resonating somewhere near to 1.6mHz. Effectively, a coil in series with a wire aerial electrically lengthens the aerial, and a capacitor is series electrically shortens it.

The aerial socket on the back of your transmitter wants to ‘see’ fifty ohms. And this is exactly what it does ‘see’ when it ‘looks’ into the AMU circuit. Before you grab some bits out of the junk box and build this matching unit, there are a couple of things to bear in mind.

Depending on the frequency you are using and the length of your end-fed aerial, the feed impedance of your aerial may be very high or very low. If your aerial happens to be one half-wavelength at your operating frequency, the feed impedance could be in the region of 5000 ohms. Does this matter? If the aerial matching unit is good enough to match this impedance then, no, it doesn’t matter. What does matter are the high voltages present at the feed point. The variable capacitor will flash over unless there is a sufficient gap between the vanes or plates of the capacitor.

Unfortunately, decent variable capacitors are hugely expensive and there are virtually none around on the secondhand market. However, you’re going to have to get your hands on one somehow. High voltages will also be present in the coil. A flimsy thing would with very thin wire on a cardboard toilet roll tube will probably catch fire. Imagine, when you are constructing your aerial matching unit, that it will be dealing with 1000 Volts or more and you shouldn’t go far wrong.

Just out of interest, the variable capacitors used in the final stages of high power transmitters are placed in a vacuum. This way, the capacitors may be relatively small. By small, I mean taking up several feet rather than an entire room. An electric arc needs oxygen to survive. Take the air away, and you will drastically reduce the possibility of arcing. A vacuum is also used for high voltage switching on the national grid. On lower voltage circuits of around 33kv, the switch contacts are immersed in oil.

What does make me laugh, and annoy me, is the way manufacturers rate their tuning units at 1kw or more when they’re using what amounts to broadcast receiver variable capacitors in their so-called ATUs. These capacitors were great in the old mains radios. But they have very little spacing between the vanes and are liable to flash over when loading an end-fed wire which is voltage-fed. The capacitors may be fine when using a current-fed aerial, but they are no good for voltage fed aerials.

This is the ultimate aerial matching unit.

Now we come to a real aerial matching unit, the mother of matching units. This will match balanced line or single wire end fed aerials. The components used are few and simple. However, trying to get hold of them is a different matter. A split stator capacitor will cost you in the region of £80 or more, and as for the coils… Let’s not get despondent before we even consider constructing this AMU.








Split-stator capacitors are used to preserve balance across the circuit. The split stator capacitor problem can be overcome by using two separate capacitors ganged together. But the cost is still immense.

One form of split stator capacitor is shown here. Basically, this type is two variable capacitors on one shaft. The rotors or moving vanes are electrically connected whereas the stators are isolated from each other. For some reason, probably due to lack of demand, these capacitors are costly.

I was lucky enough to purchase a KW107 Supermatch ATU. This fine piece of kit is British made but, sadly, they’re no longer manufactured. If you can get your hands on one, then grab it.

The KW Supermatch has two coax outputs and two balanced outputs. It also has a built-in SWR meter and a 300 watt dummy load. With no lossy baluns in sight, this really is an amazing machine.

The diagram shown on the right is the basic circuit of the KW E-ZEE match and the KW107 SuperMatch ATUs. Unfortunately, I don’t have the full circuit showing switching etc. However, this gives a pretty good idea of how the thing works.