Section 4
Feeders and Antenna

Antenna matching units

4F1 Recall that where an antenna has not been designed for the frequency being used, the feed resistance will change resulting in a mismatch and that an Antenna Matching Unit (AMU), also sometimes referred to as an ATU, can correct the mismatch and is used to ensure that the transmitter can supply energy to the antenna without damage to the transmitter.

Whilst from the above you have learned earlier, we hope, that an antenna is designed for only a single frequency because the feed resistance will change which results in a mismatch at the transmitter.

So if you want to work a particular band then it is best to make the antenna resonant on the centre frequency for that band.

This is where an Antenna Matching Unit (AMU), also sometimes referred to as an ATU, can correct the mismatch and is used to ensure that the transmitter can supply energy to the antenna without damage to the transmitter.

However by the use of an Antenna Tuning Unit the transceiver can be fooled into thinking that the antenna is the right one for it, and not be damaged and will therefore radiate much of the output power from the transceiver.

Let's look at the words Antenna Tuning Unit. Whilst you might think that the antenna is being tuned the only way to tune and antenna is to physically alter it construction. All the Antenna Tuning Unit is doing is changing the impedance of the antenna to appear to be 50 ohms which is required by most modern rigs. It is better to think of an ATU as an antenna matching unit.

The origin of some of the text on this page is from the RSGB with additions by the web master