Brats club logo


Syllabus Sections:-

5e Antenna Matching Units

5e.1 Understand that AMUs (ATUs) can "tune-out" reactive components of the antenna system feed-point impedance (before or after the feeder) and can transform impedances to an acceptable resistive value.

The main purpose of an ATU is to present to the Transmitter at its aerial socket with a resistive load of the correct impedance.

The Antenna Matching Unit or antenna Tuning Unit as it was first explained to you in the Foundation Course Click here to check back and again in the Intermediate Course Click here to check back.

You are aware that an AMU (or ATU same item) does not actually tune the antenna, that can only be achieved by altering the physical dimensions of the antenna.

What can an AMU achieve ?

The AMU as this level of amateur radio prefers to use can:

  • can "tune-out" reactive components of the antenna system feed-point impedance (before or after the feeder) and

  • can transform impedances to an acceptable resistive value

How does it work ?

In side an AMU is generally a one or two large coils that can be "tapped" and a variable capacitor or several variable capacitors.

By careful manipulation of the "tuning controls", at a low power input from the transmitter but sufficient to be able to observe the changes to the control settings, the inductor(s) (coil(s)) can "tune out" reactive components of the antenna system feed-point impedance and transform impedances to an acceptable resistive value.

As there are parallel tuned circuits in the AMU it can also filter out unwanted harmonics of the wanted frequency. Further parallel tuned circuit can develop high voltages and current passing through the components so care has to be taken in the selection of particularly the capacitors so that they will not break down or "flash over" - the wire of the inductor also needs to be of a substantial size.

Can an AMU tune all antennas ?

Quite simply No !! Whilst you do not have to know the ins and out of exactly what are the limitations and how do they come about - you do have to know that the AMU can match a fairly wide range of impedances mismatch to the usual 50 output from the transmitter but there are limits to the range.

Understand that if the AMU is located at the transmitter, it will have no effect on the actual SWR on the feeder between the AMU and antenna.

Where can the AMU be used ?

The AMU can be placed near to the antenna or near to the Transceiver.

It should be obvious t you that if it is placed at the Transceiver end of the feeder then the AMU can have no effect on the mismatch all the way down the feeder but will merely present to the transmitter a correct load. Further with a long feeder run and in consequence high SWR in the feeder, the feeder losses will also be greater.

Thus where possible it is best to place the AMU at the load (the antenna) and use a feeder that has an inherent impedance the same as the transmitter.

Identify typical AMU circuits (i.e. T, Pi and L circuits).

The AMU can come in various forms T, Pi and L styles and you have to be able to identify the types. If you look closely you will note that the "T" type looks like a "T", the "PI" type like the Greek letter "" and the last one "L" like an inverted "L".

AMU "T"

AMU "PI"

or AMU ""

AMU "L"

You have to be able to distinguish between the various circuit diagrams.


That is the end of this section

brats copyright logo