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Adjusting a T match ATU for best efficiency

Introduction

T match ATUs are very popular with HF ham radio stations. They are an adjustable impedance transforming device, used to transform the impedance seen looking into the antenna feedline into a load impedance that suits the transmitter. This article discusses the high pass T match configuration (series C, shunt L).

Commonly, the load impedance that suits the transmitter is 50+j0Ω, and the means of detecting that condition is a VSWR meter between transmitter and ATU, or as is often the case, a VSWR meter integrated into the ATU between its tx connector and the T network.

Users will have noticed that a 'match' can be obtained at more than one combination of the three T network components, and this raises the question as to whether there is an optimum combination, and how to find that optimum combination.

Concepts

There is usually a single combination that delivers the desired impedance transformation (witnessed as input VSWR equal to 1) with the least loss. For most purposes, this would be considered an optimum combination.

So, finding the optimum combination is simply a matter of finding the settings that deliver the most output power when adjusted for input VSWR equal to 1.

Relative output power can be indicated using an inline volt meter, current meter, power meter on the antenna feedline (ie antenna side of the ATU), or a field strength meter.

Note that when the T match is adjusted for input VSWR equal to 1 using different combinations of the ATU controls, the transmitter sees the same load impedance and will deliver the same power to the ATU.

Maximum power output does not necessarily coincide with input VSWR equal to 1 (contrary to popular belief). There is good reason to adjust the ATU for maximum output power coincident with input VSWR equal to 1, see:

for more discussion.

Many T match ATUs use air dielectric capacitors which have very low loss. In that case:

Estimating T match ATU efficiency describes this in more detail.

In practice

Find out which settings of the ATU controls give higher C and higher L and make note of it, do not depend on dial markings as they are commonly just logging scales and bear no relation to the quantity of C or L.

Dismiss rules of thumb that you may have heard, most are wrong as is evidenced that they are often inconsistent with each other.

Using low power, find a setting that gives input VSWR equal to 1 and note the relative output power.

Try to find a better setting. A good place to start is less L.

When you have found the settings that give maximum relative output power for input VSWR equal to 1, record the settings for future use. You might consider something like Antenna Tuner Calibration Chart for record keeping.

If you note departures of the behaviour of an ATU from the concepts set out above, it is due to the fact that components are not ideal. Sometimes, a new inductor setting may have quite different inductor loss. Stray and distributed inductance and capacitance have more effect on higher frequencies, but efficiency is usually better at higher frequencies.

A more complete ATU

You might now be wondering why ATUs with metering don't also incorporate a relative output power indicator. It is a good question.

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