L-Match ATU

Preamble

There are many ATU projects around, all a little different from each other.

What makes a good ATU?

Easy to tune,
Tunes (nearly) all bands,
Is simple to operate,
Has a useful form factor,
Can support both balanced and unbalanced lines,
Is efficient/low loss

Construction

As a construction exercise, I decided to build a simple L-match ATU based on the design from Peter Parker VK3YE. The L-match tunes either impedances greater than 50 ohms, or less than 50 ohms depending on whether the variable capacitor precedes the inductor or not. Peter's design is very space efficient and was purpose built for QRP portable operation. My ATU was for low power, luggable operation (ie my work location, etc). L-Match

The design consists of a 30 micro-Henry air wound inductor I found in the spares box at the WARS club. I used a 100 pico-Farad air-spaced variable capacitor which I subsequently found not to be large enough for the lower bands (ie less than 40m). I used a convenient toggle switch for switching between high and low impeadances, and a 12 position rotary switch to variably select a tap point on the inductor. All pretty simple really. L-Match

The exercise here though was to improve the quality and aesthetics of the housing of the ATU. I wasn't that happy with the Howes transceiver housing. This time, I drilled all the holes for the connectors first in the sheet metal box. I've found that gradually drilling larger holes successively (starting at 2mm progressing to 16mm) improves the final quality substantially. I don't have a lot of vices, G-clamps etc so this method works well for me. L-Match - Internals

An under-coat of car spray primer, followed by a couple of coats of mat-black car spray made the housing look very presentable. Finishing with black knobs sourced from the club spares made the final enclosure look very swish indeed.

Results

It works....

However, there are a few small issues I have with it. Once on the air, you quickly learn that retuning an ATU every time you switch bands, it's not a lot of fun having to re-tune. To make tuning easier for myself, it's now apparent that you need to design the successive inductor tapings to present an impedance slightly less than the maximum impedance presented by the variable capacitor. This would present a smooth transition of impedances when switching through the inductance tap points.

This can be expressed mathematically fairly easily such that you can determine the optimum tapping points in terms of turns ratios for each tap. When I get a chance to format up some equations, I will drop it in here!

Shane Magrath
VK2KEP

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As a construction exercise, I decided to build a simple L-match ATU based on the design from Peter Parker VK3YE. The L-match tunes either impedances greater than 50 ohms, or less than 50 ohms depending on whether the variable capacitor precedes the inductor or not. Peter's design is very space efficient and was purpose built for QRP portable operation. My ATU was for low power, luggable operation (ie my work location, etc).
The design consists of a 30 micro-Henry air wound inductor I found in the spares box at the WARS club. I used a 100 pico-Farad air-spaced variable capacitor which I subsequently found not to be large enough for the lower bands (ie less than 40m). I used a convenient toggle switch for switching between high and low impeadances, and a 12 position rotary switch to variably select a tap point on the inductor. All pretty simple really.
The exercise here though was to improve the quality and aesthetics of the housing of the ATU. I wasn't that happy with the Howes transceiver housing. This time, I drilled all the holes for the connectors first in the sheet metal box. I've found that gradually drilling larger holes successively (starting at 2mm progressing to 16mm) improves the final quality substantially. I don't have a lot of vices, G-clamps etc so this method works well for me.