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Portable ATU Construction

Portable ATU Construction

The portable ATU shown in my You-Tube Video costs less than 15 to build.

It is based on the famous Z-Match circuit which uses two dual-section polyvaricon tuning capacitors and will handle about 25 Watts depending on the frequency and load. The exact value of the polyvaricons is not important but the dual-sections must be equal e.g. 275/275 pF or 335/335 pF is fine, but 140/60 pF won't work properly.

The orientation and placement of controls may seem a little weird at first, but this eliminates wasted space, enabling the ATU to be kept as small as possible. Note the foam packing around the switch in the centre of the toroid, this is all that is necessary to keep the toroid in place.

Construction is straight forward but take your time marking out and drilling the box as it's easy to make a mistake. I was always taught to measure twice and cut once. After you have done that the wiring is easy.

Winding the toroid is not difficult, just keep the winding tight and remember that every time the wire passes through the centre of the toroid it counts as one turn regardless of where it starts or ends. The wire size/type is not too important, provided you can get it on the toroid in a single layer. I used some old 24 s.w.g. pvc I had to hand for both primary and secondary but, if you are going to buy the wire specially, 20 s.w.g. for the primary and 24 s.w.g. for the secondary seems the norm. I added some tape after winding my toroid to protect the windings, rather than keep them in place.

Here's the basic Z-match circuit diagram.

The most common variation is the addition of a switched 470 pF fixed capacitor across the coupling capacitor, to lower the minimum usable frequency to 1.8 MHz.

Another common variation is the use of a different size or type of toroid that dictates a significant change in the number of turns.

Beric (K6BEZ) kindly sent me some photo's of the ATU he built with the components attached to the enclosure's lid, which makes assembly and wiring much easier.

He also used BNC rather than phono sockets, and a slightly longer enclosure so they could be mounted on the front panel, which is much more practical and well worth the little extra space it requires.

Good luck with your own construction.

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