Z-match ATU covering 160m – 10m
As a new Ham, I put up a homebrew 40m coaxial dipole which wouldn’t even give me a good SWR on 40m! In frustration I looked around for an appropriate ATU circuit to build. I found this article by M0PMB which describes a design that had been previously published in Practical Wireless. This is what I originally built although I’ve made a few modifications over the years to make it more adaptable – it will tune all bands between 10m-160m. I love it!
SWR achieved is generally <= 1.1. Tuning can be on a knife-edge but very usable as long as you’re using geared drives on the capacitors (and insulated shafts). The circuit is really simple, the only tricky bit was in ensuring that the capacitors were insulated from the case – you can sometimes find wooden shift extensions at Rallies!
Another ATU using the same design was built for the Nestle Radio Club (M0NUK), of which I was a member but sadly as of September 2007 the club has now closed – to see click here. The biggest problem for building ATUs is finding air-spaced variable capacitors in good condition. New ones are virtually impossible to get hold of these days beyond 50pF unless you’re paying many tens of pounds. Rallies are a good source of ‘reclaimed’ capacitors but try to spot obvious defects such as plates touching. If you’re lucky though you might find unused, old ones (£7.50 for a dual 385pF variable capacitor with geared drive at Barnsley Rally 2004). Peter and Christine at Jabdog managed to supply an air-variable and Richard at Past Times might be worth asking too!
Below there’s a pic of the original build and to the right a pic of what happens when you use wire of too small current rating! I had two wires running in parallel for about 6 inches between the dual VC and the coil and after 12 months they fused together causing a very high SWR! I replaced these wires with higher ratings and all now seems ok but I was initially surprised to find that I had to recalibrate the ATU for the bands – two original wires had obviously added capacitance!
January 2007 – update A complete refurbishment was carried out providing a much greater range of capacitance for the input capacitor. Two switches were added allowing a dual gang 500pF+500pF variable to be used in series or parallel configurations.