VHF ATU

Why ? Just about all antenna designs are ready to go for a 50 Ohm match. Well on the 6m and 4m bands it's quite OK to mess about with simple long wires and loops for DX working, and they sure ain't 50 Ohms. Also in my case I got fed up trying to adjust J-Poles and Slim Jims when constructed out of the usual 300 Ohm ribbon cable (ever tried it ?).

The answer was to simply construct the Slim Jim to the usual dimensions, foregoing the tuning stub part and replace it with a simple matchbox ATU. Problem solved. Great for indoor antennas too. I've worked loads of DX on these two bands with no more than 5w of SSB / CW / FM modes with indoor antennas. Best DX was Slovenia on 4m FM using an Slim Jim hung up by the bedroom window.

You can tune it with a normal SWR meter, or use a VNA. The trick is to use an INSULATED trimmer tool for the variable capacitors (2.5 -25pF) . The 200nH coil can be 10 turns on a 6mm former, stretched out over about 17mm. If using a VNA, only move the trimmer capacitor very slightly until your VNA has had time to refresh it's screen otherwise you'll confuse the readings. Also do not have too wide a frequency scale. Maybe 10Mhz is good, and then you can fine tune it afterwards.

Note that some antennas often suffer from common mode currents on the outside of the coax. Using some clip on ferrites is very useful. On my 70Mhz Slim Jim I need two of them, each with two turns of RG58 coax going through them. They had a 13mm inside diameter bore size. They are cheap at about £1 each. Sometimes you don't know what you're buying though, so be prepared to experiment with different ferrite mixes, number of turns and number of ferrites placed at the feedpoint. More turns will increase the choking impedance, but also lower the frequency at which that occurs. Some sample measurements here.