Re: Non Linear Vac Cap?!

Barry - 2E0IDE



Thanks for the comments and advice. This is my first attempt at building a Loop. I used a modified “Work Mate “ to bend the copper tube as per pic(an idea from  John G6KJK) with heat shrink instead of paint. All copper fittings were hand-made, and all plastic mounts and loop fitting were home designed and 3D printed.


I did wonder about the copper band on the drive end and will take your advice and re-design.


I’m using a coax Faraday Loop coupler that is a slightly squashed circle (squashed even more than the pic - It does improve SWR). The coupler is designed to be easily removable so that I can experiment with gammas etc.


After the lubricant fix on the vac cap, the SWR on 20M is negligible but rises slightly for the higher bands.


I’ve had good contacts into Eastern Europe on my meagre 50 watts allowance.


Overall, a very enjoyable project and the controller is to be commended. Arduino based rotator next.


Once again thanks for all the help and advice.




Barry - 2E0IDE




From: <> On Behalf Of Immo, DL5KB
Sent: 07 March 2021 15:03
Subject: Re: [loopController] Non Linear Vac Cap?!


your'e welcome. This experience is part of my worklife. 

By the way: beautiful work, your tuning-box.
Perhaps a little remark to this solution. The connection to the head of the capacitor elongates the loop on this side. So the mechanical middle of the loop is shifted half the length of this connection to this side. It may (I say it may!) change the reachable best SWR, because the coupling loop now is not at the point of zero of the loop. Using a gamma-match, you should search the new zero on the loop, with a coax coupling-loop it may have no such importance.
With an earlier built, I used a gamma-match and had to shift it abt. 3 inches for good result. The picture shows my first solution with a vac-cap on the bench, just with an ungeared Nema 19. My loop today has a 5:1 geared stepper and a coax coupling-loop.
73, Immo


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