Re: Efficiency of MFJ remotely-tuned loop antennas

Posted by Alan Bloom on Jan 20, 2021; 5:37am
URL: http://elecraft.85.s1.nabble.com/Efficiency-of-MFJ-remotely-tuned-loop-antennas-tp7667578p7667652.html

I love to see people doing serious homebrew.  Bravo!

I wonder if the SWR runaway at 150W could be due to the PVC pipe used as
the insulator for the variable capacitor?  PVC is quite lossy at RF. 
Perhaps when it gets hot it detunes the capacitor.  It would be easy to
check by simply feeling the cap to see if it gets warm.

Alan N1AL


On 1/19/2021 5:39 PM, Andy Durbin wrote:

> "although I saw a design not long ago that used two coaxial copper pipes with a PVC pipe as the insulator.  The inside conductor was mounted to a long threaded rod moving it in and out of the outer conductor."
>
> Not my original idea but that's what I have on my loop (PEX insulator not PVC).  Works fine up to 100 W on 30 m but has SWR runaway at 150 W.   Capacitor is driven by a geared DC motor and is tuned from the shack.  Home brew is far more fun than buying commercial.
>
> I'll post a link to a presentation if there is any interest.
>
> Andy, k3wyc
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