Helical Magnetic Loop Antenna
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This is a helically loaded mangetic loop design perfected by K8NDS. It tunes 40m thru 15m with 1.5:1 SWR or better. I've had really good luck with it on 20m, less so on 40m but I can work 40m PSK every now and then.
I got the vacuum variable cap from the Ukraine off eBay. A small geared DC motor drives the shaft on the cap and this is how the loop is tuned. On 40m the Q is very high, so anytime you QSY a few kHz the cap has to be retuned. There is no feedback to see where it's currently tuned but the noise floor comes up very dramatically when the loop tuning is centered on your receiver.