Helical Magnetic Loop Antenna
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 15 17 and 20m, less so on 40m but I can work 40m PSK every now and then. There is more detail about these antennas at this group: https://groups.io/g/HelicallyLoadedMagLoop/topics
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.
Initially I was a little worried about tuning it and had planned to have a stepper motor and/or a 10 turn pot to provide feedback on the cap location. After using it a little though, the noise floor change combined with an SWR meter on my transceiver makes getting it centered really easy and a more sophisticated tuning control is unnecessary.