A few years ago I built a magnetic loop TX antenna. With QRP powers, the very high voltages that exist if the loop is efficient are less problematic. The key to efficiency is low loss. For example, the loop should be made of thick copper pipe or coax, if possible. The capacitors used to resonate the loop should also be low loss. Having said this, I have had good results with normal air spaced variable capacitors and trimmed coax as capacitors. Of course, magnetic loops tend to be very sharply tuned, but for things like WSPR, JT65 and PSK31 where frequency changes are small they are fine. In fact they work remarkably well even indoors. They can be small too as the photo shows. This loop was tuned to 10m WSPR but the same sized loop was used on 20m where I was spotted in Japan - from the back of a chair in the bedroom!