Testing a lightweight HF magnetic loop antenna made from 7 metres of RG58 coaxial cable. Contacts were made on 40 and 20 metres during the Lighthouse Weekend. It uses just three main parts: the coax itself, an air-spaced variable capacitor (approx 50 – 100pF max) and a T50-43 ferrite toroid. The toroid is threaded over the coaxial cable. RF from the transceiver is fed via a 2 or 3 turn winding over the toroid (experiment with this for lowest SWR). The coax inner is left unconnected; only the braid is used here. The loop is for low power use only – it will take 10 – 20 watts before the variable capacitor arcs over. Performance is, as expected, somewhere between a smaller pedestrian mobile magnetic loop and a full sized wire antenna. It would be improved if the variable capacitor was a low loss type and the coaxial cable was soldered straight to it. While there’s enough information above to make your own, it’s worth checking out G4FON’s website ( www.g4fon.net ) for similar loops. My loop is closest to his Version 2.
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