A “Small” Magnetic Loop for 40m NVIS

I have run some design iterations on a magnetic loop antenna for EMCOMM use on 40m with an eye for testing it as an easy to deploy NVIS antenna on this band. I plan to build this using RG-213 as before but with a diameter of just under 8 feet. Rather than use a variable capacitor, I will use a length of RG-8X coax and tune for a fixed center frequency of 7.285 Mhz. This should allow me to operate on any of the regular 40m traffic nets run here in Texas. It will be easy enough to cut several lengths of coax for tuning on different parts of the band fairly easily. See the file below to see my design notes for this antenna:

40m loop notes

My thought is to use a small plastic box to mount the UHF panel connectors and connect the tuning capacitor coax. A small screw eye on the top of the box will allow me to haul this up to height. Rather than use a coax coupling loop I plan to use instead either an FT140-43 or FT240-43 toroid with between 5 and 10 turns of insulated wire evenly spaced to form the feedpoint.As with my current loop, I’ll use PVC pipe and fittings to make a spreader to keep the loop as circular as possible. This antenna could be hauled up by a single line onto a tree branch. I plan to use a painters pole to start with and will experiment with the loop between 1 and 6 feet off the ground. The biggest cost on this is expected to be the RG-213 coax at $32. I expect that this antenna can be made to break down into pieces no longer than 3 feet. I’ll post more info as parts come in and I start building.


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