Testing Toroid Magnetic Loop Coupling

I am planning on feeding the 40m NVIS Magnetic loop antenna with a Toroid configured as a transformer. I ordered and received some FT140-43 and FT240-43 toroids off of eBay this week and decided to try and match my existing loop as a test. The trick is to determine the proper number of wire turns to get a good match. For me this was trial and error. I started with about 8 turns of #14 insulated solid wire on the small FT140-43 core. I’d run a sweep with the antenna analyzer and remove turns until the SWR dropped to close to 1:1. For this configuration it worked out to 4 turns. Here is what is looks like as I tested it:

Photo Apr 09, 5 11 15 PMPhoto Apr 09, 5 47 30 PM

Here are the SWR plots. First up is the Faraday coax loop I have been using on 20m:

loop20mfaraday Custom

Here is the plot with the toroid also on 20m:

loop20mtoroidsolid4 Custom

Very Reasonable agreement.

Here are the SWR plots for 17 & 15m:

loop17mtoroidsolid Customloop15mtoroidsolid Custom

The good news is this works!

Given the current design of my portable mag loop I may continue to use the coax coupling loop. I would have to change the way the loop is held up to accommodate the toroid. This will not be a problem with the 40m loop as I will feed it on the bottom as the capacitor section will be on the top. I’ll likely also change to an easier to work with wire gauge to get the windings tighter on the core. That #14 wire is pretty stiff and hard to work with. I tried some stranded speaker wire and it is hard to get it to stay put. Once I get some better wire I’ll try the larger core and see if there are any differences. Meanwhile, I have gathered up the rest of the parts for the 40m loop and hope to start building it yet this weekend.

 

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