40m Magnetic Loop Antenna

Finally got around to finishing the 40m magnetic loop antenna. This antenna is designed to be easily deployed and used as an NVIS antenna with up to 100W. The heart of this antenna is the hi-voltage capacitor I designed and previously discussed. Before setting up the new antenna I made a few changes to the capacitor:

  1. Wrapped exposed copper with electrical tape
  2. Replaced the steel 5/16″threaded rod with a two food nylon rod. This was to minimize coupling effects while tuning
  3. Made a new center post with better hole alignment. This helps keep everything from binding.
  4. Epoxied the capture nut into the center post.

Here is the capacitor in use on the new antenna:

photo-dec-28-2-55-26-pm

The 40m loop consists of 22 1/2′ of RG-213 coax. The length was selected to match the capacitance of my prototype capacitor. Here is the design points via AA5TB’s spreadsheet:

2016-12-28_16-02-07

The rest of the build involved cutting pieces of 1/2″ PVC pipe to length to support the antenna. I hauled the whole thing ip a tree branch with the feed point just about 1″ off the ground. Initially the SWR was a bit high but it settled into about 1.4 when I removed two turns from the toroid. Here is the SWR plot through 50 feet of RG-8X feedline:

40m-loop-wspr-custom

Here is a view of the antenna deployed on a tree branch.

photo-dec-28-2-55-20-pm

I have been running WSPR on 40m now for about an hour and transmitting 1W. Here are the contacts thus far:

2016-12-28_16-08-31

This seems to be working well so far. My only complaint is that I have to stand on a ladder to tune it. I’ll need to work on that. I’ll run some additional tests now that its built.

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