Magnetic Loop Transmitting Antenna

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The Loop after I made it operational. Read/look below for details on how I set it up. My first two contacts were on 40M CW into Tasmania and Hawaii during the 2020 IOTA contest! Received signal strength was 1-2 S-units stronger than my doublet antenna!

I will post more comparisons between it and my balanced line fed doublet at 35 feet.

Loop re-construction

Loop antenna, as received as a gift from a local ham. It came with everything but the vacuum variable capacitor. We had to unsolder one of the copper pipe joints to make it easy to transport.

Transporting it home, it just barely fit into my station wagon!

I had to clean all the grime and dirt from the parts. PNW winters encourage moss to grow on everything. After cleaning I went ahead and painted the mast with black acrylic to prevent further UV damage to the fiberglass.

The next challenge was to find a way to fit my vacuum variable capacitor to it since they are different sizes. My design goal was to find a single copper piece replacement for the 90 degree fitting in order to minimize resistive losses and so that I didn't have to re-drill any holes. This part took the longest, as I had to think a bit about what I wanted, and then it took some extra time to fabricate it using hande tools. But I am pleased to say it worked out!

The geared-down motor had probably sat stationary long enough that the commuator corroded, such that it wouldn't run and had apparently infinite resistance when measured. But after removing the gear train and hand-spinning the motor shaft it started and clean up the brush/commutator contact enough to start up on its own again.

I ended up buying a replacement motor, but the replacement runs at 16RPM, which is tougher to fine tune versus 2RPM but makes it easier to switch between 80M and 40M. I may get another replacement 2RPM or something in between.

To couple the motor shaft to the fiberglass drive shaft I use made a "bushing" out of an aluminum pie pan.
Udpate: I have ended up using the end cut off of a plastic twist tie, which is much simpler to construct and more reliable.

The gamma match as made didn't seem to want to tune up to 1:1 SWR, so I replaced it with 8 gauge solid copper wire. I may come back and replace it with a wider diameter pipe. For now it matches 1:1 SWR on both 40M and 80M.

Tuning Control

I have an old variable voltage power supply, which I rigged together with a reversing switch to tune, either buy transmitting a low power or with the MFJ SWR meter. I will eventually construct a better/single housing for the whole assembly.

Created 23 July 2020
Edited 23 July 2020
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