Bicycle Wheel Mag Loop
A while back I stumbled upon VK3YE's excellent video about building a mag loop antenna out of a bicycle wheel.
Building some type of loop like this has been in my mind for a long time so I shopped for variable capacitors that could handle this on eBay and got some junk. They weren't cheap. Overall I spent way too much money on this antenna compared to something off the shelf like a number of Am-Pro sticks could have been bought but that's not the fun (or frustration) part of it.
I have had some MTB wheels in the garage for years, collected from bikes that have gone to the Rust section of the Silicon Heaven.
So finally this Saturday I got off the couch and wandered down to the garage.
After checking out the two front and two back spares, I picked the one that's in worst shape, a front wheel with scored rims which came out of an old Raleigh bike i had picked up from Freecycle which was beyond any meaningful repair. The rim was in a relatively good rust-free shape and the spokes were mainly OK so I removed those carefully, losing only a couple of spokes in the process, lots of spare parts were generated as a result which is good.
Then I cut the wheel into two using my little hacksaw. These two steps were the most time consuming parts. Using a wire brush I cleaned the inside of the wheel which was very dirty, and it was ready to be taken upstairs.
Since this was just a test, I didn't solder anything, in any case soldering a metal to such a large piece of metal definitely requires a blowtorch, you can't do this using a normal soldering iron so physical connections were done using screws. I put my capacitor that came out of an ECM into a box and created a dial using some cartoon out of an ice cream pack, linked that to the wheel using crocodile clips. It's not ideal but it's OK for a little QRP work around 1-5W.
Then I screwed the ground in the middle and created a gamma match as VK3YE described, built out of a spoke which was screwed back into the wheel.
Overall this gave a reasonable match. It was close but still needed an ATU on most frequencies. I could hear a lot of places but 28MHz and similar upper HF bands were dead so I left it like this.
In this setup it is basically a folded very short dipole with a capacitor to make it in tune. It's efficiency is very low but works.
Later in the evening, I tried the "magnetic" part of the loop, which is actually caused by a magnetic coupling. I put a thick wire in place with the help of some wires and also created an ugly balun since I wasn't sure how balanced this thing was. Both helped and also 28MHz opened up.
On the other hand when I tried it on Monday, it wouldn't work. Hearing is fine but causes way too much RFI and the USB interface to FT-818 shuts down, which terminates the radio TX. I need to figure out why it's causing so much interference with anything over 0.5W - and with that no one can hear me.
