Tag Archives: toroid

Testing the Hi-Voltage Capacitor on the Magnetic Loop Antenna

I did some preliminary testing of the Hi-voltage capacitor I completed last week and found some interesting things.Rather than commit the 40m loop design I decided to try and adapt the existing loop antenna I have previously described. To make this work in a configuration similar to what I have in mind with the 40m loop, I installed the capacitor at the top and used a toroid instead of the shielded coax loop for the coupling at the bottom of the loop. To do this is needed to replace the vertical PVC supports to accommodate the configuration. The reducer I used for the 1/2″ pipe on the 40m loop was replaced with a 3/4″ version. This configuration worked well. The capacitor is in the range of being able to tune this loop on 20m. Previous experiments indicated a good match with 5 turns on the FT140-43 toroid. It turned out that 6 turns would give me a perfect match 1:1 SWR at resonance. Here is the SWR plot on 20m.


TheĀ  bandwidth at the 2:1 points measured at 45kHz. This compares favorably to the original capacitor which was indicating a bandwidth of around 68kHz. This indicates that the Ohmic losses are indeed lower and therefore efficiency should be higher as well.

I did not have enough time this weekend to run all the measurements I had planned but I did operate the antenna indoors at 45W with the FT-817ND and amplifier.The Califonia QSO party was underway and I made numerous contacts in California without difficulty with this loop. The capacitor handled the power without any issues at all!


Update on Toroid Coupling

I spent some time yesterday working with the toroid coupling. This time I used #18 insulated wire on the FT140-43 toroid. It still worked out to 4 turns being the best match on 20m. The SWR worked out to 1.02 on this band. I was getting SWR of 1.2 on both 17m and 20m. The smaller gauge wire is much easier to deal with. Also found that I can move the toroid to just against my vertical PVC support and hold it in place with the Velcro tying the feedline to the PVC. May be a solution for field work after all but I’d need to test this on the 12m and 10m bands as well. During testing I managed two 5W SSB contacts from inside the shack with good signals reports. One was in Georgia and the other in Cuba.