I’ve had a lot of fun using the LDF5-50 based MagLoop. By adding and swapping one or more high-voltage doorknob style capacitors it’s been possible to cover the 80m <> 20m bands. The only two negative things being that the loop no longer fits in the back of my new car to go out portable, and as there’s no protection against rain etc. it’s very much a fine weather antenna.
To protect the trombone tuning capacitor a length of large diameter plastic tube was needed and this would have to be cut, shaped, and fitted around the upper part of the antenna. I had a length of suitably sized tubing with end-caps but the tube material was a black polythene plastic type material. I was uncertain about the suitability of this material due to the potential high carbon content. I decided to cut the tube to size and do a set of VNA measurements before and after fitting the cover.
Here are two VNA screen captures before fitting the cover. The loop is tuned to the low end of 80m and the measurements done using the VNArduino analyser.
The second image is with the VNA “zoomed-in” and adjusted for the minimum SWR point.
Here’s a photo with the cover fitted:
Checking again with the VNA showed that the addition of the tube had caused the resonant frequency to change by approximately 10kHz on 80m. This can be seen in the following to screen grabs.
The doorknob capacitors can still be changed by removing the top cap. Two holes allow screwdriver access to secure the capacitor(s). The trombone can be adjusted by reaching up from the bottom of the tube.
The MagLoop complete with coupling loop.
In hindsight I guess it would have been better to have done the before and after tests with the 20m setup. The next step is to check the tuning for all the bands from 80m <> 20m.
Assuming all is good, it should now be possible to run extended outdoor tests without fear of rain affecting play. 🙂
WARNING: Because of the very high Q, some capacitors can arc over at power levels as low as 5 watts. Remember also that even with only a few watts of RF power, magnetic loop antennas produce very high voltages across the capacitor(s) and can cause nasty RF burns if touched while transmitting. Care must be taken not to touch the loop when transmitting and to keep a safe distance from the antenna.