When I first made this MagLoop my goal was to be able to use it on the 40m band, and this was achieved. Recently I began to wonder if it might also work on 80m.
So the MagLoop was setup indoors (temporarily) to allow the tests to be done in the warm. 🙂
Fortunately I have a very understanding XYL 🙂
Using a handfull of mica-capacitors and my VnArduino antenna analyser it did not take long to establish how much extra capacitance was needed to get the MagLoop to tune on 80m. With an extra 500pF, in parallel to the 120pF needed for 40m, the VSWR graph looked good. Having established the value needed, the next step was to search the shack spares-box for a suitable high-voltage RF style door-knob capacitor. Fortunately I had just what was needed.
This is the arrangment for 80m:
I knew that the VSWR looked reasonable and I could put RF into the antenna, but would it actually radiate and be heard / seen anywhere? To test this I used my Ultimate3 MEPT configured to transmit alternate periods of FSKCW6 and WSPR. The output power was ~200mW into 10 metres of RG-58 connected directly to the MagLoop, no ATU was used.
Given the current band conditions the results were very encouraging. The following photo shows the WSPR spots over a 12 hour period:
Here are some QRSS results. Note that the first photo has been edited and the letters G4HSK added above the FSKCW to help you find my trace:
By using various combinations of the three door-knob capacitors (500pF / 120pF / 57pF), in parallel with the trombone-capacitor, the MagLoop can now be used on all bands from 80m through to 20m.
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.