A new LDF4-50 Magnetic Loop Antenna

Having recently changed my car I realised this week that the diameter of my LDF5-50 Magnetic Loop (ML) is fractionally too big to fit in the back of my new car!!! So something needed to be done if I wanted to try some more /P operation. To try to reduce the size of the 5-50 loop would require a fair amount of work with a gas blow-lamp and most likely result in the ML no longer performing well on 40m

So after some rummaging around I found some old well-used LDF4-50 connectors and an off-cut of LDF4-50 cable. After running a few online calculations I decided to try a loop with a circumference of 8.5ft (~259cms) this should in theory cover 14MHz <> 30MHz albeit slightly sub-optimal for 20m.

The following two photos show the new ML being tested.

The tuning box has two N-type connectors and two variable capacitors. The capacitors are wired in parallel. The large value capacitor being used to set the band and the lower value for fine adjustment.

The initial measurements using my Arduino based Vector Network Analyser look very promising

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Unlike its LDF5-50 ML this loop is constructed for low power operation (~5W) due to the small fairly close spaced capacitors.

How the ML will actually radiate remains to be seen… more to follow in the coming days. WIP  🙂


WARNING:  Because of the very high Q, some capacitors can arc over at power levels as low as 10 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.

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