19 Jan 2013

A mag-mount as an E-field probe for 137kHz

Next week, assuming the snow has gone, I want to drive around the local area to plot the polar pattern of my earth-electrode antenna working on 137kHz.  My 7-8W beacon transmitter can send a range of QRSS and CW messages, so I want a way of driving to different places using my FT817 fed via a preamp and measuring the S/N using Spectran whilst remaining inside the car.

Portable RX loops are fine, but these are directional and need to be set up each time a measurement is taken outside of the car.  Ideally I need an omni-directional RX antenna like an E-field probe. So, I'm looking for a way to drive to a spot, take a quick measurement from inside the car, then move on; a loop works, but when it is -2 deg C outside and the ground is icy, it is not ideal.

Either I put the electronics for an E-field probe right at the base of the mag-mount (mechanically not ideal) or I have the electronics in the car and somehow tune out the coax cable capacitance.  At the moment I am still experimenting to find an arrangement that works with good sensitivity by checking the strong Greek commercial signal just outside the 137kHz band.

Based on 472kHz results, the earth-electrode antenna behaves like a loop with loop-like directionality. I 'd expect to see the same pattern on 137kHz.


Anonymous said...

Your car might already have what you need.. the built-in car antenna!

Car MF/LF antennas are also of course capacitive probes as they are extremely short for the wavelength. The 'coax' used for these radios has an extremely thin inner conductor to minimise the loading.. exactly as you are trying to do. For 198KHz Radio 4 they work amazingly well
Hugh G6AIG

Roger G3XBM said...

Yes a fair point, but when the radio and its antenna are so well integrated, as in my Kia C'eed it will be more convenient to use a purpose built solution.

I will have to do a straight comparison of 80cm 30t loop (previously used) against the probe(s) to ensure the EFP gives similar sensitivity.

73s Roger