9 Jan 2017

Earth-electrode antennas at 472kHz

Some 4 years ago I posted this based on my results at 472kHz:

These were my conclusions in Jan 2013:
  1. The earth-electrode antenna at 472kHz (2 earth rods in the soil 15-20m apart fed from the TX output) behaves like an H-field loop transmitting antenna. At the present QTH the spacing is less.
  2. It has directivity, with strongest signals in the line of the loop and weakest signals at right angles to it.
  3. It works as an effective RX antenna too.
  4. In my environment the loop looks like about 50-60 ohms resistive, so a good match to my transverter directly without matching. At this QTH it looks a higher impedance, presumably because the soil and rock are different.
  5. The structure works because much of the return current flows deep within the soil and rock beneath the earth-electrode antenna. In my case Rik OR7T calculated that the loop area in the ground is effectively 290m sq with a radiation resistance of 0.017 ohm and a loss resistance of 66 ohms.This is a BIG loop! I think the loop is smaller at this QTH.
  6. Performance compared with my 9m high top loaded Marconi antenna averages only around 8dB down, not a bad figure at all, even with the connecting wire on the ground and not elevated at all.
  7. Where no other antenna option is available, the earth-electrode antenna is well worth trying both on 472kHz and on 136kHz both for RX and TX. Although it works well here, your geology may be different and results not the same.

1 comment:

Hugh said...

The subsoil has a lot to do with it. The Americans deliberately chose to build their VLF submarine transmitter on a ridge of highly insulating rock.. presumably to enhance the loop. At my QTH it should be interesting.. underneath my house are quarries and cavities.. should make for good ground loop.... just need time to install it! Hugh F/G6AIG