2 Aug 2010

14.9km on 136kHz QRP (earth electrode antenna)

This morning I did some more tests using my little 136.93kHz 2W QRSS3 beacon feeding into the 20m spaced earth electrodes. The TX end was unchanged, but this time on receive I used my 30t 80cm loop resonated with about 700pF and fed straight into the high impedance input of my E-field probe. Results were as follows:
  • At 2.4km good copy by ear (12wpm CW would have been very solid)
  • At 8.6km good copy using Spectran
  • At 14.9km clear copy of my callsign and locator in QRSS3 with Spectran on the Gog Megog Hills near Cambridge (see screenshot)
All these locations are more or less in the line of the earth electrodes that run NE-SW from the home QTH (JO02dg).  Another RX test some 11km to the south of the QTH  resulted in no visible trace, suggesting the TX earth electrodes are behaving as a loop antenna with some directionality. This was not the case when the same earth electrodes were used at 0.838kHz some weeks ago and when the mode was mainly conduction through buried utilities. Remember, this is ultra simple stuff:
  • 2W out crystal controlled QRP 136kHz beacon TX on a 5 x 5cm board (see picture)
  • Invisible ground "antenna" that can't be seen, even when a few metres away from it.
  • No attempt to match the TX to the earth electrodes.
  • Simple loop + FET impedance matching circuit and deaf FT817 at the receiver end.
It would be interesting to work out what the ERP must be and interesting to speculate what would happen with such a system at 73kHz. Pity there is no allocation there today.


andy said...

Is the ft817 deaf due to the quoted sensitivity being 32uv at that frequency. If so, would it not be better to build and ultra simple receiver for that frequency? My latest 3 transistor 80m dc receiver has sensitivity of -110 db which would be far better than the ft817 i think.

Roger G3XBM said...

Thanks Andy. With the E-field probe ahead of it I think the S/N is sufficient on the FT817, but I agree a DC receiver for the band is not a bad idea especially for a simple approach.

andy said...

Another possibility is finding an old receiver like the lowe hf225.This tunes down to 30khz with a sensitivity from 60khz of 0.8uv
Its quite an old receiver now so you may be able to pick one up quite cheaply.
This approach would be much better than the ft817.

Steve said...

G0AKN's (SK) experiments on 73kHz and 136kHz all those years ago with Earth Electrodes make interesting reading. His writings were a reference manual when I was playing around with 8975Hz experiments.

I might hunt around in the undergrowth here for my 80m spaced electrodes I used for 8975Hz and play with 136kHz like John did.