8.76kHz tests with loop + earth electrodes on RX

Today I did a test at 5.1km to compare reception of my 8.76kHz 5W transmission using (a) an 80cm loop antenna and (b) a small earth electrode antenna (14m spacing) using a variety of preamps.  Although quite solid reception (17dB S/N in 0.18Hz BW) was achieved using the loop laying flat on the ground, there was absolutely nothing detected using the earth electrodes, which was both surprising and disappointing. I used the very same preamp in both cases and also tried the lo-Z input tuned preamp with the earth electrodes that successfully copied DK7FC at 648km last weekend.

Several possibilities for the failure with the earth electrodes:

• The electrode spacing was far too small
• The barbecue skewers used as electrodes were too short and not making decent soil contact
• The soil was too wet, so the pick-up was lower than with dry, low conductivity soil. The test site is in the fens where the soil is a dark peat.

Next step is to repeat the test up on the chalk uplands to the south of the QTH where soil conductivity will be lower and to try a larger earth electrode spacing on receive.

8.76kHz field testing in the snow at 1.5km

With a temperature around 0 deg C I did a field trip today TXing on 8.76kHz VLF to test two things: (1) to check the rebuilt 4-5W TX based on the TDA2003 could be copied at moderate range and (b) to try out the new tuned preamp with earth electrode receive antennas and see how these compared with a loop. The usual first test site, a National Trust car park about 1.5km from home was chosen. Today it was totally deserted with more sensible people curled up in front of warm fires. Using the 80cm receiving loop or with the earth electrode antenna (electrodes placed about 13m apart) decent

copy was achieved in QRSS3, using Spectrum Lab this time rather than Spectran on the PC.  For the earth electrodes I just used barbecue skewers pushed in about 10cm into the very cold ground. Reception was also possible with the two earth electrodes just 2m apart. The first picture shows the signal received with a loop. The second is with the earth electrode antenna on receive. The ground was covered in a thin layer of wet snow and results when the soil, and soil surface, are dry should be better on earth mode conduction using earth electrodes at each end. The roads in the Fens were too icy to try reception at a more distant point 5-6km away. Also, I need a much longer baseline earth electrode pair in the car in case this is needed at greater range.

New front-end for 8.76kHz VLF earth mode tests

Today I completed the design of a new "front-end" for my tests on 8.76kHz when using earth electrodes as the RX antenna. Up to now I've used an 80cm loop/preamp at the far end of the test, usually with the loop on the ground for maximum signal pick-up. Now I want to compare results with an electrode pair antenna at the RX, laid across the road to form a pickup loop within the ground around any buried pipework and cables. The electrode pair is about 30-100 ohms, so I needed a low input impedance. I decided on a grounded gate FET amplifier with a simple LC tank circuit in the drain tuned to 8.76kHz fed via an emitter follower into the PC sound card. It has sufficient selectivity to reject 50Hz and lower harmonics and reject the stronger VLF and LF stations which could cause intermod and overload. I've also added a small LC lowpass filter on the input to the FET. This will be field tested at a site 5.3kms from the home QTH as soon as the weather improves: it is currently below freezing day and night!

Vertical loop versus earth electrodes antenna

Just done an interesting test comparing my vertical wire loop with an earth electrode antenna. I removed the lower part of the loop and connected the ends to ground. Matching was totally different (low Q antenna) but the performance was only 5dB down on the loop, based on reports from 2 local and one DX station. A similar test on 136kHz as the only station on in range (M0BMU 69km) was unable to copy my signal on the earth electrode antenna suggesting at least 5dB down on that band too.

Another report on the 136kHz QRPp beacon

G3XIZ has sent me a nice screenshot of my 10uW ERP 136kHz beacon as received in Biggleswade, about 35km away. Chris believes I'd be perfect copy with QRSS10 (10 second dots).

62km with QRPp and earth electrodes on 137.675kHz

Today I ran an earth electrode transmission test with Mike G3XDV in Welwyn Garden City 62km away. I put my QRSS3 8-10uW ERP beacon on and Mike took regular screen shots through the day until mid afternoon. Although unable to copy the message in QRSS3, when using QRSS30 speed for reception the start and end of the message sequences (9 mins 7 seconds apart) could clearly be seen. He was able to precisely detect when the beacon was turned off. Mike estimates another 6-10dB should allow him to read the message properly. Signals were weaker later, possibly because of rain here altering the effective loop size within the ground.

Resonating the earth electrode antenna on 136kHz

The 20m spaced earth electrodes "antenna" has about 30m of wire coming back ato low height to the feedpoint upstairs. I wondered if I could increase the current flowing into this structure by tuning out the small inductance presented by the wire. Using a small capacitance switching box I selected the value that gave most current with the 2W beacon TX. It was 100nF although because of the resistance of the ground return path the Q is low and the peak small. This suggests the inductance of the feed wire is only 13.5uH. I also tried matching the loop using a 3C90 transformer with various secondary taps but this made little difference. So, conclusion is that resonating the loop and matching it makes very little difference to the current (and ERP) because of the low Q.

500kHz WSPR with 20m spaced earth electrode antenna

The effectiveness of the 20m spaced earth electrode "antenna" (virtually nothing is in the air) continues to amaze me. Last night I copied PA0O on 500kHz WSPR and this morning I've got 4 reports of my sub-1mW ERP signal from M0BMU around -19dB S/N at a distance of 69km. This level suggests the (non) antenna is only around 6-8dB down on my best top loaded  short vertical used to get reports from OK2BVG 1232kms away last winter and the ERP is around 200-250uW.

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.