6km DX on 17.52kHz earth mode

My 5W QRP signal at 17.52kHz is clearly visible at 6km

Last evening I continued the earth mode tests at 17.52kHz by going out to the test site 6km from home and looking for the signal with a variety of antennas in QRSS3 and with an interrupted long carrier transmission.

No copy was achieved using QRSS3 and Spectran. However, using a continuous carrier with a 2 minute ID break I got extremely good copy (about 6dB S/N) with Spectrum Lab in 37mHz bandwidth with the horizontal 30t loop but no copy on the earth electrode antenna parallel to the road. Moving the loop 10m away from the road resulted in no copy, as did mounting the loop vertically or a few metres above the ground.

It would appear that at 17.52kHz it is easier to detect the signal at distance with a loop than with an earth electrode antenna whereas at 1.095 and 8.760kHz either scheme works. Perhaps someone who understands the theory will be able to explain this.

My next test is to repeat these tests at 137kHz to see to what extent 137kHz is aided locally by utilities assisted earth mode. At the test site 6km away I can see how strong the signal is with tight coupling into the ground at the roadside, and with an E-field probe some distance away from the road.

3.6km with 17.52kHz earth mode

This morning I did an initial test at 17.52kHz using my 5W QRSS3 TX and earth electrode TX "antenna".  A test at 3.5km from home showed the signal ~20dB over noise (0.73Hz BW QRSS3) using the 30t loop on the ground and with a 1t loop (about 20m sq) laying in the road (see attached Spectran screenshot). Signals were also copied with the earth electrodes parallel to the road but weaker than on the loop. They were also copied weaker still with the RX earth electrodes strung across the road. Signals are definitely coming through the ground/utilities with no evidence of any radiated signal and they are every bit as strong (or stronger) than at 8.76 or 1.095kHz.  However, at another test site 4km from home where I usually manage to copy 8.76kHz QRSS3 reasonably well there was no sign of my 17.52kHz signal either with the 30t loop or the earth electrode antenna. No idea why.

Later today I may try the 17.52kHz test at my 6km distant test site with a long duration carrier, although I suspect I may copy it in QRSS3, which I'll try first. I may also try a test using my 80m sq vertical loop as the antenna at the TX end, although I don't expect any great range by pure induction.

1.095kHz earth mode test at 6km

1.095kHz earth mode signal at 6km

Today I repeated the earth mode (through the ground) "long carrier" test of 17.7.11 this time at 1.095kHz rather than 8.760kHz. The same test location was used in the fens not far from the River Cam. This is now a very long way from home! As before, I got my wife to cut the carrier for periods of 1 or 2 minutes to positively identify my signal. I need to automate this so she can continue to do the gardening.  4 different receiving antenna arrangements were tested, using my low impedance MPF102 tuned preamp into the PC:

(a) an earth electrode pair parallel with the road,
(b) an earth electrode pair diagonally across the road,
(c) a single turn loop in the road, and
(d) a 30t 80cm diameter loop at the edge of the road. 

Best results were with (a) and (d). With (b) there was no detectable signal.  At 1.095kHz the signal was again around 10dB S/N in 11mHz bandwidth on (a) and (d) and marginally weaker on (c). This is a similar result to 8.76kHz although I was expecting the signal to be stronger at this lower frequency.  I'm now tempted to do an earth mode test at a higher frequency, possibly 17.52kHz which I can still see with Spectran and Spectrum Laboratory and I can derive with my 4060 divider circuit. Using earth mode I believe I'll still be legal as very little radiated signal can be present. The image shows the signal with the RX earth electrodes parallel to the road, i.e. set-up (a), at a distance of 6km from the home QTH where the TX was running 5W into 20m spaced earth electrodes.

Using Spectran to listen to HF CW

Usually free Spectran software is used here to see VLF signals when doing my earth mode tests. However it also makes a very useful add-on to the FT817 as an adjustable width audio filter. Select the NDB settings and then use the filters (adjustable edges with right and left mouse buttons) with the menu on the left set to "BandPass". It is useful on SSB and on CW and allows the bandwidths to be screwed right down to as low as need be. There is also a CW peak function which puts a narrow filter around 800Hz.

Internet data usage measurement?

Although my ISP doesn't appear to set limits on the amount of data I download, can anyone tell me how I can measure the amount of data I am actually downloading each day or each week?

The financial crisis - living beyond our means?

The financial problems that have become apparent since 2008 can be put down to individuals and nations living beyond their means. How can the US government continue with an exponentially increasing debt? Why do people crank up huge credit card bills when they have no likelihood of ever paying them off? Why do many wealthy individuals in places like Greece make tax avoidance a way of life?

What happens next is hard to figure out but we are in for a period of pain and readjustment. Just maybe the outcome, eventually, will be a good and fair one but I can't help thinking we are entering troubled times where the threat of violent revolution is not far away.  This is a time when moderate people with good judgement need to make the voice of reason clearly heard.

HF noise levels

It is sad how much the noise floor has risen at my QTH in the last few years. Listening on 160 and 80m is now almost impossible on a vertical because of the wideband noise that obliterates the band to an S8-9 level. The noise is there right through to above 17m. Thankfully 10m with the horizontal halo is still pretty clear. Apart from my experiments at VLF and LF I am now tempted to move back to the VHF/UHF bands for serious operation.

VLF beacon transmitter schematic

G3XBM's VLF 5W Earth Mode Beacon

This is the schematic of my TDA2003 5W ULF/VLF beacon transmitter used in recent earth mode tests. The frequency is derived from a 4.48MHz crystal that is divided down to either 8.760 or 1.095kHz using a 4060 IC. This is keyed in either 10wpm CW, QRSS3 or QRSS30 with a K1EL beacon keyer IC. The output of the TDA2003 is matched to the resistance presented by two earth electrodes using a tapped toroid transformer. Best DX is now 6km across the fenland near me.

Braintree LF/VLF talk slides

The slides from my LF/VLF talk at the Braintree and District Amateur Radio Society on Monday August 1st 2011 are now available on-line. BADARS is a small but active radio club in Essex and I was given a very warm welcome by the members.

10m tropo

Weak signal modes never cease to amaze me. This evening I've been exchanging 5W WSPR reports with G4CUI 173km away near Sheffield on 10m by what I presume is tropo. Running WSPR for hours on end certainly shows unexpected paths popping up from time to time. Even 5W is enough with this mode, and I assume modes like JT65.

If properly exploited by using weak signal modes like JT65 and WSPR 10m could be a great inter-G band even with QRP and small antennas. I wonder who will be the first G station to Work All Britain on 10m JT65 with QRP?