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.