Jean-Marie Polard - F5VLB has been experimenting with so called "earth probes" to transmit RF on our amateur bands.
He mentioned it to me a couple of month ago and that was the first time I had heard of using the earth as an antenna. I google a bit on the subject and it seems like on very low frequencies like MF or LF there had been some experimentation on the subject. But now Jean-Marie have done more test and this time he tested higher frequencies and in particular on 7MHz. He sent me the following text in an email the other day.
Hello Harry,
here are two pictures, I will send more separately.
My system is as follows :
Two rods of 2m long spaced 27m oriented E-W in my garden. Each rod has a shielded cable connected (only the copper wire, the shield is left unconnected). Antenna are buried to the top. Cables run at 40cm deep.
Each top of rods are protected by a concrete box with cover. Inside earth and gravel protection and go to the shack with the black cable.
I enter the two 'wires' in the antenna output of the LPF filter after the Zachtek TX.
I removed the half part of the WSPR-LP1 PCB as it was not needed and I needed it to fit a metal box I had. I use a separate box with the LPF filter for 7MHz.
Since a few days, I transmit on 7 MHz in wspr and keep track of reports
I can see that during the days, I get reports from 500 ... 800 km and no reports from long distances. This is a confirmation that he HF penetrates the d-layer and bounces on the E-layer. The distances reached allow me to think that this is only an NVIS phenomenon instead of a real rebound.
At sunset, layer d disappears and the E-layer follows but does not disappear so quickly. And so I get reports on long distances up to 1500 km and no reports on short distances.
Once the E-layer has disappeared, the F1 and F2 layers recombine to form the layer F. These layer disappearances and the recombination of the layer F occur at different altitudes. During night, some part of e-layer is present, very tiny, and some erratic bounce can be detected, or maybe via the f-layer.
And, as the MUF is around 6.24 MHz makes reduce the chance to get report during night.
Last record with Zachtek was :
2019-11-30 00:28 F5VLB 7.040034 -26 0 IN88in +23 0.200 EA8BFK IL38bo 2397 1489
2397km !
Jean-Marie is preparing a PDF write-up on the subject that he also sent to me. I lifted the following section that I think is interesting. It shows a comparison of the signals received using his earth probes and a Mini-Whip active antenna. The earth probes has higher signal to noise ratio! I was a bit surprised to see that.
A screenshot is better than a lot of lines to show the difference between the reception of
the signals.
On this screenshot, one can see the difference between the earth probe antenna at left and
the e-field reception with a PA0RDT system. I used the stereo user file of Spectrum Lab,
adapted to my measures. Both signal are entering an usb soundcard UMC202HD and both
signal were adjusted to have the same level at the entry of the UMC.
I will finish off with a picture that shows Jean-Maries workbench.
Really nice and interesting work Jean-Marie!
We are looking forward to the finished write up.
Jean-Marie is active on the Facebook group "WSPR - Weak Signal Propagation Reporter" and "Eartprobes" where he post updates once in a while, hopefully he will post the finished write up there.
Thank you Jean-Marie for sharing your experiments! I might be tempted to try this myself if I can ever find a good set of grounding rods and some free time ... :-)
73 to you all.
//Harry