The objective of today's experiment is to investigate if the earth dipole is directional. This time, I used a new MEPT which is set for QRSS30 and has an output of 3 W. This should give approximately the same S/N to the other MEPT that has about 25 W output and is set up for QRSS3. The setup consisted of a fixed ground electrode connected via a short wire to the impedance matching autotransformer which was in turn connected to a 100 m wire, terminated with a ground rod at the end. The impedance was found to be around 450 Ohm, purely resistive. So, this should work as it did last time...
Indeed, Dale VK1DSH reported reception with a 30 dB S/N ratio at his location some 30 km away.
The initial earth dipole setup run roughly from North to South. Here is the resulting capture from my grabber:
I then moved the 100 m wire to 90 degrees from the original direction, this time running from East to West. After I moved the wire, I walked back to the transmitter to get the earth rods and then I thought I should check what happens when the cable is simply on the ground but not connected to a rod at the other end. To my surprise, the scopematch indicated a rough match with a small reactive component, but not horribly bad, so I let it run for a few minutes. The following is the resulting capture from my grabber:
The signal is not as strong, but is it detectable! How about that for a wire on the ground and 3 W! The 136 kHz band is turning from the most difficult band to the easiest one, all you need is to lay a cable on the ground for an antenna!
Dale also captured this signal (the "VK" at the beginning of the following spectrogram).
I then connected the remote end of the 100 m cable to a couple of galvanised rods and found the impedance to be again around 450 Ohm, purely resistive. This is what it looked like in my grabber:
This looks approximately the same as the North-South dipole, maybe a bit weaker. The grabber is about South from the TX location, so, assuming that the earth dipole has a gain at 90 degrees, I would have expected a better signal from the E-W dipole compared to the N-S.
The day after, I plotted the peak signal amplitude over time, as recorded by spectrum lab running on my grabber:
In this plot, from 00:30 UTC to 02:25 UTC the N-S dipole was used, then from 02:35 UTC to 02:45 UTC it was the E-W unterminated dipole, and finally at 02:55 UTC to 03:15 UTC it was the E-W terminated dipole.
This shows that the E-W dipole produced slightly better signal in Canberra compared to the N-S. This makes more sense.
Hammer time: the new antenna construction technique!
If you find all that interesting, I recommend to visit the links in the top of this page.
Go back to the main earth antenna page.
Dimitris Tsifakis, VK1SV