The purpose of the fourth experiment is to do a direct comparison of the performance of the earth dipole to that of a "typical" above-ground, amateur LF antenna. Of course, there is nothing typical about amateur experiments on the LF band and one has to search really hard in order to find two installations that are very similar. In this case, typical is taken to mean a simple inverted-L antenna that could potentially fit in a backyard.
The transmitter of choice today was the 2 W MEPT, a simple crystal controlled, single MOSFET, all-weather transmitter, based on the VK1SV MEPT design. The MEPT was configured for QRSS30.
The above ground antenna consisted of:
Now, one could say that this is not really a very efficient setup. And I would certainly agree with that. I know that I am loosing a bit from the slopping horizontal segment, from the low Q variometer and probably from the less than optimal ground. Perhaps all these together, with a fair bit of effort could result in a 10 dB of improvement?
Tuning was straight forward. Once I discovered that the required inductance was a bit less than that calculated with MMANA-GAL, then everything was fine. I think I used 20 m of horizontal segment in the calculations, maybe that's why. Anyway, here is the setup at the base of the antenna.
So how did this antenna perform? Not bad it seems. Here is a capture from my grabber.
This should not come as a surprise. Steve, VK2XV, also reported a good reception of my signals in QF56ik. Steve has a grabber too which he will occasionally put to service. He is 217 km away from my TX location.
David, VK2DDI, also reported a good copy of the signals. Again, that's not a surprise given David's excellent RX setup.
Given the previous lessons learnt, the earth dipole was made longer (~250 m) and I used better rods this time: copper plated steel rods which are 1.5 m long and are used as "earth stakes" in a home electrical installation. I planted (with a lot of effort) two of these near my shelter and at the remote end one. The remote end is very very near (but not in) the dam. This should help (or should it not?). The wire was simply left on the ground surface and not elevated. I just didn't have the time to elevate it, so this is something that can be improved in the future. The earth antenna looks like this:
The impedance of the earth dipole was found to be inductive and not purely resistive. So here is a second point for improvement of this antenna.
So did it work? The signal certainly made it to my grabber.
The signal is strong. I don't know why it looks a bit wobbly compared to before. Maybe because of the mismatch things got a bit hotter inside the box? Anyway,here is a capture that has both the above and below ground signals. The raw data can be downloaded here.
Shocking stuff, the earth dipole appears to be stronger than the inverted-L! The plot shows a difference of about 3 dB! VK2XV reported the same.
Steve mentioned that the signal from the earth dipole is about 1 dB better than that of the inverted-L. I surely did not expect that! This has to be some sort of a record - 217 km covered on LF with 2 W of output power and no (traditional) antenna!
Again, David VK2DDI reported reception of the signals. But in this case, the signal produced by the earth dipole appears to be weaker. The following capture shows the change.
The following table shows distance and bearing from the transmitter of the three stations that reported reception
| VK1SV | VK2DDI | VK2XV | |
| Bearing (deg.) | 202.2 | 78.9 | 42.3 |
| Distance (km) | 20.02 | 138.6 | 217.84 |
The results were surprising. Even with a possible improvement of a few dB on the above ground antenna, the earth dipole seems to come very close to the inverted-L. This, at least in my mind, makes experimentation with such a simple antenna something worth doing.
I am also thinking that there is scope for some more record breaking here. Can we reach Melbourne with the 25 W MEPT? Can we reach Brisbane or Adelaide with one hundred Watts or two? I have done some calculations and I think this is entirely possible. That is the beauty of LF. For the first couple of thousand kilometres or so, the spreading loss (6 dB per doubling of distance) is what determines coverage.
Anyway, I still cannot explain the good performance of the earth antenna so I would love to see others experimenting with similar setups. Is this something peculiar to my block? There are just too many variables but in any case, I hope this experiment will inspire others to get on LF.
Steve, VK2XV, has managed to cover a distance of 135 km using a 70 m earth dipole! He used 2 W and QRSS3. The receiving station was VK2DDI and the signal was very strong. Steve has documented his experiment here. I am very happy that there is more evidence that the earth antennas are not a waste of time and are indeed something that is worth more experiment time!
Go back to the main earth antenna page.
Dimitris Tsifakis, VK1SV