One man's ground is another man's antenna

23/01/2013

The goal of today's experiment was to investigate the impedance of the earth antenna on 475 kHz, an antenna that has been successfully used before, on 136 kHz. In order to do this, I used my compact, 1 W output QRSS3 beacon and a z-matching unit (a ferrite toroid with multiple taps used to match the impedance of the antenna but not to tune the antenna). I also used my trusty AA-30 antenna analyser.

Here is a map (snapshot from the Google maps web site) of my grabber location (A) and the transmitter location (B). They are a bit over 20 km apart.

Test one: 250m wire, terminated at remote end

In this test, a wire of approximately 250 m length was placed on the ground, on a north-south direction. On the south end, I connected the transmitter output. The transmitter was also connected to the local ground system, consisting of two, 1.5 m long, copper plated earth rods. The remote end was connected to a single earth rod.

The antenna analyser showed an impedance of 111+j1. This is a resistive load which is something I did not expect. Using the impedance matching transformer, I achieved a perfect match to 50 ohm.

The following is a capture from my grabber:

Despite the 1 W output, the little QRSS3 transmitter appears to produce a solid copy signal, at a distance of 20 km. The vertical lines on the spectrogram are an indication of QRN. David, VK2DDI also tried to copy my signals but his system was rendered useless by S9+20dB QRN.

Test two: 250m wire, unterminated at remote end

The remote end of the antenna was disconnected from the earth rod and the wire was simply left on the ground. The impedance now has changed to 700-j500. The impedance transformer was used to match the new antenna to the 50 ohm transmitter, as close as possible. To my surprise, the signal on the grabber appeared to be much stronger:

It is interesting to notice that the carrier appearing from 00:06 to 00:08 on 474.95 kHz is from the antenna analyser! After checking the analyser's manual, I discovered that it produces +13 dBm, or 20 mW. That's impressive!

Test three: 150m wire, unterminated at remote end

I removed the 250 m wire and replaced it with a 150 m wire, also on a N-S direction. I scanned the new "antenna" with my analyser and got the following swr-frequency chart:

This picture looks familiar, I thought. This is what you would expect to see when scanning a resonant antenna. The impedance is a bit over 50 ohm, so in order to get the SWR as low as possible, the impedance transformer was used.

The signal is a little bit weaker on the grabber, however not by a huge amount. I estimate the difference to be a couple of dB by visually comparing the peak hold (green line) on the two spectrograms:

The carrier in the following spectrogram is again my AA-30 antenna analyser. It is amazing that the signal is clearly detectable 20 km away.

Conclusions

Previous tests on 136 kHz have shown that a terminated antenna produced better results compared to the same wire but unterminated at the remote end. The opposite seems to be true for 475 kHz. More testing is necessary and will take place on the first opportunity.

Go back to the main earth antenna page.

Dimitris Tsifakis, VK1SV