Sweet Spots & Dead Zones

Nothing is ever straight-forward when it comes to antennas. While the theoretical aspects hold up well, the practice often seems to suggest the contrary.

In my own example, how can it be that a simple three element band 2 beam, just one metre above the ground, out-performs a professionally crafted and carefully optimised 9 element beam on a rooftop mast? Well, it can't. But the location is probably more important than many of us realise.

This unexpected discovery only came to me when I erected a Triax FM3 on the garden for the purpose of having a 'quiet' antenna for meteor scatter DXing. The facts which follow do not suggest that a Triax FM3 can outperform a Körner 9.2.

When I bought the fabulous Körner 9.2 antenna I was very impressed with its performance across band 2. Not only was the gain noticeably superior to that of my Triax FM5, it also exhibited far better directivity than I had expected. I used my garden as a test site before getting it on the rooftop mast. I was hearing fully quieting signals from troposcatter signals in excess of 700 km. They were actually noise free at times too. The 9.2 was only on a makeshift mast at only two to three metres above the ground. This was surely going to provide a huge improvement to my band 2 DXing, and indeed it did!

The problems I faced related to locally generated electrical noise. I won't go into that again as I have probably thrashed that topic too much as it is, but once the 9.2 went on the roof, I was aware that it received considerably more noise than it did when it was under analysis on the garden. Regular viewers will be aware that I am not allowed to have an antenna on a mast on the garden due to overzealous housing regulations. All antennas have to be professionally erected on the property.

To cut a long story short, I erected a Triax FM3 on a small tripod base in the middle of the garden. It was only one metre above the ground. I checked the reception and immediately noticed a super-quiet FM band. I say "super-quiet" because the noise levels registered 4dB across the band. This is as low as the XDR-GTK software goes. This is not high-grade laboratory standard test equipment so these readings might be best considered to be approximate, though it gives me a very good reference - something I did not have on the Sony XDR-F1HD which the software controls. Knowing how this software behaves, I would expect a reading down to 1dB were it able to go that low. I have no idea why it stops at 4dB.

While I was conducting this initial reception test, I noticed that many continental transmitters were coming though, fading in to noise free levels and then dropping out again. Typical scatter conditions, but wait a minute ... this seems to be picking up more continental signals that the rooftop 9.2. How can it be from this most basic of antenna setups? It must be because the noise levels are much quieter. RDS is now forming with signals as low as 20dB! I can hear meteor bursts coming in to 26dB already! This bodes well. But can this really be dragging in more DX than the 9.2? It certainly seems to be. Time to make a comparison.

Switching between the 9.2 and the FM3 provided me with some BIG surprises which I never expected. Both antennas received roughly the same amount of signal, though continent signals were in greater abundance on the FM3, quite clearly due to the FM3 receiving less noise, but it was apparent that the FM3 was picking up the same amount of signal level as the 9.2. Again, how could this be?

I then spent the next half an hour or so moving the FM3 around the garden. There was a very well-defined 'sweet spot' close to the middle of the garden. Moving the FM3 two metres further away from the property quietened the noise levels down even more, but also reduced the signals. Even with the FM3 one metre away from its original position saw a reduction in general pick-up. I had found this sweet spot by accident. It occurred to me that the rooftop 9.2 was in a dead zone. It's the same as receiving weak signals in a moving car. You will find you can get better reception by driving the car a metre or so forwards or backwards, etc.

So I find myself in the less than fortunate position of having the 9.2 antenna in a poor location. No wonder it performed so much better on the garden - it was very close to that sweet spot when I first tried it. I am now considering moving the 9.2 elsewhere on the property. I'll have to think about this carefully and conduct more tests. Bearing the housing situation and our neighbours in mind, this could be problematical.

John