antennas and noise level

Author Post

Johan, ON4IQ

Fri Dec 10 2010, 08:03AM
After reading the G3WOS article on his new array and my personal observations on arrays I was puzzled.
When you elevate your antenna array the noise level goes down (6-7dB). Could it be that the majority of
noise pickup is really in the front lobe?. Looking at the elevation patterns you will notice the sidelobes get
somewhat worse, so if noise pickup is really from the sidelobes the average noise level when elevating
should be higher right? I AM TALKING ABOUT NOISE not spurs, birdies whatever..

When I compare my 2x6el (yu7ef) versus 4x9 (M²) the latter is always the one with lowest noise level (and beamwidth)
The towers are identical (height) and spaced 42m apart.
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Justin, G0KSC
Fri Dec 10 2010, 06:24PM
Justin, G0KSC
Registered Member #64
Joined: Thu Mar 20 2008, 04:10PM

Posts: 49
Hi Johan,

First of all looking at the plots on your site which show elevation, it is clear to see these are inaccurate. I have also spoken with Roy Lewallen (author of EZNEC) and Jerry Burke (Author of NEC) and both confirmed EZNEC/NEC will ONLY give accurate results if the antenna is parallel with ground. As soon as the model is elevated, what you see is NOT representative of what would occur in the real world. The recommendation is to use freespace plots which will give a much closer representation of what will be seen from your antenna in the real world.

Next, there is very little you can do to get rid of noise in your main lobe, it is only from other directions (assuming in these directions your pattern in highly suppressed) that you will see a drop. Keep in mind that the plots you see are a cross-section of the real plot. The antenna pattern is cylindricle so appears above and below the antenna too.

If your antennas are working properly and you have well supressed sidelobes then as you elevate, your noise level with drop. I cannot understand how you would expect noise to increase.

Finally, You simply cannot compare 2 x 6 element Yagis with 2 x 9 element Yagis. The beamwidths are completely different and much, much wider on the 2 x 6els ( because beamwidth is dependant on boom length, nothing else) which ultimatley will result in more noise. When comparing 2 antennas with 15M long booms against 2 antennas with 6M long booms the (much) longer of the 2 should outperform the shorter antennas in every respect, by quite a margin. That is unless there is something very wrong with your longer antennas!

If you seek to test one of Pop's low noise antennas, you will need to look towards his longer VHF/UHF antennas. The short 50MHz antennas are optimised for bandwidth and stability rather than suppressed noise levels. If you took time to study the polar and SWR plots within EZNEC you would understand this.



[ Edited Fri Dec 10 2010, 06:52PM ]
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Johan, ON4IQ
Fri Dec 10 2010, 07:12PM
Johan, ON4IQ
Registered Member #52
Joined: Wed Mar 19 2008, 12:21PM

Posts: 14
you have raised an interesting topic, yagi analyses when elements are not parallel over ground, that is when elevated or having sloping ground. Guess only few have flat ground around the array extending long enough.
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Justin, G0KSC
Fri Dec 10 2010, 07:23PM
Justin, G0KSC
Registered Member #64
Joined: Thu Mar 20 2008, 04:10PM

Posts: 49
Hi Johan, I think you know exactly what I mean and also what is fact so I will not humour you and explain in more detail.
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