[Moon-Net] Question about polarization a noise. Odd behavior.

Bob Atkins ka1gt at hotmail.com
Sun Nov 20 00:54:45 CET 2016


Based on a number of emails I've received I think I have a working theory of what might be going on. I had made an naive assumption that the pattern of the antenna was symmetric and that may well not be the case (in fact it almost certainly isn't symmetric)


Based on a few emails and thinking more about it, I think what I'm seeing can be explained by two things:


(1) A broadband noise source in the area

and

(2) Asymmetric antenna side lobes


If the pattern of the side lobes is irregular then the strength and polarization with respect to the noise source will change as the antenna polarization is rotated (each boom rotates). That could explain why there appears to be a difference with 180 degree polarization change. The feed isn't entirely symmetric (folded dipole/T-match) and the stacking distance is constant, irrespective of polarization. Add all those factors together and if there's a local noise source, the strength of the pickup will indeed change as the antenna is rotated through 360 degrees with +180 having a different side lobe pattern to -180.


Where the noise is coming from is a different question. It's more than just ground noise since it rising to 3dB over a 50 ohm load.  I've dried direction finding with no success. It seems to be in one direction, but when you follow it it disappears. Maybe multiple reflections? It's not in the house as far as I can tell. Direction finding doesn't indicate that and turning everything off makes no difference (a few spurious signs disappears but the noise floor doesn't change).


So I think the question may have been answered. The solution to the problem is a more difficult challenge.


Thanks to everyone for their comments.


Bob

KA1GT




________________________________
From: moon-net-bounces at mailman.pe1itr.com <moon-net-bounces at mailman.pe1itr.com> on behalf of Bob Atkins <ka1gt at hotmail.com>
Sent: Saturday, November 19, 2016 3:13 PM
To: moon-net at mailman.pe1itr.com
Subject: [Moon-Net] Question about polarization a noise. Odd behavior.


I just got my polarization rotation running properly for the contest last night.  I observed two things, one I understand (?) and one I don't.


I know I have a background noise issue on 432 (which isn't coming from my house). Last night I noticed that at one polarization (say horizontal) sky noise was about 3dB LESS than a 50 ohm load - as it should be. However at the orthogonal polarization (say vertical) "sky" noise was up to 3dB MORE than a 50 ohm load. So whatever is the source of my local noise, it's polarized and it's reducing my sensitivity by up to 6dB (despite which I made 16 QSOs last night).


So far, so good (more or less...except for the noise)


However, I also found in some cases that 180 degrees rotation CLOCKWISE from, say, horizontal to vertical (+180 degrees) gave a DIFFERENT noise level than a 180 degrees ANTI-CLOCKWISE rotation (-180 degrees), something that I'm not sure I understand. Both rotations go from, say, horizontal to vertical, but the end up 180 degrees out of phase. How can that affect the noise level?  Vertical polarization is vertical polarization right? The phase only matters when combining antennas. The array is 2x28el and both booms rotate at the same rate in the same direction and so stay in phase.


What don't I understand here?  This excess noise is annoying and seriously limits weak signal work (unless the signals happen to be at the polarization that picks up least noise!). I'm still trying to figure out where it's coming from. Maybe this odd "phase" issue is giving me some sort of clue?


If anyone has any thoughts on this, I'd appreciate hearing them.


Bob

KA1GT
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