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

Edward R Cole kl7uw at acsalaska.net
Sun Nov 20 02:26:35 CET 2016


Bob,

I was going to suggest if stronger inverted vs upright (whatever that 
means) then it might indicate "Australian Noise" vs "Alaska Noise" - ha ha!

But one mentioned shift in cabling well that might also indicate some 
common-moding over the coaz shield?

My Noise is stronger in Hpol than Vpol - yeah big surprise.  Almost 
10-dB stronger at times.  I actually think there may be as much as 
three sources with different characteristics:
1)  broadband white noise from 50-170 MHz up to +30 dB over quiet noise floor.
2)  bands looking like modulation envelope of SSB spaced around 100-KHz apart.
3)  wavy fuzzy bands tracing a sine wave pattern on about 120 KHz 
wavelength...that was running last night
last two seen on 2m.

Also think occasionally I get ac line noise when its rainy or 
windy.  That responds well to K3 NB.

No nearby TV stations (>100 km) or FM broadcast (>30 km)

73, Ed - KL7UW

At 02:54 PM 11/19/2016, Bob Atkins wrote:
>Content-Language: en-US
>Content-Type: multipart/alternative;
> 
>boundary="_000_DM5PR14MB1755CDCC9EB4DBD9940E8B9FE7B30DM5PR14MB1755namp_"
>
>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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73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag Business e-mail:
     dubususa at gmail.com 
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