[RFI] Sources of wideband hiss

[Jim Brown]

That cavity in front of the first gain stage strongly suggests IMD in 
the RX system.

I was part of a team that maintained VHF and UHF repeaters in Chicago 
for our club, where ALL TV and nearly all FM is on one of two buildings 
that are slightly more than a mile or from other. There was also a huge 
number of commercial 2-way systems there. It was virtually impossible to 
receive VHF from anywhere within the Chicago Loop, thanks to massive IMD 
from all of those transmitters. It sounded like broadband noise.

Nearly all systems were set up for TX on one of the tall buildings, with 
multiple voted RX sites at least 4 miles away. 220 and 440 were no problem.

73, Jim K9YC

On 12/23/2023 5:08 PM, Gary NA6O via RFI wrote:
> At the W6TCP hilltop EME site, this exact problem occurs on 2m and also on 220 MHz whenever the array is level and pointed towards one of the nearby towns. There is NO character or modulation detectible, just a broadband increase in the noise floor. It was bad enough on 220 to cause serious problems. The solution there actually surprised me. He upgraded the LNA from a P220VDG (from DB6NT) with a fairly broad LC bandpass filter to one from WD5AGO with a much sharper cavity bandpass. That change alone was worth almost 20 dB in noise reduction! All I can figure is that there must be a great many VHF transmitters out there and with our high antenna gain, some kind of complex intermod was occurring but I really don't know. So I will always cast some suspicion towards out-of-band sources, in addition to the usual buzzing switchmode converters.
> 
> -Gary NA6O
> 
>>
>> What are some sources of wideband hiss?  I'm talking about an unstructured,
>> apparently unmodulated many-dB increase in the noise floor across the 2m
>> band*  No associated birdies or other noticeable signals, just a nasty
>> hiss.
>> I see it in the direction of high-voltage power lines 2km away and also
>> toward a couple of houses I just a few hundred meters distant.  The
>> mechanisms of those sources are not necessarily the same of course.  Ideas
> 
> 
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