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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