I think the previous posts have covered most of the possibilities. I
would hazard to guess that the crud you are hearing is the main culprit and
the 3rd harmonic of the AM station is getting the bad rap for it. Had a
similar issue when I lived in the Detroit area with 3 AM stations mixing to
create a birdie in the 80M band. They were all a couple miles from my
house. A single birdie I can live with but broadband noise takes out the
whole band so focus on that. Remove any preamplification and lower the
gains of any RX settings or antenna preamps to see if that makes it go
away. After I was content that the noise wasn't being created by overload
in my equipment, then I would move on to see if it was being generated by
mixing with another very local noise source or from the dissimilar joints
mentioned. I would look for the local noise source.
You will probably spend a lot of time flipping breakers or even driving in
the mobile to see if it changes to prove out whether it is a widespread
area problem or just at your house. Not sure if you live in a neighborhood
or out in the country which would affect the number of noise sources you
would come into contact with.
It is interesting that the crud goes 3600-3960 which is 90 kHz above and
270 kHz below the 3870 kHz frequency. If it was related to the 1290 kHz
3rd harmonic I would expect it to be more balanced around the carrier
frequency. So my money says there are two noise sources and the wideband
hash is being generated nearby and the AM station is getting blamed.
Good luck. Let us know how the investigation goes.
Earl
N8SS
Date: Sat, 5 Dec 2020 16:14:53 +0000 (UTC)
From: Byron Tatum <bjtatum1@att.net>
To: Towertalk Reflector <towertalk@contesting.com>
Subject: [TowerTalk] AM Broadcast Harmonic Interference to 75 Meters
Message-ID: <812396963.3030097.1607184893245@mail.yahoo.com>
Content-Type: text/plain; charset=UTF-8
Hello-? ?Can someone point me in the correct direction, in finding
technical help and someone to make measurements, regarding a problem I have
at my new QTH involving harmonic interference from a local AM broadcast
station? Here are some details - The station is licensed for 5.0 KW daytime
/ 1.0 KW nighttime power levels. The transmitting site is 2.5 miles from my
QTH. The transmitting antenna is approx 240' of 65G tower on an insulated
base with steel guy wires broken up with insulators. An old FM broadcast
antenna is mounted on top of the tower but is not in use I was told. The
station broadcasts on 1290 Khz.? ? My problem is an extremely strong
harmonic at 3870 Khz both day and night, plus there is crud every 10 Khz
from about 3600 Khz to 3960 Khz. I have received this interference in a
very similar manner on my FTDX-5000MP in combination with the Micro-Tune RF
filter and on a well aligned R-390A. I don't believe it is my equipment. I
contacted the station owner and he told me tha
t he made measurements and is within the FCC limits. I do understand that
the station can be emitting a signal within FCC limits of harmonics and the
harmonics can be generated in surrounding objects such as dis-similar
metals or corroded joints. The crud every 10 Khz indicates to me some
serious mixing products being generated somewhere.? ?I feel that I need to
find a Ham who has analyzing equipment and knowledge of this subject who
can come to my place and make measurements and guide me onlooking into
this.Thanks,Byron W5FH?
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