An actual harmonic of a station on 1290 would fall at 2.580 with the third
harmonic being at 3.870. Can you understand what is being broadcast on the
transmitter, music, voices etc for positive identification?
I helped when I was a OO with a Canadian station on 640 putting a harmonic a
few years back on1.920. There you could hear music and voices but not easily
understood for me, but S9 stronger for others. I reported this to the ARRL who
went thru proper channels and shortly thereafter I received an email from the
50,000 watt station in Toronto. When commercials were playing the engineer
switched transmitters, switch antenna patterns even went to lower power with a
back up transmitter but the harmonic was still there. While this was happening
he was emailing me and I was emailing back to him, despite all testing nothing
changed the harmonic was still there. They were getting ready to did up the
ground system around the station when they discovered the reason for the
problem. On the tower they rented a space for another firm to place an antenna
on the main tower much like the FM antenna you noted, that was the cause of the
harmonic. This problem was not all across the band as you note but only 1.920.
How strong is the strongest interfering signal you are able to hear. The
problem you report of a wide area of interference suggest something that is not
within FCC rules.
I would take a portable radio or if you have actual measuring equipment to the
station and listen to the signal to make sure it gets stronger as you approach
the station if you cannot positivily identify the station by its programing you
are hearing.
73 Tim
WB8UHZ
On Saturday, December 5, 2020, 12:16:02 PM EST, <k1ttt.dave@gmail.com>
wrote:
You might want to query the rfi reflector... also consider something close to
you, even a neighbor's house, that might be picking up the primary, rectifying
it, and modulating it with a 10khz switcher harmonics. Had something similar
to that here where a wall wart was generating and modulating harmonics.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: telnet://k1ttt.net:7373
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Byron Tatum
Sent: Saturday, December 5, 2020 11:15
To: Towertalk Reflector <towertalk@contesting.com>
Subject: [TowerTalk] AM Broadcast Harmonic Interference to 75 Meters
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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