[TowerTalk] AM Broadcast Harmonic Interference to 75 Meters

amfone amfone20000 at yahoo.com
Sat Dec 5 13:54:41 EST 2020

 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 


    On Saturday, December 5, 2020, 12:16:02 PM EST, <k1ttt.dave at 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 at arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: telnet://k1ttt.net:7373

-----Original Message-----
From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of Byron Tatum
Sent: Saturday, December 5, 2020 11:15
To: Towertalk Reflector <towertalk at 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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