[TowerTalk] AM Broadcast Harmonic Interference to 75 Meters

Gary Schafer garyschafer at largeriver.net
Tue Dec 8 15:49:23 EST 2020

I had a similar problem with a Kenwood TS430. It has lots of relays in the
front end to change filters etc.
Sometimes just hitting the transmit button momentarily would make the
products go away, other times not.
It ended up that there were bad contacts on some of the relay where mixing
would take place.

Look for marginal connections in the antenna circuit. Connectors, relays
antenna and other things close by that could re radiate the signals.

Gary  K4FMX 

> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On 
> Behalf Of Earl Morse
> Sent: Tuesday, December 08, 2020 2:37 PM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] AM Broadcast Harmonic Interference 
> to 75 Meters
> 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 at att.net>
> To: Towertalk Reflector <towertalk at contesting.com>
> Subject: [TowerTalk] AM Broadcast Harmonic Interference to 75 Meters
> Message-ID: <812396963.3030097.1607184893245 at 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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