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Author Topic: Question about NOAA interference  (Read 724 times)
KC2MAJ
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Posts: 7




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« on: December 04, 2017, 07:32:35 PM »

I don’t really post here so please forgive any noob errors. Hoping you Elmer’s can help.

KWO35 is the NOAA station for Manhattan, NYC broadcasting at 162.55 MHz. It is currently out of service over a five year issue about reported interference with Coast Guard communications on Marine Radio channel 16 at 156.800 MHz. To me that seems like a lot of frequency separation for intractable interference.

Any help understanding this would be greatly appreciated!!! A link to a Wikipedia article covering the history of the issue is below. Thanks!!!

73

KC2MAJ

https://en.wikipedia.org/wiki/KWO35
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WB6BYU
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Posts: 17184




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« Reply #1 on: December 04, 2017, 08:07:02 PM »

This probably isn't just a problem with overload, unless the Coast Guard transmitter is close
to the NWS transmitter.  (One would assume that that aspect of the problem has been
eliminated by moving the transmitter multiple times.)

The article doesn't give detailed enough information to decide what the actual problem is,
but those who have worked at repeater sites know there are many different possible forms
of interference when you have transmitters on different frequencies.  For example, you can
get a mixing product between two different transmitters that generates a spurious signal
on a different frequency:  the third harmonic of the NWS transmitter mixing with a TV
transmitter on 644.450 could produce a spur on 156.800 that could radiate from either of
the two antennas, or could be generated in the Marine receiver.  Or you can get spurious
signals from a single transmitter due to harmonics in the frequency multiplication chain or
mixing products, or a spurious oscillation somewhere.

If the Coast Guard radio is a "remote base", then the uplink frequency may be subject to
interference.  If the interfering signal is heard over a wide range of locations then the
problem is less likely to be in the Coast Guard system, but the article says that the audio
comes in on the Coast Guard audio (as opposed to acting like a separate signal).  That would
suggest a whole range of other, generally uncommon, problems with the audio feeds to
the two transmitters, but the article isn't written at enough of a technical level to assume
that is really the case.

So it may just be a case that there are multiple strong signals on the air in NYC, including
the NWS transmitter, that mix together in any convenient bit of rusty metal that will act
as an antenna (or in the front end of any receiver connected to a good antenna that doesn't
have a selective front-end filter) and generate a local signal on 156.800 MHz.  That's a very
difficult situation to solve, because no one transmitter or receiver is at fault
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KC2MAJ
Member

Posts: 7




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« Reply #2 on: December 04, 2017, 08:16:18 PM »

Thank you for an awesome reply. I think that you’re right about the scattered nature of both transmit and receive on the Coast Guard end. 16 is their emergency frequency so it’s important could be coming out of or going to anything from a CB quality HT or a high end off shore rig.

Again many thanks.

73
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WA3SKN
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Posts: 6523




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« Reply #3 on: December 05, 2017, 03:01:13 PM »

Sounds like the National Weather service is having trouble finding a suitable location for their KW035 transmitter!
They could have technical problems, funding problems, or just finding a good rentable site for their station... sites can be at a premium in some populated areas (like NYC!).
Best to ask the NWS about this as they would have the answers.  Since they replaced the original eqpt AND also moved locations, I doubt it is a technical problem!
73s.

-Mike.
WA3SKN
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WB6BYU
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Posts: 17184




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« Reply #4 on: December 05, 2017, 03:13:48 PM »

Did the trouble start happening about the time that the local TV stations went to digital?

If so, it might be a 3-way mix that would be eliminated if any one of the stations goes off
the air.  Once the NWS station (which may be the only one whose audio will demodulate in
the marine receivers) went off the air, the interference went away, and any time it tries to
come back on the interference will reappear, even if nothing is wrong with the transmitter.
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KC2MAJ
Member

Posts: 7




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« Reply #5 on: December 06, 2017, 08:45:42 PM »

WA3SKN & WB6BYU

Thank you for your analysis. Yes, I do believe location is an issue. I don't know about the timing versus digital TV. The budget point is also a good one but NOAA radio is one of the most basic building blocks of emergency communications. When people's internet and electricity are down, battery powered receivers will still work. The coverage area of KWO35 has been through some really tough events. I hope NOAA and the Coast Guard will re-prioritize this appropriately and get the FCC involved if necessary. 5 years is a long time to work it out.

Again, many thanks for everyone's input.

73

KC2MAJ
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