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FM Co-channel interference observations

B

Buckeyes2001

When there is a co-channel interference situation on FM, why is it that sometimes the 2 (or more) stations seem to flip-flop with each other and you hear a mix of the 2 signals and other times the 2 stations' signals seem to cancel each other out? Does it have to do with the source of the 2 signals relative to your position or more to the relative signal strengths of the 2 signals as to which phenomenon you hear?
 
I'm sure someone else here could answer it more specifically than I can but I'd guess it has to do with the high frequency of FM waves as compared to AM and which FM station tops the other I'd think would have to do with signal strength.

Here's a nice example of co channel interference on an FM frequency during E Skip where two stations battle it out!


http://www.youtube.com/watch?v=MLmPdUvOrrA
 
This "capture effect" is a fundamental characteristic when FM signals are demodulated by an FM receiver, in that only the stronger one dominates at the receiver output.

Very good FM broadcast receivers can reduce the program audio of a weaker, co-channel station by 30 dB or more as long as the incoming radio wave from the stronger station is at least 1 dB greater than the other.
 
I don't know, but sometimes when I was stationary on the car radio the station would seem to change by which way the wind was blowing. - A quick story my mom used to take care of an elderly lady who lived right near Stop & Shop on Pine Street in Forestville, Connecticut. It was a much shorter drive to go there than to go to my mom's so I'd meet her there to give her my laundry to do. I'd sit in the car waiting for her and I'd mess around with the radio dial. I could pick up 97.5 WALK from Patchouge, Long Island, but if the wind changed direction I'd get Bomba 97.5 a translator from Bolton, Connecticut. This no longer happens as Bomba has moved to 97.1 FM.
 
I get that all the time on 101.7 in Vallejo, CA in the Glen Cove Area

KKIQ Livermore, CA and KHTH Santa Rosa, CA

Sometimes on 98.5

KRXQ Sacramento, CA and KUFX San Jose, CA
 
It depends on the phase of each signal as they arrive at your receiver. They can cancel out if they arrive out of phase.
During e-skip or on an airplane when multiple FM stations are on a frequency, I can get different stations based on how I hold the antenna.
 
Usually during Es here, because there's fewer stations out here in the West, I usually can get one station, in and out. Sometimes I do get two duking it out, but that's more rarer. Never have gotten three deep on a frequency IIRC.

-crainbebo
 
I live in Burlington, NC. Just east of Greensboro. Ever since they moved 92.3, licensed to Asheboro, NC, 20 miles south and went to Class C, they have always fought WXLK, Roanoke, Va. Also a full C. The Roanaoke signal is on a high mountain north of Greensboro probably 80 miles, although I think the Asheboro station is somewhat directional. It can be a mess
 
I may not be 100% sure of the original question, due to the "canceling out"....but I offer that if one or both stations feature HD, *that* could be an issue as well.

I own the Sony XDR-F1HD, and when DX conditions come in strong, if my antenna is pointed NW, my semi-local WIRK 107.9 will get totally taken over by WSRZ...WSRZ runs HD, WIRK does not.

During E-skip, my local WKCP 89.7 was run over a couple of times by Boston's WGBH, or at least WGBH was bothersome enough to display its RDS readout. I count RDS readouts as loggings.

cd
 
WPST and WDAC, their transmitters are less then 60 miles within each other, and the battle is horrid once you head out of Philly towards the west. Normally PST is an incredibly strong signal.
 
This is a problem that plagues Snohomish County in WA state often when dealing with co channel stations in Vancouver, BC and Olympia/SW Washington. Especially on hills

88.1 CBU-1-FM Vancouver, BC/KWAO Ocean Park, WA
96.1 CHKG Vancouver, BC/KXXO Olympia, WA
97.7 CBUF Vancouver, BC/KOMO-FM Oakville, WA
99.3 CFOX Vancouver, BC/KDDS Aberdeen, WA

The only three-deep hash I ever got was on Camano Island, WA between CKLG-FM Vancouver, BC, KGY-FM McCleary, WA and KWPA-LP, Coupeville, WA a few years ago on 96.9 MHz.
 
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