FM Station overloading VHF Scanner?

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n9lea

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Feb 12, 2005
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I have a Radio Shack 1000-channel scanner hooked up to a Radio Shack 20-176 Ground Plane Antenna. The antenna is mounted to a mast on the roof and is connected to the scanner by coax. The system is definitely light on grounding.

Since I installed this antenna system, I've had sporadic problems with hearing a local FM radio station on the VHF bands. The problem is intermittent and seems more common at night.

When the problem exists, I can clearly hear the radio station, which broadcasts from a tower about 2 miles from my QTH on 105.1 MHz at 6 KW. As I mentioned, it's intermittent. When I hear it, I can hear it the strongest on 159.15 MHz, but it splashes across wide chunks of the 154-159 MHz range, including a few frequencies I like to listen to. When it's on, I can't squelch it out, though the scanner's attenuator will usually get rid of it. Of course, I don't like to run with the attenuator so I can hear other distant stations.

I don't know if this is a true interference issue or if the signal is just so strong that it's overloading the receiver.

Can anyone suggest a solution to this problem? Would some sort of grounding solution be best or a filter of some sort? Or am I just relegated to the attenuator?
 

SkipSanders

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Joined
Dec 19, 2002
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1,058
Even worse than the interference, FM Broadcast transmitters often desense your receiver if you have an outside antenna connected. It's almost a basic requirement to put an FM Trap filter inline with an outdoor antenna in a major urban area to minimize this problem.
 

gmclam

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Location
Fair Oaks, CA
You can use any old FM trap designed for (cable) TV to do the job. One issue is the type of connectors it has. If you want to monitor the aircraft band, you might need something a little better.

When I measured signals directly from my discone the FM broadcast signals were the strongest at a typical strength of 20dB hotter than the signals I want to scan. I am no where as close to a transmitter as you are.
 

gdsteele

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Nov 11, 2004
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I had the same problem. I live 2 miles from the broadcast site for 104.9 and it swamped my scanner front end so bad that I couldn't receive the NOAA broadcasts.

I agree with fineshot, the PAR filters are the way to go. Their customer service is second to none. When you contact them, let them know what station(s) cause the the most interference and they will notch those out for you.
 

jim202

Member
Joined
Mar 7, 2002
Messages
2,559
Location
New Orleans region
I will ask a simple question here, do you have a channel 2 TV station near you? I smell a couple of
problems here and it all boils down to overload and intermod.

Jim




I have a Radio Shack 1000-channel scanner hooked up to a Radio Shack 20-176 Ground Plane Antenna. The antenna is mounted to a mast on the roof and is connected to the scanner by coax. The system is definitely light on grounding.

Since I installed this antenna system, I've had sporadic problems with hearing a local FM radio station on the VHF bands. The problem is intermittent and seems more common at night.

When the problem exists, I can clearly hear the radio station, which broadcasts from a tower about 2 miles from my QTH on 105.1 MHz at 6 KW. As I mentioned, it's intermittent. When I hear it, I can hear it the strongest on 159.15 MHz, but it splashes across wide chunks of the 154-159 MHz range, including a few frequencies I like to listen to. When it's on, I can't squelch it out, though the scanner's attenuator will usually get rid of it. Of course, I don't like to run with the attenuator so I can hear other distant stations.

I don't know if this is a true interference issue or if the signal is just so strong that it's overloading the receiver.

Can anyone suggest a solution to this problem? Would some sort of grounding solution be best or a filter of some sort? Or am I just relegated to the attenuator?
 

radact

Member
Joined
Oct 16, 2007
Messages
22
Location
Las Vegas, Nv
I had the same problem with a station being 4 miles away and my antenna outside just barely clearing the crest of the house. I went and put it in the attic of my garage and wilth all the wood up there it reduces the signals just enough that it's not a problem any more.

I haven't noticed any loss of signal strength in the Public Service, Civil Air, MilAir or other bands. I also use my scanner to listen in on the downfeeds for some of the Amateur Radio Satellites as they pass over my location.

I also enjoy listening to Red Flag excersises as they take place all up and down the Nellis Test Range, which in some directions can be well over 100 miles and I can still here the planes talking to each other.

Good antenna and keeping the coax as short as possible really helps. The excess coax can also pick up unwanted signals.

Happy Scanning.
 

n9lea

Member
Joined
Feb 12, 2005
Messages
7
No Channel 2 nearby. For price reasons, I'm going to try one of the cable TV trap things and see what happens. If that doesn't work, I'll have to look at the more expensive options.

Thanks to all!
 
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