Constant Oscillating Sound on Public Service Frequency

[HangEmHigh]

When I monitor one of my town's public services frequencies, there is a constant oscillating sound whenever no one is transmitting. I dont think its a question of the squelch on the radio, as I've tried all the possible settings and get no relief. Any idea what this could be? The signal is nice and clear when someone is transmitting, but other times, the oscillating sound drives me crazy. Can post a mp3 file if that would help diagnose. Thanks!

 

[drgrant]

Do you have another radio to test this against? (different brand, model) My guess is your RX has a spur/birdie or something in it.

If it's one of those Baofeng jobs everyone is crowing about, I wouldn't be surprised if those things are loaded with spurs.

-Mike

 

[cockpitbob]

What's the frequency? I assume it is an FM signal on VHF but want to be sure.

How strong is the signal. Does your receiver have an S-meter that can give a clue as to the strength?

By "oscillating" do you mean a constant tone? An mp3 might help.

 

[HangEmHigh]

Thanks for the lightning fast responses!! I don't have another rig to test it against. Good call, Mike .. it is a Baofeng UV-5R. Signal seems strong to me, Bob, but I don't have an S-meter to check (unless there is one built in to my HT). Will record and post an MP3 in the next half hour. Freq -> 470.3125

 

[jpm]

definitely post an mp3 so we can hear it. Hard to tell what it could be just from your description but hearing it should identify it.

 

[jpm]

Actually I think I know what you're hearing. There's a data/control channel on an adjacent frequency (470.325) which is probably coming thru when the receiver opens up on the .3125 frequency you're listening to. Your receiver is probably not setup for narrow band reception or you're too close to the data channel transmitter for it to reject this adjacent channel when the primary is being received. If you have an attenuation switch (either physical on the back or electronic via an ATT button on the keyboard) try attenuating the receiver to see if it filters that adjacent channel out. The only other option would be a notch filter if you don't want to upgrade the receiver.

 

[cockpitbob]

HangEm just sent me the .mp3. It sounds like an electronic version of a diesel engine or jack hammer with full quieting but voice transmissions are perfect. I've heard this sound once before on a 2M ham repeater. I bet our experts here will know what it is.

 

[jpm]

HangEm just sent me the .mp3. It sounds like an electronic version of a diesel engine or jack hammer with full quieting but voice transmissions are perfect. I've heard this sound once before on a 2M ham repeater. I bet our experts here will know what it is.

That's exactly what the data channel I found sounds like. (see post just above yours)

 

[HangEmHigh]

Thanks, JPM! Looking at the UV-5R documentation to see if an attenuation setting is possible. Will report what I find.

If you have an attenuation switch (either physical on the back or electronic via an ATT button on the keyboard) try attenuating the receiver to see if it filters that adjacent channel out.

 

[HangEmHigh]

I did find a Wideband (25 kHz bandwidth) or narrowband (12.5 kHz bandwidth) setting in the menu. Tried to set it to NAR, but still getting the same interference. Maybe Im asking too much from a $29 radio?

 

[jpm]

yeah it could just be that the receiver isn't capable of rejecting the signal, tho I kinda suspect its proximity to the transmitter on that data channel that's doing it. Do you get the same thing when you're in a different area at least a few miles away?

 

[HangEmHigh]

I'll bring it with me the next time I'm out in the car driving around. I live quite a distance from the police station, but its possible that their transmitting equipment is not colocated at the station. Will let you know what I find.

yeah it could just be that the receiver isn't capable of rejecting the signal, tho I kinda suspect its proximity to the transmitter on that data channel that's doing it. Do you get the same thing when you're in a different area at least a few miles away?

 

[drgrant]

I did find a Wideband (25 kHz bandwidth) or narrowband (12.5 kHz bandwidth) setting in the menu. Tried to set it to NAR, but still getting the same interference. Maybe Im asking too much from a $29 radio?

If you have it hooked up to a real (external) antenna its possible that thing is getting overloaded.

-Mike

 

[HangEmHigh]

Nope. Just the rubber antenna that came with the HT.

If you have it hooked up to a real (external) antenna its possible that thing is getting overloaded.

-Mike

 

[jpm]

Nope. Just the rubber antenna that came with the HT.

this is why I'm thinking proximity to the data transmitter. That's most likely not associated with the PD by the way, its just an adjacent channel. Their system is conventional anyway so it wouldn't use a data channel like that.

 

[JFR2]

Does it go away if you remove the antenna? If not, it's internal or VERY close by.

PM me your location if you want and if you're close by, I'll look at the spectrum thru my external antenna.

 

[jpm]

Good point, also try this. Aim the antenna connector like a gun barrel at a target and turn in a circle to determine the direction to the transmitter. When you hear the strongest signal, you're pointed at it.

 

[HangEmHigh]

Tried removing the antenna and can't hear the data chanel with it off.

Also noticed that I can hear the same sound over multiple frequencies. From 470.3125 up to 470.340.

I am located in Cambridge, JFR2, just outside Central Square.

I tried to pinpoint the direction, jpm, but it didn't seem significantly stronger in any one direction.

 

[jpm]

well its probably close since you're getting it on a pretty wide spread. Its probably on one of the buildings in Boston or Cambridge. Not a lot you can do really other than trying a more selective radio or a notch filter, which is probably more expensive than the radio itself.

 

[JFR2]

470.3125 is a Camb PD freq. Since he's in Camb, it's very likely he's got some mixing going on with an overloaded front end of a cheap Chinese radio. I'm sure many of you have heard NWS broadcasts on other VHF freqs, especially with a GRE scanner.

PS- I own one of those cheap Chinese radios. It's $30 so it's hard to knock it for what you get.

 

[Gillham]

I'm across the river. 470.3125 isn't giving me any funky noise. Heard some peeps transmitting - no noise.

I'm not using the stock antenna though. I'm using a 14" whip.