Scanner background noise

GB46

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Here's something I've always wondered about: Sometimes I keep my scanner's squelch open to look for signals that may be too weak to open the squelch on their own. When no signals are present, however, the level of background hiss is so high in every VHF/UHF scanner I've ever used that I have to turn the volume down. Why is this? Is it caused by the circuitry in that type of receiver? I've also noticed this when the squelch is opened by a signal from a vehicle in motion. The hiss is very loud during fades.

It's never that high on shortwave receivers, even in an environment with lots of RFI.
 

majoco

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There's just as much noise whatever the detection method, AM, WFM, NFM or SSB - I just tried it on my Icom R7000 receiver. There is some noise generated in the first stages of a receiver due to the random movement of electrons but most of the noise is 'cosmic noise'.

" Cosmic Radio Noise Intensities in the VHF Band* "

 

GB46

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There's just as much noise whatever the detection method, AM, WFM, NFM or SSB - I just tried it on my Icom R7000 receiver. There is some noise generated in the first stages of a receiver due to the random movement of electrons but most of the noise is 'cosmic noise'.

" Cosmic Radio Noise Intensities in the VHF Band* "

Good post, Martin, and thanks for that interesting article! I guess one could say that there's an astronomical amount of noise at those frequencies. :)

I always blamed circuit noise and also thought that FM detection was the culprit, since it's also present between FM broadcast stations, but now that you mention it, I get it on the air band too, which uses the AM mode.
 

Ubbe

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AM mode always uses AGC, automatic gain control, to set the recievers gain so the signal isn't too weak and not too strong either so it clips off at the top. That AGC "idle" gain can be set by the circuit designer so it is on the weak side and you will hear very little noise or it can be set a bit higher and the noise starts to reach the levels of the FM mode.

To check your enviromental RF pollution you simply take the antenna off and compare to the receivers own noise level.

/Ubbe
 

GB46

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To check your enviromental RF pollution you simply take the antenna off and compare to the receivers own noise level.
I've tested that both on a VHF frequency and on WWV at 10 mHz. The noise level is about the same with the antenna either off or on, except that the sound of it is different as I toggle between NFM, WFM and AM, with more treble on AM. I figure that's due to the difference in bandwidth in each mode.
 

majoco

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Gerry said:
The noise level is about the same with the antenna either off or on
You're very lucky to live in a relatively noise-free area then! If I tune to Radio Greece on 9.420kHz AM with the bandwidth set to either 6kHz or 2.7kHz the "S" meter shows about 10dB over S9. However if I off-tune by 10kHz still on AM then the meter shows zero with the narrow bandwidth but S7 on the wider. Unfortunately I am plagued with a wideband noise from about 8.2 to 11.8MHz which raises the level by about 12dB and it's there for 24/7. It's not coming from my house as it's still there in a power cut - I suspect it's from a telephone sub-station about 50 yards away - which also radiates a large signal on 13.560MHz which is an ISM frequency around here so I can't do much about that.

Often the AGC doesn't develop much voltage on an SSB signal as there's no carrier which is why you often get a cleaner SSB audio by turning the RF gain down - it reduces the background and the up-and-down noise and the carrier insertion better matches the incoming signal - unfortunately a lot of radio's don't have RF (or IF) gain controls. Turning the AGC off makes matters worse as the radio tends to go to max gain.
 
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Ubbe

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You can't demodulate a SSB signal so you add your own carrier in the receiver to be able to do AM demodulation. The AGC controls the gain in the receiver at several places. Your gain control, and attenuator and preamp, are different controllers and are not enough to reduce the signal where it doesn't top out and hits the roof. If you also have IF gain controls then it might be possible to manually set a gain where it will produce an acceptable audio quality. But it is a very small window in the gain setting where it will work and usually HF signals fluctuate too much in signal strenght that makes it almost impossible to have time to adjust RF gain, IF gain and perhaps also volume level to match the different RF levels.

/Ubbe
 

GB46

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You're very lucky to live in a relatively noise-free area then!
I think you've misunderstood. I was mainly referring to my VHF/UHF scanner, which seems to have its own onboard noise source. An HF receiver behaves quite differently. If I disconnect the antenna from my R75, for instance, the bands are dead quiet. Reconnect it and up comes the noise. It's the same with my Sangean portable if used with an external antenna. If I disconnect that, the bands are only dead quiet when the whip is not extended. Extending the whip increases the noise, but not as much as an external antenna does.

The scanner, however, has very high background noise at all frequencies, with or without the antenna, unless a strong signal overrides the noise. That's obviously the reason for the squelch, but what I don't understand is why each of the four VHF/UHF scanners I've owned behaved the same way, even the Radio Shack Pro 2001 which I had back in the 80s. That was the one with 16 programmable channels, each with its own pushbutton to toggle it in and out of the scan cycle. The buttons were in a horizonal row with an LED above each button to show when the scanning passes that channel. This, of course, was a desktop scanner, because there had to be room for the keypad and the frequency display, as well.
pro2001.jpg

Unfortunately I am plagued with a wideband noise from about 8.2 to 11.8MHz which raises the level by about 12dB and it's there for 24/7. It's not coming from my house as it's still there in a power cut - I suspect it's from a telephone sub-station about 50 yards away - which also radiates a large signal on 13.560MHz which is an ISM frequency around here so I can't do much about that.
We're more or less in the same boat, although my noise sometime covers up the 40m ham band. From roughly 11 to 15 mHz there's a new noise source, which I can't identify. It's a rushing sound that seems to switch on and off at 1-second intervals. It's very strong, so probably in my vicinity. That ISM frequency is sometimes active here too, and in my case it may be from the shopping centre across the street, possibly from the point-of-sale terminals.

But to return to the topic, I was originally referring to VHF/UHF scanners rather than full-fledged communication receivers.
 

WB9YBM

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Here's something I've always wondered about: Sometimes I keep my scanner's squelch open to look for signals that may be too weak to open the squelch on their own. When no signals are present, however, the level of background hiss is so high in every VHF/UHF scanner I've ever used that I have to turn the volume down. Why is this? Is it caused by the circuitry in that type of receiver? I've also noticed this when the squelch is opened by a signal from a vehicle in motion. The hiss is very loud during fades.

It's never that high on shortwave receivers, even in an environment with lots of RFI.
There are a lot of noise sources out there--everything from washing machines with a "noisy" motor to ignition noise from a souped-up hot rod. The first thing I'd recommend is seeing if there's a connection between when the noise appears versus what's going on in the immediate vicinity.
 

GB46

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I know about RFI, but wasn't referring to external noises. This is noise I can hear on scanners even without the antenna. It's a constant rushing noise heard with the squelch open any time of day, and regardless of frequency, quite unlike what you're describing. Most radios have some circuit noise, of course, but it seems a lot louder on scanners. My present scanner is the Icom R6, but I've observed the same thing on other makes and models. If you have a portable scanner, try removing the whip or external antenna. Then go to a clear channel and open the squelch. Maybe you'll hear what I hear.
 

WB9YBM

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I know about RFI, but wasn't referring to external noises. This is noise I can hear on scanners even without the antenna. It's a constant rushing noise heard with the squelch open any time of day, and regardless of frequency, quite unlike what you're describing. Most radios have some circuit noise, of course, but it seems a lot louder on scanners. My present scanner is the Icom R6, but I've observed the same thing on other makes and models. If you have a portable scanner, try removing the whip or external antenna. Then go to a clear channel and open the squelch. Maybe you'll hear what I hear.
hmm...not sure I can add anything...sounds like a unique problem. Good luck!
 

nd5y

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I think the OP is talking about the normal white noise on an FM receiver with no signal is present and the squelch open.
The audio level of the noise on an FM receiver with no signal present is always at a high level.
It's a characteristic of how FM receivers work. That's why FM received audio level doesn't vary with signal strength the way it does on AM (or AM with a BFO for receiving SSB & CW). When a weak unmodulated carrier appears and gets stronger then the noise at the speaker will get lower. When the unmodulated carrier reaches a certain signal strength the there will theoretically be no noise out of the speaker. That's where the term "full quieting" comes from.
 
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GB46

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I think the OP is talking about the normal white noise on an FM receiver with no signal is present and the squelch open.
The audio level of the noise on an FM receiver with no signal present is always at a high level.
It's a characteristic of how FM receivers work. That's why FM received audio level doesn't vary with signal strength the way it does on AM (or AM with a BFO for receiving SSB & CW). When a weak unmodulated carrier appears and gets stronger then the noise at the speaker will get lower. When the unmodulated carrier reaches a certain signal strength the there will theoretically be no noise out of the speaker. That's where the term "full quieting" comes from.
Thanks, that's what I was referring to. I suspected circuit noise all along but wasn't sure, and as you explained, it's due to FM circuit design in particular. Since I always monitor with headphones, opening the squelch to see if any weak stations are not getting through can potentially damage my eardrums, since I use a comfortable volume setting for modulated signals, not for the background noise, which is much louder. The R6 has a button at the left side for temporarily toggling the squelch open. The first time I used that I got a blast that nearly made me jump out of my chair. :eek:
 

majoco

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The NZ amateur radio organisation publish a magazine every two months and in the current mag there was a very interesting article about how they discovered that the sun was a radiator of RF noise via the WW2 search radars. Unfortunately there is no 'on-line' version - I could scan the article, it's only 2 pages but they'd probably cancel my membership! Anyway, a pertinent paragraph goes like this...

Quote "It was with some surprise then, that ...the officer in charge of the RNZAF radar station...on Norfolk Island observed on 28th March 1945, a large increase in the noise showing on his radar screen whenever the radar aerial pointed in the direction of the sun. This noise, which was only noticeable during the half hours after sunrise and before sunset, and appeared whether the transmitter was on or off, had not previously been observed at Norfolk Island or anywhere else in New Zealand." unquote.

This was called "The Norfolk Island Effect" and is in this paper "F.E.S. Alexander. Report on the investigation of the Norfolk Island effect R.D. 1/518, Radio Development Laboratory, DSIR, 1945" but Googling doesn't find it. Radar at the time was on 200MHz. I have been up to the radar site on Norfolk Island and it's quite eerie - no equipment or sheds of course, but some very large blocks of concrete and some very heavy pieces of machinery. Not tropical at all, often up in the clouds and dripping wet! Dr.F. E (Elizabeth) S. Alexander later went on to develop radio astronomy and was the first female radio astronomer.

 
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GB46

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This reminds me of my first experiences with satellite TV. We had a 10-foot TV dish that the former owner of our house left behind with the property. The converter still worked, and we were able to use it without a subscription to a satellte TV service. The only signals we could pick up were from one of Canada's Anik satellites. Anyway, around the same time each morning reception would suddenly fade out completely, replaced by a screen full of white noise. What we learned was that the sun was in just the right position at that time to block the signal with its radiation.

By the way, since you mentioned 13.56 Mhz in your previous post, I think I've finally pinned down the source of the signal I receive on that same frequency. It's one of those RFID devices in use in one of the stores at the shopping centre across the street from us. I can hear a constant idle signal all day long, interrupted occasionally by a short burst of data, which I guess is when the device is reading one of those security tags attached to some merchandise. The interruptions are not occurring now, but it's currently at 9:10 PM local, and the store is likely closed for the night. The signal is not very strong, as it's a low power device with a limited range, so it doesn't interfere with anything on my end.
 

GB46

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Have you considered a DSP speaker, that all but eliminates the white noise?
No, but then I never use a speaker, as I listen exclusively through headphones or earbuds. My R75 has the DSP module, but its noise reduction feature doesn't accomplish much. It masks the noise somewhat, but reduces the signal's audio a bit at the same time. In extreme cases, turning up the noise reduction tends to distort the audio.

As for the scanner, I don't have to hear the noise at all if I avoid keeping the squelch open and let the signal open it instead. The only reason I brought that up was out of curiosity about high noise levels on VHF/UHF scanners, but I can see now that it's the nature of the beast.
 
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