Rolling noise on 2m

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Rolling noise on 2m

Postby cprobertson1 » Tue Mar 13, 2018 11:17 am

Good Morning folks!

TL;DR - rolling noise - any ideas?

I've not been scanning for a while - but got back into ham radio recently and decided to pick up my Uniden 3500XLT and have a bit of a scan of the 2m band (specifically 144.000-148.000 inclusive).

Being in the house, I was expecting noise - but I was getting a clicking noise (tchp...tp-tp-tchp...repeating) on a lot of different frequencies - so I shrugged my shoulders and said "meh, must just be EMI" and thought nothing of it.

And then things started getting annoying: I tried to lock out the frequencies, but noticed they were shifting downwards - urgh! So I gave up for a bit - figuring I'd go for a walk once the weather cleared up.

That brings us to today, when I went for a 3-mile walk - and discovered the noise stayed with me! (I'll try to get a recording of it so you can have a listen in).

So - the symptoms:
-Rolling noise across 2m band - drifting down in frequency
-Strong signal (always opens squelch, even at full)
-Dissapears if the antenna is disconnected (external source?)
-Doesn't stay long enough on a frequency for the "Hold" function to be effective
-So far been unable to get a hold of the signal on my FT-991 rig (may be due to antenna placement) - but that's not to say it's not there, I've just not tried very hard! (I'll be doing that tonight!)

How would I go about ascertaining if it's an external radio signal - or if there is something wrong with my scanner?
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Re: Rolling noise on 2m

Postby Chris P » Tue Mar 13, 2018 11:53 am

posting your location may help others in your area to check if they can hear anything . Also by connecting a good dummy load to the antenna may eliminate front end self oscillation as if the noise still occurs with a dummy load this could be the reason for the noise (a dummy load should not pick up external signals if it suitably screened but will appear to the front end as an antenna )

I can often hear a repetitive clicking on certain frequencies in the am vhf airband which I discovered to be a wx satellite causing interference on an image frequency as it passed overhead although these signals did not drift across the band

PS as an aside did you have a mobile phone or similar device ( I have a camera that emits odd signals at UHF )with you when you went walking (ie a possible mobile signal source close to the scanner )
Regards Chris aka G8FFF nipper or tazmin88
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Re: Rolling noise on 2m

Postby cprobertson1 » Tue Mar 13, 2018 12:23 pm

Chris P wrote:posting your location may help others in your area to check if they can hear anything .


Whoops! Actually meant to do that :P - I'm in Irvine, North Ayrshire, Scotland - my walk took me eastwards towards the GSK plant. I've not checked the opposite direction yet though.


Chris P wrote:Also by connecting a good dummy load to the antenna may eliminate front end self oscillation


Ah-ha! Now that is a good call! I shall give that a go tonight - I'll need to make up a patch lead but that won't take long.

Chris P wrote:PS as an aside did you have a mobile phone or similar device ( I have a camera that emits odd signals at UHF )with you when you went walking (ie a possible mobile signal source close to the scanner )


Now, that is an interesting one. I had my iPod in my pocket, and my phone on a holder in my hip - but now that you mention it, they've always been within 10m of me. That will definately be something to check - in fact, I will take a walk at 1230 without my excess electronics, and see if I can still hear it... see if it's been my phone this whole time!
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Re: Rolling noise on 2m

Postby cprobertson1 » Tue Mar 13, 2018 2:10 pm

Eek! Ran out of time to edit the previous post while I was editing it - guess I'll have to double-post instead :embarrased:

Hmm! I was scanning from 1230-1250 - and I could not for the life of me find the noise again!

Although I might be in a dead spot (certainly not impossible) - but this might be a clue!

The noise, so far, has only been encountered when it's been late in the evening or early in the morning - which makes me wonder if what I'm hearing might be the LED street lights - of which there is one right outside my house, and plenty along the walk I took (which could explain why I heard the noise across the full walk!)

I shall take a longer walk around 1600ish and see if the signal remains - and then check periodically across the evening to see if it changes as night falls (I shall also try a dummy load as you suggested ;))

Thanks for the advice so far, much appreciated!
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Re: Rolling noise on 2m

Postby m0lsx » Tue Mar 13, 2018 3:24 pm

Are there over head power lines along the route of your walk? Someone running some nasty cheap over the powerline technology could explain why it's over a wide area.
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Re: Rolling noise on 2m

Postby cprobertson1 » Tue Mar 13, 2018 4:26 pm

m0lsx wrote:Are there over head power lines along the route of your walk? Someone running some nasty cheap over the powerline technology could explain why it's over a wide area.


Afraid not - nearest overhead line is ... let's see... about five miles away as the crow flies. Would underground lines be able to radiate EMF, or does the ground attenuate it enough that it's not an issue?

There is also a bunch of industry to the southeast - but my experience with scanning in that area has been either wideband noise, or small, sharp peaks and maybe a few harmonics - I've not encountered any drifting noise like this before I'm afraid!

Of course, that's not to say some industrial unit is not the culprit - just that I've not heard them doing it yet ;)
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Re: Rolling noise on 2m

Postby m0lsx » Tue Mar 13, 2018 6:20 pm

cprobertson1 wrote:
m0lsx wrote:Are there over head power lines along the route of your walk? Someone running some nasty cheap over the powerline technology could explain why it's over a wide area.


Afraid not - nearest overhead line is ... let's see... about five miles away as the crow flies. Would underground lines be able to radiate EMF, or does the ground attenuate it enough that it's not an issue?


Normally if it's underground lines, then it peaks & fades around inspection covers etc, as the soil does attenuate signals.
If the signal was not there at lunch time, that suggests, either it's a signal that was around your person on that trip or it was something that was further afield & simply not active at that time. So something scanner generated can be discounted.
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Re: Rolling noise on 2m

Postby cprobertson1 » Wed Mar 14, 2018 9:48 am

m0lsx wrote:Normally if it's underground lines, then it peaks & fades around inspection covers etc, as the soil does attenuate signals.
If the signal was not there at lunch time, that suggests, either it's a signal that was around your person on that trip or it was something that was further afield & simply not active at that time. So something scanner generated can be discounted.


I've now pretty much ruled out the scanner itself - managed to see the signal on my FT991's waterfall display; though I didn't have the presence of mind to take a picture of it!

There seemed to be a number of broad, ill-defined signals perhaps 5-10kHz in width - spaced around 300kHz apart.

I also appear to have been mistaken in thinking the signals were only drifting down in frequency - it would appear they drift down for a 100-200 kHz and then reverse direction; there may have been an overall tendency to drift in one direction, but I wasn't paying enough attention to notice I'm afraid. The rate of drift seems to be fairly constant, but I may be mistaken.

I also set up BCTools+ and logged the frequencies I was seeing, but I wasn't picking up much of anything (in fact, I was barely even picking up Scottish Information on 125.730) - I'll need to get that installed on my laptop at some point so I can get the scanner into a better position!

Speaking of installation, the http://www.bctool.org website seems down - anybody know what's up with that? Hopefully I can still get the files on github, but failing that I'll have to use something else for programming/scanning/logging :P

I'll also need to get a recording of the signal - I'll try and get on that tonight in fact. I'm also visiting Glasgow, so I'll take it on the train and see if I can still hear my noise source over the noise from the train itself - and if so, maybe get an idea of its range!

You know something...?

There is a large outdoor 30ft display in the middle of Irvine, near the townside exit to the mall... and the last time I saw it, it had great green bars across it. I wonder if something is up with that... something in that region definitely opens the squelch in my radio when I'm walking by it - I wonder if it had a much larger EMI range than I had previously suspected... I guess I'll walk towards it tonight and see!

That would be a good case-use for a directional antenna actually - I've been looking for an excuse to build one ;)

Right! Note-to-self:
-If noise near 30ft display is similar to observed noise: check if noise strength varies with distance
-If able to hear noise on train: note distance
-Find/install BCTools on Laptop
-Build 2M Yagi
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Re: Rolling noise on 2m

Postby cprobertson1 » Fri Mar 16, 2018 12:52 pm

I rather wish you could edit posts past the 'x' minute mark! Decades on the internet have conditioned me to fear and loathe double posting :biggrin:

First up, I went out yesterday evening and made anote of where I could receive the signals (if you can't see the map let me know and I'll see what I can do!

I've also managed to exclude the 30ft displays as a possibility - it's easy to pick up for a good 50+ meters; but it is a very distinct noise totally dissimilar to what I'm hearing.

Speaking of which, I've still to make a recording of that, jeez - I'm sure taking my sweet time!

I've also discovered the noise is worse when you have earphones in - but it's still there without it it - probably just creating a loop - though it might also deliberately be using the earphones as an antenna (I am unsure! There's nothing in the menu options about it as far as I can see!)

Anyhoo - I'll update over the weekend - I'll hopefully get a directional receiving antenna up in the garden, and from there I can use an SDR to get an idea of just how widespread the signal actually is!
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Re: Rolling noise on 2m

Postby m0lsx » Fri Mar 16, 2018 2:34 pm

cprobertson1 wrote: I've also discovered the noise is worse when you have earphones in - but it's still there without it it - probably just creating a loop - though it might also deliberately be using the earphones as an antenna (I am unsure! There's nothing in the menu options about it as far as I can see!)


The earphones cable is possibly acting as an antenna, or more likely as a counterpoise. . And I bet the earphone cable is around 1 meter long? So a nice half wave.
So in a way they could have just confirmed you belief that it's on 2 meters. As one possibility is that it's a strong out of band signal that your scanner is picking up & mixing with internal signals to place on 2 meters.
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