BBC3 on 27.99625MHz in South London

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synerr
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BBC3 on 27.99625MHz in South London

Post by synerr » Thu Aug 18, 2016 8:02 pm

Hello,

Stumbled across this forum whilst looking if I can find any information as to why I can hear BBC3 on 27.99625MHz.

I'm in South London and was procrastinating playing with my new SDR dongle and Gqrx when I tuned into that frequency and heard classical music. Figured out it was a BBC channel because they read out some BBC news and trying out all BBC radio channels matched it to Radio 3 :|

Thanks and apologies if this is a stupid question. Been on this planet for quite a few decades but embarrassingly new to all things radio.

Cheers,

B

DJ Mikey
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Re: BBC3 on 27.99625MHz in South London

Post by DJ Mikey » Thu Aug 18, 2016 8:47 pm

Super Tropo due to weather conditions i'm receiving all sorts tonight mainly a lot off BBC stations miles outside their transmission zones prob from up north / west of england

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teckniqs
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Re: BBC3 on 27.99625MHz in South London

Post by teckniqs » Thu Aug 18, 2016 9:11 pm

My money's gonna say that they aren't on 27.99625 in South London at all and it's all down to your SDR's lack of filtering. Try it on something else like a decent scanner.

bristolpirates
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Re: BBC3 on 27.99625MHz in South London

Post by bristolpirates » Thu Aug 18, 2016 11:05 pm

Simple test - If you move the centre frequency of the display (known as the IF) up or down, does Radio 3 stay on 27.99625, or does it move somewhere else. The images you get with cheap SDR dongles tend to change frequency depending on the IF.

An FM notch filter will help get rid of unwanted FM images outside of the FM broadcast band.

You will also probably find the you get mirrored images within the FM broadcast band itself. If you have a strong signal, it will appear as a weaker signal on the opposite side of the display, exactly the same distance from the centre of the display.

Unless you spend a lot of money (a lot of money!) on an SDR, expect unwanted signals. Even an SDRPlay or Airspy will have image problems.

The SDR dongles are a cheap way to get into using SDRs, and can be very effective, but they need a few modifications depending on what you want to use them for.

Albert H
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Re: BBC3 on 27.99625MHz in South London

Post by Albert H » Fri Aug 19, 2016 12:49 am

I've recently been getting into SDRs. There are some boards that can be used as signal generators, and are capable of handling spread-spectrum and various complex modulation forms...

I now have a working 8-stereo channel DAB multiplex working on my workshop bench. I've used Raspberry Pis as the audio encoders (using bolt-on audio cards) and a knackered old desktop PC as the multiplexer. The SDR module I'm using is called an "RF-One" and comes from the 'States.

I have about 150mW of spread-spectrum multiplex on Band III on 5A. It's perfectly receivable on all my DAB radios in the house. Now I need to build a very big, very linear, broadband power amp!

It's just "proof of concept" at the moment, but a proper DAB pirate set-up won't be too far away. This will really piss off the authorities!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

synerr
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Re: BBC3 on 27.99625MHz in South London

Post by synerr » Fri Aug 19, 2016 2:49 pm

bristolpirates wrote:Simple test - If you move the centre frequency of the display (known as the IF) up or down, does Radio 3 stay on 27.99625, or does it move somewhere else. The images you get with cheap SDR dongles tend to change frequency depending on the IF.
I think you hit the nail on the head.
bristolpirates wrote:An FM notch filter will help get rid of unwanted FM images outside of the FM broadcast band.
I'm quite happy this happened, think I've found something to do with my life :-)

<snip>

bristolpirates wrote:The SDR dongles are a cheap way to get into using SDRs, and can be very effective, but they need a few modifications depending on what you want to use them for.
I paid £5.18 back in the days when the pound was still great at buying stuff.

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Re: BBC3 on 27.99625MHz in South London

Post by Gum » Fri Aug 19, 2016 11:47 pm

You need to turn down the RF gain of the dongle in your SDR software until those images disappear...

Never used Gqrx myself, but in SDR# it looks like this:

Image

bristolpirates
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Re: BBC3 on 27.99625MHz in South London

Post by bristolpirates » Sat Aug 20, 2016 12:53 am

Good point, yes it's essential to get the gain set at exactly the right level to give maximum sensitivity and minimum images. You will have to discover this by trial and error. Whether it will get rid of the images completely or not depends on aerial position, strength of the signals that are causing the images, etc...

I use my SDR with a tablet when traveling, and this week whilst I was in a hotel I found that hanging the aerial by the bedroom window gave me a huge amount of mixed images (where you get two stations on top of one another, and neither of them are supposed to be there). Moving the aerial to the bathroom window got rid of those images completely. If I picked the aerial up and moved it just a yard away from the bathroom window, the images came back. The bathroom window position was the only spot in the whole of the hotel room where the images didn't happen.

synerr
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Re: BBC3 on 27.99625MHz in South London

Post by synerr » Mon Aug 29, 2016 8:35 pm

Thanks again :-) Truly appreciated!

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