458.725 MHz

Post by **barf** » 04 Aug 2006, 18:55

Hello All,

Can anyone help me with this, is been bugging me for years -

On 458.725 MHz there is a two tone alarm, its on constantly, and receivable everywhere i have tried, Selby, Leeds, Doncaster, M1, M62, and i know now, here at work near Huddersfield.

I think its some form of telemetry, but there never seems to be any encoded data, i see nothing other than the two tone alarm on the analyser.

Not too odd, you may think, just an alarm, but heres the odd bit -

the tones vary, not with time, but with bearing and antenna polarity! So far i have identified 3 seperate tone pairs, and now from work here atop Emley moor i can here loads, but without my RDF equipment working i cant work out where these signals come from. Its like its a grid or web covering the country. And at UHF, its either very powerfull or lots of very local tx's.

So, what the 'eck is it? some form of navigation aid? I cant ID it, and ive been a radio engineer for over 7 years!

Someone please tell me what it is, or someone please RDF it and find a transmitter site!

please if you have any info email me direct

Martin

Post by **Sandhound** » 04 Aug 2006, 20:28

G'day.

TETRA. 458.725 MHz

Post by **barf** » 04 Aug 2006, 20:34

How'd you get that info?

TETRA is digital, no discrete carrier for an analogue receiver to receive. Also, I worked on the initial Airwave rollout, and have been trained on TETRA, theres no mention anywhere in my notes of such a signal, and ive certainly never heard it (or seen it on a test set) from a TETRA handset or mobile

I will of course check with my fellow engineers, in case this is some part of the system i am unaware of

Post by **Sandhound** » 04 Aug 2006, 21:48

barf wrote:How'd you get that info?

TETRA is digital,

Correct.

no discrete carrier for an analogue receiver to receive.

What platform is digital sent on as it uses RF and any receiver will receive digital data if it can tune to the frequency and mode that is being used to transmit it, but will not be able to decode it with out the correct software intervention.

Post by **barf** » 04 Aug 2006, 23:07

So, how does that account for a two-tone signal, with each tone around (estimate) 1/2sec long, and each transmission having a different pair of tones?
TETRA received on an analogue unit has the characteristics of harsh noise, as will any digital platform signal, whether TETRA, GSM, TDAB, etc

To have a two tone signal at the audio path in a NFM receiver requires that an analogue(sinusoidal) signal be the modulating input to the transmitter, and the modulation be analogue.

Also, as the 458MHz band is predominately telemetry, it is unlikely that TETRA could account for this. I see all sorts of bizarre effects from TETRA (i no longer work on the equipment, now in TV transmission) but never anything like this from it

Post by **Sandhound** » 11 Aug 2006, 19:12

barf wrote:So, how does that account for a two-tone signal, with each tone around (estimate) 1/2sec long, and each transmission having a different pair of tones?

G'day.

I would have thought it quite logical, why two-tone signal is used and there are multiple pairs being used or available for use.

TETRA received on an analogue unit has the characteristics of harsh noise,

Well yesish, but it looks nice displayed on a spectrum water-fall when filtered to it's band-width.

To have a two tone signal at the audio path in a NFM receiver requires that an analogue(sinusoidal) signal be the modulating input to the transmitter, and the modulation be analogue.

What's wrong with using digital NFM transmission

Post by **barf** » 12 Aug 2006, 10:55

Not sure what your getting at here - why should two-tone be logical? maybe 30years ago for paging, not used much nowadays, and certainly not as a continuous alarm.

Besides, as i think i mentioned earlier, i worked on TETRA during its rollout for the police, especially on cross-system repeaters. TETRA does not do anything of this sort. Also, no one is authorised to operate TETRA at this frequency.

To add further info (and complications) - i managed to get my RDF rigs running and tried to hunt this signal down. You cant, not easily. The best i could trace is in the region of a golf course near selby. This is not a single high power transmission, but seems to be a local signal emmitted in many areas, with a range of it seems half a mile or so. I have identified many locations with one of these signals present, bu no tracable path between any of them. Ive taken bearings using yagi antennas and with doppler RDF, and ive done a local field strength grid survey. These are seperate transmissions, but whatever is producing them is not easy to locate.

Try taking a long walk with your receiver and see if you pick it up, chances are your located in a 'dead' area where the signal s are weak.

Martin

Post by **Sandhound** » 12 Aug 2006, 21:01

barf wrote:Not sure what your getting at here - why should two-tone be logical? maybe 30years ago for paging, not used much nowadays, and certainly not as a continuous alarm.

G'day.

Logical or what ever you want.

The first tone 5 mSec wakes a pre selected receiver in the group, the second tone is the data being sent to it which will inc a tx tone and frequency change for re transmitting to another cell with in the group. Hence the multiple pairs.
The data is sent up to a max of 10Kbps and as low as DC.

The RF O/P is 5 - 500mW, (automatic varible to conditions). Modulation F3D. Bandwidth +/- 3.5KHz. Modulation rate DC - 10Kbps Frequency deviation +/- 3KHz

Post by **barf** » 13 Aug 2006, 16:49

I think we need to re-establish some details here -

this is a 2-tone constantly repeating alarm, it is always the same two tones from any indevidual transmitter, and the tones have a duration of half a second. I have analysed them and they contain NO other modulation. It is not data, it has a 'baud rate' of 2. It never changes from the same two tones. Any indevidual signal never changes in relative signal strength, and no amount of RDF tracks any of these signals to anywhere near either a TETRA base station (and i am well versed in these installations) nor to any possible TETRA mobile/portable terminals.

Theres also the simple fact that i have checked the TETRA site and user license database, and none of it matches. That and having also run the signal through a TETRA communications service monitor unit.

Have you managed to listen to this signal? Can i ask what your professional involvement with TETRA has been?

Martin

Post by **rattledrat** » 13 Aug 2006, 19:00

barf wrote:I think we need to re-establish some details here -

Like why is this discussion in the SCANNING RADIO FREQUENCIES Martin

this is a 2-tone constantly repeating alarm, it is always the same two tones from any indevidual transmitter, and the tones have a duration of half a second.
It never changes from the same two tones.

Well it wasn't the other day when I read you offerings. Aug 04, 2006 - 07:55pm you had So far identified 3 seperate tone pairs,

Any indevidual signal never changes in relative signal strength,

It must be a X### data TX network.

and no amount of RDF tracks any of these signals to anywhere near either a TETRA base station (and i am well versed in these installations) nor to any possible TETRA mobile/portable terminals.

I think sandhound is winding you up with TETRA

and his last message as I read it knows what the system is that you are banging on about.

Martin

Rattledrat.

Post by **barf** » 15 Aug 2006, 01:32

Hmmm, seems that no one on this site is prepared to provide any real substantiated qulatative data on this. Neither does it seem anyone who replies is prepared to discuss their professional level.

I shall take this elsewhere.

For the record, I AM a professional radio engineer, previously involved in public safety communications, specialising in secure data and telemetry, and in a previous professional capacity in signals intellegence.

Well, there we go, i will leave you all to listening to your taxis

Post by **rattledrat** » 15 Aug 2006, 21:09

barf wrote:Hmmm, seems that no one on this site is prepared to provide any real substantiated qulatative data on this. Neither does it seem anyone who replies is prepared to discuss their professional level.

My professional level is private and confidential of which I don't discuss on a forum..
The answer that you want is - the sigs are F3D, self scencing X--- system.
To display their data a Laptop computer is required with interface and correct software.

For the record, I AM a professional radio engineer,

Is that Plug & Play or do you repair and modifie transmitters

Post by **Andy** » 16 Aug 2006, 16:27

.
"Well, there we go, i will leave you all to listening to your taxis"

Blimey, Barf - don't slam the door on your way out! Just because no-one here knows - or wants to know - what a set of tones on UHF are, doesn't mean we deserve your contempt. The vast majority of 'scanning enthusiasts' are not particularly knowledgable, and many have only enough knowledge and skill to operate their clobber. They learn 'on the job'.
As they learn, they find more stuff to listen to, and a bleeping noise is not of great interest to anyone except the most curious.
For your information, here in East Manchester (tucked in on the west side of the Pennines), I can hear on 458.725 a *very* weak carrier with a once-per second pip. Sounds like boring water company's telemetry to me.

Post by **Guzzy** » 16 Aug 2006, 17:13

Technical boffins having a squable.

Never pretty, always too many sharp pencils and protractors being thrown about for my liking!

Now, where were we ... "Charlie to base, P.O.B."

Post by **sparkles** » 17 Aug 2006, 00:23

458.72500 NFM Jersey BGS Seismic Telemetry ?may be