Tetra Reception

[G8OEO]

I’m lucky to have a light rail Tetra system in my area of NE England which is not encrypted and my AR5700D does a good job receiving the main repeater output with the mode set to Tetra-TC.
I still haven’t definitely found any repeater input frequencies on which the trains themselves are transmitting and I am not sure which mode to select on the receiver while searching/recording.
Would their transmission into the system be classed as Tetra-DM or would it be the same as the repeater output?

 

[racingfan360]

I don't believe either modes available on the AOR can decode the uplink (repeater input) frequencies. I've tried my DV10 with both modes, and while I didn' expect the Direct Mode (terminal to terminal) to work, neither would decode the uplink in the end. I doubt you're missing anything by just monitoring the downlink frequencies.

 

[G8OEO]

Thanks for that, I can generally hear the stations but the trains are rarely on talkthrough.
Listening and searching on FM I haven't heard anything at all on the frequencies I think are in use for uplinks but will keep trying.
As far as I can work out I would expect the uplink to be 14.5 MHz below the output frequency, with the 70cm Amateur band in between.

 

[G7HID]

Thanks for that, I can generally hear the stations but the trains are rarely on talkthrough.
Listening and searching on FM I haven't heard anything at all on the frequencies I think are in use for uplinks but will keep trying.
As far as I can work out I would expect the uplink to be 14.5 MHz below the output frequency, with the 70cm Amateur band in between.

Try 6.5MHz above.....

 

[IK2GNP]

Does the AR5700D automatically detected those Tetra transmissions? Or did you set it manually?

 

[Ubbe]

Tetra input frequency of the basestation are always 10MHz lower in EU.
It's usually impossible to even see the carrier of a mobile transmission as their output power are regulated from what their input signal level are from a basestation. If the signal are strong then the mobile might transmit with 100mW.

/Ubbe

 

[G8OEO]

Try 6.5MHz above.....

I’ll give that a try thanks

Does the AR5700D automatically detected those Tetra transmissions? Or did you set it manually?

The 5700 doesn’t seem to recognise Tetra transmissions when set to DALL. With the mode set to Tetra-TC and the slot set to Auto it works fine.

Tetra input frequency of the basestation are always 10MHz lower in EU.
It's usually impossible to even see the carrier of a mobile transmission as their output power are regulated from what their input signal level are from a basestation. If the signal are strong then the mobile might transmit with 100mW.

/Ubbe

I’m monitoring 440.150 MHz, in which case the input frequency would fall just inside the 430-440 Amateur band so it’s either somewhere a little below 430 or higher than the output as @G7HID suggests. If the mobiles are all intelligently transmitting with the minimum power required, in the same way as mobile phones operate, it would explain why I haven’t even had a sniff of their signals so far.

 

[Boas]

Hello G8OEO,

so that I understand correctly. You set a specific frequency range and let the AR 5700D scan. In T-TC mode and which grid do you set, 12.5 kHz or 25 kHz?

Thanks and 73
Boas

 

[G8OEO]

Hello G8OEO,

so that I understand correctly. You set a specific frequency range and let the AR 5700D scan. In T-TC mode and which grid do you set, 12.5 kHz or 25 kHz?

Thanks and 73
Boas

Yes that should work ok and check Tetra Slot is set to Auto under Options.
I set the Step to 12.5kHz because I have found some encrypted channels appear to be using frequencies that would suggest this spacing. I would have thought Tetra required channels spaced at 25kHz or more but I suppose it’s possible they may be using a 12.5kHz offset for some reason.
Where I live I know for sure there is only one operator using unencrypted Tetra and have access to only a single channel at a reasonable signal strength. So it didn’t take much searching to find them.

 

[Boas]

Thanks G8OEO!!!

 

[Ubbe]

I’m monitoring 440.150 MHz, in which case the input frequency would fall just inside the 430-440 Amateur band

That's a very odd frequency as you cannot order a standard system using that frequency as the output from a basestation. You will have to order external duplexfilters and combiners from a 3:rd party. Looking in the WTR database I find 440.150 and 440.650 as 25KHz and 100W so that are probably Tetra and that license are paired with 425.650 and 426.000, which is -14.5MHz as you stated.

/Ubbe

 

[G8OEO]

Ok I have an additional scanner looking for activity on several possible channels now and will report back if anything is found.
@Ubbe do you think it would be worth subscribing to the WTR database for this sort of business DMR and Tetra stuff In the UK?

 

[Ubbe]

Go to Steve Holloway's facebook page and he has programs that import data from the WTR database and presents it in formats to be loaded into different scanners and you can select to only include frequencies in your own area. It seems to give the same result, or even better, than a subscription. You can probably use that data in csv format and import in Excel to be used as a lookup database.

If you are going to try and scan for tetra signalling it probably will not work using a standard noise squelch, as the signal are of a high energy that will be detected as noise. You will probably need to instead use a RF level squelch that most Icoms have, or a SDR dongle and a level detect frequency search as can be found in SDR#.

/Ubbe

 

[oe5kbo]

I’m lucky to have a light rail Tetra system in my area of NE England which is not encrypted and my AR5700D does a good job receiving the main repeater output with the mode set to Tetra-TC.
I still haven’t definitely found any repeater input frequencies on which the trains themselves are transmitting and I am not sure which mode to select on the receiver while searching/recording.
Would their transmission into the system be classed as Tetra-DM or would it be the same as the repeater output?

Hi i am oe5kbo karl from austria.

I have a question. Did you catch a uplink Tetra Signal. Its very fast only 14 milliseconds. I find out that it is possible but i dont know if a scanner is better or a SDR Stick.

Let me know if you have a Tetra Signal in your scanner on the monitor and maybe i can see a screenshot from this Signal

Waiting for your answer.

 

[G8OEO]

Hi Karl I haven’t been able to find any Tetra uplink signals at all. I think they are too weak to be received in my home location and I’m still not sure about the frequencies in use.

 

[Ubbe]

Hi Karl I haven’t been able to find any Tetra uplink signals at all. I think they are too weak to be received in my home location and I’m still not sure about the frequencies in use.

The license says it's using that -14,5MHz split. But be aware of that tetra mobiles are adjusting their output signal in sync with the signal strength they receive. The higher the RX signal the less TX output they will use. They can go down to 10mW if their RX signal are strong enough. I have the same situation that I never see anything on the input frequency from the closest tower to me.

On our different tetra systems they have their towers very close to each other, almost like cellular towers are located, so I doubt that they ever transmit at full power, that usually are 10W, and the step below that are probably 1W and then 100mW and finally 10mW. You'll need to have a clear view to the mobile vehicles antenna to be able to receive anything.

/Ubbe

 

[G8OEO]

Thanks @Ubbe I think I was wasting my time trying to hear the mobile transmissions. I’m not even sure the receiver would be able to handle them in the unlikely event they might occasionally be within range of my home.

I live slightly out of the city, not far from Newcastle Airport, and often wish I had an antenna on higher ground and closer to the city centre. I should probably try to get hold of a suitable UHF Yagi and point it in that direction to see if it can pull in a few more interesting DMR users.

 

[Ubbe]

If I have a tetra radio at my bench and a scanner like a UBC125 or BCD436 with its antenna off I can hear those transmission blips clearly.
But a scanner like Unidens SDS or Whistlers TRX do not have its squelch working quick enough to catch those transmissions and can only be heard when squelch are set to 0 and fully open. A spectrum program like SDR# and a SDR dongle will show those blips if you are within range.

It looks as if Newcastle airport uses an old MPT-1327 system but will probably soon go with IP radios that cannot be monitored.

Newcastle upgrades air traffic service

Newcastle International Airport has installed MEP's Voice Switch TCS990 digital switch within its air traffic service

www.airportsinternational.com

/Ubbe

 

[G8OEO]

Are the Tetra blips simply a brief handshake between mobile and base to keep track of their whereabouts etc in a similar manner to mobile phones? I’ll have a listen for them with open squelch.

Much of Newcastle Airport did use an MPT-1327 system for operations staff comms but this changed over to DMR a few years ago.

I wasn’t familiar with VCCS at all, hopefully these developments are purely for the ATC side of things.

 

[Ubbe]

There's 4 timeslots in tetra signalling and a mobile usually transmits in one, so there's a short 15ms blip each 50mS or so. Tetra mobiles tends to send a lot of single blips and more so if they have a GPS. Our commuter buses that use tetra have a vehicle computer that sends a lot of data over tetra, so much that they cannot use the control channels and are instead on specific data channels spread out over the timeslots in a site. So I would easily see and hear those transmissions if I where close enough to a bus.

/Ubbe