UHF Digital (DMR) voice repeater

Technical

Channel	DVU59
Transmit Frequency	439.7375MHz
Receive Frequency	430.7375MHz
Antenna	Stacked dipoles
ERP	14dBW (25W)
Access method	Colour code 3
Base Station	Motorola DR3000
Coverage	Click here to view predicted coverage (60km). Plot provided by the ETCC.

On the 12th September 2015, the first DMR (Digital Mobile Radio) repeater operated by the TVRG became operational. This is a UHF repeater with the callsign GB7BK and is co-sited with the VHF analogue repeater GB3RD on a site about 11 miles WNW of Reading town centre.

The philosophy with DMR repeaters is that they are part of a network linked by the internet, in this case the DV Scotland Phoenix network. The Phoenix network was connected to the world-wide MMR MARC network. MARC stands for Motorola Amateur Radio Club,who originated the concept. The subject of DMR is quite involved and information can be gleaned from anumber of sources.

A basic overview:-

A DMR transmission occupies a 12.5kHz bandwidth channel but can carry two simultaneous separate transmissions, thus being spectrum efficient. The transmissions uses two timeslots to achieve this. The DV Scotland Phoenix repeaters are all connected to the internet and this allows connection to the various talkgroups. Talkgroups are effectively virtual places where users can go to and talk to other users on the same talkgroup.

GB7BK offers access to following talkgroups on the two timeslots. These are the same for most Phoenix repeaters but a few have additional local geographical talkgroups. For a complete list and latest updates please see DV Scotland Phoenix

The following table shows the talkgroups that can be accessed from GB7BK. Those which are shown in red are User Activated (UA) talkgroups. Those shown in black are not UA and are sometimes referred to as “Always On”. This means that if there is a signal on one of these talkgroups, the audio will be relayed through GB7BK., bearing in mind you can only hear one talkgroup on a timeslot at any one time.

In contrast you will not hear audio from any of the UA talkgroups unless the particular talkgroup has been activated. To activate a UA talkgroup, you simply need to transmit on that talkgroup. Then, as soon as there is a break in the transmission of the UA talkgroup any subsequent audio will then be heard. The concept of UA talkgroups can be confusing and I hope the description above has been of some help.

Please always bear in mind that when you use the “calling channel” talkgroups such as TG1, TG2, TG13 and TG235, you will be activating a large number of repeaters when you probably only need to use two in most cases. That is why you should move to a UA talkgroup once you have established contact.

If you want to see what is happening in real time, then go to the DV Scotland Phoenix dashboard here

Timeslot	Talkgroup	Description
1	TG1	Worldwide. Use as a calling channel then use one of the user activated (UA) talkgroups (TG119 or TG129) to continue a contact.
1	TG119	UA worldwide – use either of these to continue a contact.
1	TG129	UA worldwide – use either of these to continue a contact.
1	TG13	Worldwide English speaking. Use as a calling channel then use one of the UA talkgroups, (TG113 or TG123) to continue a contact.
1	TG113	UA English speaking worldwide – use either of these to continue a contact.
1	TG123	UA English speaking worldwide – use either of these to continue a contact.
1	TG2	Europe. Use as a calling channel then use one of the UA talkgroups (TG119, TG129, TG113, TG123) as appropriate.
1	TG235	UK wide. Use as a calling channel then use one of the UA talkgroups (TG80, TG81, TG82, TG83 or TG84) to continue a contact.
1	TG80	UK wide UA working channels. Use to continue a UK contact.
1	TG81	UK wide UA working channels. Use to continue a UK contact.
1	TG82	UK wide UA working channels. Use to continue a UK contact.
1	TG83	UK wide UA working channels. Use to continue a UK contact.
1	TG84	UK wide UA working channels. Use to continue a UK contact.
1	TG9	Local talkgroup (secondary). Use if TG9 Slot 2 is in use.
1	TG2351	UA talkgroup, which is linked to the CQ-UK talkroom used by the Fusion system.
2	TG801	UA Regional: South East England
2	TG810	Regional South West England
2	TG820	UA Regional: North West England
2	TG830	UA Regional: Midlands
2	TG840	UA Regional: East of England
2	TG850	UA Regional: Scotland
2	TG860	UA Regional: North East England
2	TG870	UA Regional: Wales and the Marches
2	TG880	UA Regional: Northern Ireland
2	TG9	Local talkgroup (primary)
2	TG9990	Echo server (To check your audio)

How GB7BK came about:-

IIn late 2013 the first DMR repeater in the south of the U.K. became operational, which was GB7NS at Caterham in Surrey. This could be worked from parts of Reading and the surrounding area. Some amateurs obtained DMR radios and the usage of DMR increased considerably throughout 2014. The number of DMR repeaters increased dramatically especially in the Kent, Essex and London area.

TVRG considered the possibility of putting on air a DMR repeater but decided to wait and see how the system evolved. At a meeting in January 2015 it was decided to put an application to OFCOM for a DMR repeater (GB7RU) located at a new site in Tilehurst, west Reading. This was to be co-sited with a new analogue 70cm repeater, GB3RU. In April we received the go ahead for GB3RU but learnt that GB7RU had be rejected because of the frequency requested. We then suggested other frequencies but these had to all be in the same DVU frequency band. Eventually we were told that the Primary User (MOD) would not permit any local frequencies to be used in the allocation used for U.K. DMR repeaters. This meant we had to look for a new site.

By far the most difficult part of putting any repeater on air is to find a suitable site at an affordable price. We also had to consider that the proposed site should be sufficiently far away from MOD sites otherwise the frequency would be rejected again. This ruled out all of the town of Reading and its suburbs. Any site to the East of Reading would overlap too much with GB7WL and GB7NS. We therefore considered the existing GB3RD site on the Berkshire Downs. This wasn’t ideal as it wouldn’t cover Reading that well and we wouldn’t be able to have a high antenna because the GB3RD antenna is mounted on a pole not a mast. There could be also problems with internet access from such a remote site, as proved to be the case.

However, we decided to go-ahead and put in the application to OFCOM. We changed the callsign from GB7RU (Reading UHF) to GB7BK (Berkshire) as this was more sensible for the site which is 11 miles away from Reading.

Approval was received and the repeater became operational in September 2015. Coverage was OK and more or less in line with predicted coverage map . We obtained internet access using a 3G wireless router but this proved to be erratic and unsatisfactory. Subsequently, we have been given permission to use the site owner’s broadband which has improved matters considerably. However, it must be remembered that the site is fairly remote and outages occur from time to time.