Amateur Television Repeater GB3TM - by John Lawrence GW3JGA
In 1993 a group of local Radio Amateurs interested in Amateur Television transmission met to discuss the building of an ATV Repeater for North Wales. They were Brian GW4KAZ, Barry GW8FEY, David GW8PBX and me, John GW3JGA.
We met initially and then continued to meet, each Wednesday lunchtime, in the Electronic Engineering Department in Dean Street, Bangor, where Barry and I were on the staff. We discussed the technical design, allocated tasks between ourselves and set target dates for completion.
Brian concentrated on site work and aerial installation, Barry on building the driver/modulator, David became the Repeater Keeper and developed the technical specification and I built the logic control and rack system We also had help from John GW3MEO. Very sadly, Barry died in February 2010 after a long illness
ATV repeaters are different from audio repeaters in that they transmit continuously, in beacon mode when not repeating. The transmit frequency allocated to GB3TM is 1316 MHz and the receive frequency is 1249 MHz, a separation of 67 MHz.
In Beacon Mode a series of captions are displayed for identification and test purposes. There are two news pages. These can be updated remotely by keying an input audio tone. An audio morse code identification is also provided on the outgoing signal
GB3TM comes on air
The repeater and its aerial were completed, installed on site and GB3TM came on air in July 1994. The station has proved remarkably reliable and has been operating continuously since that date. We have replaced the cooling fan and a couple of components during that time.
GB3TM is located on the North East coast of Anglesey at SH 471 906 which is almost opposite the old Anglesey Radio site. GB3TM started life in a metal cabinet, outdoors and was much affected by the weather. It also made servicing difficult and virtually impossible in the rain. At present it is located in a stone built building which it shares with some commercial radio link equipment.
GB3TM utilises two omni-direction vertically polarised aerials, one for TX and one for RX. They are positioned to minimise cross coupling and filters are fitted to both the transmitter and the receiver to virtually eliminate any coupling between them.
The first 15 Years
For the first 15 years of operation the output transmission was frequency modulated with video. The accompanying sound was carried on a 6 MHz f.m. sub-carrier. This was most convenient because many of the analogue Sat. receivers of the day could receive and decode the transmission with only minor modifications. At times, up to 12 ATV amateurs were regularly using GB3TM. Under good propagation conditions ATV signals could be sent to and from stations in EI, GI, GD and G. For a short period a receiver on 3cms 10.340 GHz was provide for experimental purposes. A 2m receiver carrying the ATV chat channel on 144.750 MHz was connected to the alternate audio channel to provide talk-back.
Digital ATV Transmission
In 2009 it was decided to build and install a digital ATV transmitter. Funding was obtained from the Arfon Repeater Group and encoder and modulator boards were obtained from a German University ATV Group and work started on a Digital ATV transmitter.
The transmitter was designed and built by members of the ‘technical group’ comprising Brian ‘4KAZ, John ‘3JGA and David ’8PBX they are seen here in Fig.2, alongside the GB3TM equipment rack. The Digital ATV transmitter was completed by the end of 2009 and is shown in Fig.1. It generates a digital ATV signal on 1316 MHz having a Symbol Rate of 4.167 MS/s and includes stereo audio. New digital test facilities were also installed on site.
Fig. 2. Fig. 1
GB3TM digital came on air with DATV early in 2010. Initially, both transmitters for f.m. and digital were available to transmit on 1316 MHz and were selected by a DTMF tone on the audio input.
By March 2012, as the analogue transmitter was in little use, it was decided to close it down and retain a single DATV transmitter on 1316 MHz. The analogue receiver on 1249 MHz remained in use. This is the situation today, January 2013.
Work is under way to provide a reduced bandwidth (2MHz) digital ATV input to GB3TM on 70cms, 437 MHz. The receiver will incorporate an up-converter so that a digital Sat-Receiver can be used. Two local ATV amateurs are preparing to use this new channel when it becomes active.