Update on GB3DA Noise Issues (April 2013)

Danbury Church Green TX Site
Site used by GB3DA

As has been reported, the Danbury 2m repeater GB3DA has not been a well repeater for some weeks. We saw a flurry of messages today indicating that GB3DA had apparently been fixed and was now working. Monitoring the repeater today (Monday the 15th) did indeed indicate that the repeater was unusable, but from 19:30 local time, 2 second bursts of “noise” were appearing after each over.

We tried to run the Monday Night Net on GB3DA at 8pm, but the noise deteriorated such that by 8:40pm, we had to abandon it, and migrate the net to the 70cm repeater GB3ER.

As was announced by a representative of the ERG last month, the Essex Repeater Group is aware of the problem, and investigations are ongoing, not helped by the hoops that need to be jumped through to get access to the transmitter site that hosts GB3DA. See Status of GB3DA (April 2013)

Reasons for the noise

Thanks very much to Justin G0KSC for giving his take on the problems to the attendees on the Essex Ham Monday Night Net tonight, and for allowing us to reproduce his thoughts here.

Justin believes that the problems are with the cavities at the GB3DA site. Cavities are effectively notch filters used to prevent interference between the transmit/receive frequencies used on a repeater. (For a pic of the GB3DA cavities, see the Essex Repeater Group Gallery)

Justin outlined that, in his opinion, the following has happened:

“The transmit side of the cavity group has shifted somewhat, and in so doing. there’s now some of the transmitted signal that‘s getting into the receiver, and you have a ringround. So it’s keeping the squelch open, and that’s why there’s a kind of screeching effect that you’re getting on there.

You’ll probably noticed that the repeater was better today and that’s because of the change in temperature, which has altered internally the tube lengths within the cavities which had gone out of tune – that’s brought it more inline with what it should be. It does still need a tweak, but as we have more warmer weather, then the repeater will behave a lot better as it goes into its correct pass band”

Thanks to Justin G0KSC for giving a great explanation for the problems that we’ve been experiencing on GB3DA.

 

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4 Comments

  1. Avatar Peter G1FOA 16 April 2013 Reply
  2. Avatar Steve 16 April 2013 Reply
  3. Avatar Martyn 16 April 2013 Reply
  4. Avatar Murray G6JYB 17 April 2013 Reply

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