About three weeks ago I sent round an email saying that GB3RD would be taken off air to have the logic modified and be given an overall “heath check”.
This has now been done and the repeater returned to air on Friday 12th February.
The reason for this work was that for a period of time after the New Year RD was being held open and emitting a nasty raspy noise. This was annoying and needed investigation.
Firstly, we took a test set to the site and checked the repeater there. Everything - power output, frequency, squelch setting and receiver sensitivity were exactly as it should be. Then the repeater was removed and tested on the bench. Again everything was OK and no changes were made.
This means we are convinced that the interference is external and there is no problem with the repeater equipment. Finding the source is almost impossible as the interference is not regular and it cannot be predicted when it will appear. This interference is not new, its been heard at times over the years RD has been operational from its present site. Recently, its been on for much longer periods. Having said that, since the repeater came back a week or so ago, all has been well (apart from one possible occasion). In case it returns we have modified the logic to minimise the effect. This has meant adding a timeout so that when the interference is present the repeater will not be held open all the time.
The following is an explanation of how the timeout works, please take time to read it as it’s not quite the same as other repeaters.
Firstly, if you use the correct CTCSS tone (118.8Hz) then you will never time out.
If you only can access using a 1750Hz tone burst, then you will have 2 minutes before the repeater times out. The timer is only reset when it hears either 1750Hz or 118.8Hz CTCSS. The timer is NOT reset when you drop carrier. Why is this?
When the interfering signal is present its strength varies and sometimes it briefly drops below the repeater squelch, thus RD gives a “K” (having had a break in carrier) and then the interfering immediately comes back – if we then allowed the repeater to reset the timer, the interference would still hold the repeater open.
So, if two stations work one another and both use CTCSS, then the repeater will not time out.
If one of them uses CTCSS and the other doesn’t, the latter will time out after 2 minutes unless he finishes his over or sends a 1750Hz tone burst.
The worst case is two stations, neither of which is using CTCSS, working one another. Then the repeater will shut down after two minutes unless it hears a 1750Hz tone.
So, if you can, please use CTCSS tone on transmit. A couple of things you may not know:-
a) If you use CTCSS then at the end of your over you will get a morse “T” (dah), whereas if you don’t use CTCSS you will get a “K”(dah-di-dah).
b) If you enable CTCSS decode on your rig, then you can monitor GB3RD and not hear the morse idents. You will only hear the repeater when it has been accessed, which can make for a quieter life.
So, that’s what we’ve done, lets see what happens. If the interference returns we’ll see how annoying it is with the timeout. The next step would be to tighten the squelch slightly and finally if all else fails we will make RD CTCSS access only. As I previously said, we are reluctant to do that, so let’s hope this isn’t necessary.
That’s gone on longer than I intended but I hope it’s helpful.
Whilst sending this email, I’d like to finish with a quick request. The Thames Valley Repeater Group (TVRG) operates 3 voice repeaters (GB3RD, GB3BN, GB3AW) and the APRS digipeater (MB7UJ). These cost money and we’d like to encourage users to make a voluntary contribution. I’ve looked back through the finances and if I go back to the beginning of 2008, then in the past two years we have only had 12 donations from 10 individual amateurs.
So please have a think – when did YOU last make a donation? It may be longer than you think. We’d like to think the users feel the repeaters are a useful service and would like to assist in their upkeep.
If you would like to send a donation, then please make a cheque payable to “TVRG” and send it to:-
TVRG
18 Wincroft Road
Caversham
Reading
RG4 7HH.
Thanks in anticipation!
Chris, G4CCC – o/b/o TVRG.