Hi Brian,
I am not sure if this has been said already as I haven't been back
through the thread however it seems that you may have a couple of
issues here.
Firslty your new equipment has significantly more OP and better RX
sensitivy. This will certainly test the isolation afforded by the
duplexer.
Secondly your new TX may be noisier off freq than your original TX
and so require more isolation at the RX freq. Given the increase in
TX power this is almost certainly the case.
Thirdly
It would be interesting to work out if the desense is being caused by
(i)fundamental overload of the RX
(ii) TX sideband noise falling in the RX passband
(iii) Other noise being generated by the TX operation, which falls
in the RX passband
The first 2 can generally be solved by improving the duplexer
through additional cans/tuning etc. The third is trickier and
really needs to be addressed at the source. For example if you have
a noisy antenna (say water ingress has caused corrosion) filtering
in the duplexer will not fix this problem. The same can be said for
noise coming from other sources, eg connectors, metal on metal
moving contacts in the TX field etc.
A test you can do to isolate the problem is to terminate the OP of
the duplexer in a 50ohm load. If desense goes away the problem may
be noise from the antenna or feeder system or that the
antenna/feeder system is presenting a high VSWR that has the effect
of reducing isolation in the duplexer. If the desense persists the
problem is probably in the duplexer, or lack of isolation between
the TX and RX directly (ie leakage between them).
This is a rough test but if you patch around one of the TX cavities
(ie remove it from cct) and the problem doesn't change then it could
be the RX cavities aren't rejecting TX carrier well enough. If you
patch around one of the RX cavities and the problem doesn't change
then it could be the TX cavities not rejecting the TX noise in the
RX band well enough. If in either case the desense does get worse
these tests aren't really valid. This test should be done with a
50hm termination on the antenna port.
Be careful with all of the bits and pieces you test with. I was
testing a 2m repeater recently and could not get rid of a desense
problem. It turned out the 50ohm termination and some of the RF
adaptors I had were noisey. Once I removed these everything worked
well.
With respect to your flaky TX power problem, that cavity you
identified as the cause may also be generating noise which could be
causing your desense issues. If possible pull it apart and look for
corrosion, loose parts etc. Also check all the cabling and make
sure there is no chance a connector is loose (ie not terminated
properly) or a cable damaged. Shake the cavity and look at the
transmission loss and VSWR - see if anything changes.
Personally I would start by looking for something generating noise
in the system rather than suspecting the cavity performance. Look
hard at that crook TX cavity.
Hope this helps. I have found these problems tricky and the best
way to solve them is to logically eliminate as much as you can. The
trial and error method can be very frustrating...
Regards
Brett (VK2CBD)
--- In Repeater-Builder@yahoogroups.com, Brian <[EMAIL PROTECTED]> wrote:
> Ok, today we went out on the hill and did much testing. We ended
up
> running the new repeater on 2 antennas as this seems to work and
we
> are going to see how it goes for a bit.
>
> Several things were learned while testing things.
>
> 1. Our old machine was only putting out about 4 watts before the
> duplexers. We thought 10 to 12 watts.
>
>
> when we hooked up the new machine into the line with duplexers and
> all we noticed an odd thing.
>
> 2. The new machine took a couple of seconds to come to full power
> through the duplexers.....key up needle on meter moved to about 15
> watts kind of slowly and sporadiclly then boom suddenly jumped to
> full full power. We ended up through much swapping things around
> isolating this to one of the duplexer cans on the transmit side.
> Without that one can it keyed up instantly.
>
>
> Has anybody seen this and I assume it means that the can is bad or
faulty....
>
> This problem only showed up on the new repeater as the old one
didn't
> put out enough power to seem to cause this problem. Anyway I
really
> appreciate this list everyone here has been great.
>
> Brian
> --
> [EMAIL PROTECTED]
> Amateur Radio Callsign: KC0DWX
> WARN (Weather Amateur Radio Network) member
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
