I like to tune the duplexer with service monitor for best peak and notch
like you did then hook up repeater and check reflected power at transmitter
output as you did and compare foreward and reflected readings with duplexer
inline and out of line. If reflected power is more with duplexer inline
then most likely there is an impedance mismatch between the duplexer input
and the transmitter output. If the difference in reflected power is small
with duplexer in/out, I just tweak the first pass adjustment (tx off) for
minimum reflected and usually can get a good match equal to reading with
duplexer out of line. If the difference is great like you described then i
try a different jumper a little shorter or longer. Wacom had a procedure
for finding the right length jumper it was to add right angle conectors or
jumpers increasing in length an inch at a time (two inches at a time on two
meters) until you have added 1/2 wave total and the lowest reflected
reading was obtained then make a new jumper with the new optimized length.
I did this on my two meter repeater and it does work. Problem is when you
take out your meter things change a little even if you cut the jumper
allowing for length of the meter but should be close enough to work
properly.
You could have had a bad cable or it may just cut to a length that made the
transmitter mad.
If your tx output and duplexer input were 50 ohms then any length 50 ohm
cable would work. But in the real world if your service monitor is not
exactly 50 ohm and transmitter output and duplexer input is not exactly
50 ohm Changing length of the jumper will
improve preformance as you described because your jumper is acting like a
transformer.
Feel free to add to this or correct me if I am wrong.
tom n8ies
> [Original Message]
> From: Tim S. <[EMAIL PROTECTED]>
> To: <Repeater-Builder@yahoogroups.com>
> Date: 10/1/2004 7:38:08 PM
> Subject: [Repeater-Builder] Duplexer story update
>
>
> Thanks to all the help here and repeater-builder.com I got it figured out.
>
> Quite interesting actually.
>
> I verified the input to the duplexer had 42 watts by using a coupler on
the
> duplexer input cable and hooking it up to my service monitor.
>
> And hooked it all back up and I had 42 watts into the service monitor
> measuring the duplexer output.
>
> So 42 watts in and 22 watts out was -2.8 db loss.
>
> Next I checked the antenna reflected power with a bird. Bare with me here
> because the bird readings are vastly different then my recently calibrated
> monitor.
>
> The ant showed 38 out and 1/2 watt back with the bird. Pretty good.
>
> So I checked the input to the duplexer. 65 watts out 12 back. Not good.
(I
> guess) I say I guess here because changing the length of the duplexer
input
> cable seems to affect it.
>
> I decided to check the duplexer tuning once again and pulled the unit.
> Retuned the plungers all the way up and down and could not find a better
> peak. Reset the notches and reinstalled.
>
> Same kind of readings as before.
>
> Then I tried a new cable from the TX to the duplexer input that I had. It
> just happened to be about 3 inches longer than the original.
>
> That gave me on the bird 65 out and 1 back on the antenna.
>
> The service monitor showed 32.5 watts out of the duplexer. Reverified the
> 42w into the duplexer on the new cable.
>
> For a total insertion loss of -1.1 db.
>
> Also rechecked the densense and could not measure any.
>
> Now for a homework assignment. The original *bad* cable is 24 inches
long.
> And the TX freq is 483.0875Mhz. So what wavelength would that be that
> caused the problem. And I assume it is the original cable when the
repeater
> was installed 25+ years ago. Was it always wrong or did it not effect the
> original duplexer the same?...
>
> Thanks again!
>
> -Tim
>
>
>
>
>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
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