Mark,
The problem with exceeding the power rating of your
duplexer is complex. For starters, the duplexer cans are going to get hot,
and this will cause temperature cycling that may cause "pumping" of the tuning
rod. This will ultimately cause noise and eventual failure of the transmit
cavities. Let's say your duplexer has a 1.5 dB insertion loss, which is
more or less typical for a four-cavity BpBr duplexer. 1.5 dB loss equates
to about 29% of the signal, so about 44 watts will be dissipated in the
duplexer. That may not seem like much, but it quickly builds up. If
you have a six-cavity duplexer, with an insertion loss around 2.2 dB, the power
lost in the duplexer increases to about 60 watts. Of course, the power is
distributed among the two or three cans on the transmit side. The isolator
will suck up a small amount of power in the forward direction if it is properly
tuned.
If I were you, I'd be looking for a duplexer with a power
handling capacity at least 25% higher than your PA rating. Choose
wisely...
73, Eric Lemmon WB6FLY
From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of N9WYS
Sent: Wednesday, February 08, 2006 7:02 PM
To: [email protected]
Subject: RE: [Repeater-Builder] Duplexer questions
Hi, Eric.
Thanks for the insight... I plan on adding a voting
receiver system to the machine; in fact, I'm currently working on two of the
remotes as I write. (Well, sorta... hehe) I have a Motorola Spectra
TAC comparator and 4 receivers, but three of them are on VHF Lo right
now. Once converted with UHF receiver sections, they'll be added to
the system. So I'm planning on utilizing the wide-area talk-out coverage I
have now - and I'm coordinated for that level, so.....
:-)
My next question becomes - how bad can/will I hurt the
current cans if I continue operation at this level? Right now, the
repeater is not used a whole lot - but that could change as more and more people
become aware of its existence. And yes, the first isolator dummy is a 75W
unit.
Mark - N9WYS
From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Eric Lemmon
Sent: Wednesday, February 08, 2006 7:56 PM
To: [email protected]
Subject: RE: [Repeater-Builder] Duplexer questions
Mark,
Bandpass/Bandreject, also known by the short name "BpBr" is the same as a
Bandpass/Notch duplexer.
The names are somewhat misleading, because the bandpass effect is
relatively modest, although the notch is quite sharp. It is a good idea to
have a pure bandpass cavity or two between the duplexer and the receiver,
especially if you have a preamplifier.
Gee- do you really need 150 watts? If your duplexer is rated at 100
watts, why abuse it with more than its rating? Even with an isolator after
the PA, you are definitely looking at grief down the road- maybe in the next
block! With so much power, your repeater will likely "talk" much farther
than than it "hears." I'd suggest running the PA at 80 watts or so, and
make sure that the first load on the isolator is rated at 75 watts or
more.
Remember, the range of a repeater is *usually* limited by its ability to
hear the stations in the field, not by its output power.
73, Eric Lemmon WB6FLY
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