Brent,
Although there are those who will disagree with me on this, I think your
WP-639 duplexer is simply not capable of sufficiently isolating your receive
and transmit frequencies. While the WP-639 may "get by" on a low-powered
system without a preamp, you may be demanding too much in your installation.
Since the operating range of a repeater is limited more by its receive
capability than by its power output, it makes sense to maximize the receive
capability and run the minimum output power needed.
First of all, the phrase "factory tuned" needs to be qualified. I will
readily acknowledge that the majority of big-name duplexer manufacturers have
the equipment (network analyzer) and expertise (highly-trained technicians)
to properly tune a duplexer exactly to your stated operating frequencies.
Very often, a factory-tuned duplexer is accompanied by a rather pompous
statement of the form, "This duplexer has been carefully tuned on
laboratory-grade equipment, and NO FURTHER TUNING IS REQUIRED." Yeah,
right! Unless I drove to the plant and picked up that duplexer myself, I can
guarantee that it will be jostled, dropped, and bounced around during
shipment to the point that it MAY be detuned enough to adversely affect its
operation. I always check duplexers, isolators, and cavity filters on my own
laboratory-grade equipment before installation, and I find perhaps 30% of
them needing realignment. Please understand that the detuning most likely
occurred during shipment, and is not the fault of the manufacturer.
Whenever you put a preamp in front of a receiver, you really should put a
very narrow bandpass cavity filter in front of it. It always comes as a
surprise to neophyte repeater owners that a "bandpass/bandreject" or "BpBr"
duplexer has almost no bandpass action at all. The notch or reject action is
the major player, and the bandpass effect is minimal if nonexistent.
Moreover, nearby carriers can sail right through the duplexer and cause major
desense to your receiver. When you add a preamp, you are not just opening
the barn door wider, you are amplifying all of the intruders!
My personal preference is for two 8" bandpass cavities set for a total of 1.0
dB insertion loss after the receive side of the duplexer, and before the RF
preamplifier. Don't be tempted to use one 10" or one 8" cavity set for 1.0
dB instead of two set for 0.5 dB each; the out-of-band rejection by two
cavities is much greater than one cavity by itself. This is something I
learned by experience- you can believe me now, or go off and find out for
yourself. Trust me, you WILL settle on a minimum of two cavities.
In the present climate of terrorist activity and natural disasters, it does
not make any sense to deploy a repeater system that "barely gets by" or is
"good enough" for the majority of users. It makes me very proud to be a
member of an Amateur Radio group that designs and installs repeater systems
that are as good as, and in many cases are better and more reliable than,
commercial systems operated by public safety organizations. I sincerely wish
that all repeater operators felt the same way!
73, Eric Lemmon WB6FLY
Brent wrote:
> One of my 2m repeaters has a problem. It is a GE Mastr II base/repeater.
> I have a set of Wacom WP-639 on the machine, and an ARR preamp. ..a user
> 2 air miles from the site is wiped out of the
> receiver while the Tx is On, I turn the PA down below 20watts and he ( or
> all users) are clear. I have tried two different antennas before thinking
> about the duplexers, and want to double check other options before the
> cans.. these were factory tuned..cans.
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