Mathew,
There may be more than one problem to consider. It is wise to check for
desense, but also be aware that some amateur-grade handhelds are
notorious for excessive PL deviation. Alincos are the worst, in my
opinion. My DJ-S11T had 1400 Hz of PL deviation out of the box, when
400-700 Hz is ideal. When a commercial-grade repeater (MICOR, GE, etc.)
receives an input from a user with excessive PL deviation, the user's
voice may over-deviate the carrier, causing the PL to be clipped. When
this happens on a PL-required repeater, the repeater shuts down on voice
peaks. Of course, this symptom is made much worse when the voice
deviation is too high, as well. One way to check this is to use a
commercial-grade handheld radio to check for the same symptoms.
To check for desense, you can use an "iso-tee" to inject a low-level
signal into the antenna feedline at the receiver frequency, while
monitoring the receiver audio at 12 dB SINAD with the repeater
disabled. Then enable the repeater so that the transmitter turns on.
The SINAD reading should drop no more than 1 dB. Some service monitors
will change modes when RF is detected, so you may want to use a separate
antenna, instead of the iso-tee, to get the test signal into the
receiver.
If your repeater is at a site with other transmitters, you may need to
add some bandpass-only (NOT pass/notch) cavities between the duplexer
and the receiver input. As has been noted many times on this list, a
pass/notch or "BpBr" duplexer has almost no bandpass selectivity, and a
nearby transmitter many MHz away can easily cause desense in your
receiver if not filtered out with a dedicated bandpass cavity.
73, Eric Lemmon WB6FLY
w9mwq wrote:
>
> I need a little advice here. I want to test my repeater's sensitivity
> through the duplexer, while the transmitter is keyed up, into my Cushman
> Service Monitor. What is the best method of doing this? Do I need an
> isolator of some sort, or will the service monitor handle both the incoming
> power and the outgoing signal
> generator? It's a Cushman 6030 by the way. The repeater is working
> excellent for the most part. The problem that I am having is on weak
> signals, the audio is being chopped out, almost like the PL deck is shutting
> down. On the bench, the receiver is at about .25 microvolts sensitivity at
> 12 dB Sinad. Audio out of the transmitter
> is about 4 kHz wide, and clean at 100 watts. Any suggestions? Handhelds are
> able to get in from about 4 miles away, mobiles have no troubles at 30 miles
> away. The antenna is only up at 60' right now, fed with 120' of 7/8" Andrews
> hardline. But it seems the handhelds are the ones having the problems, and
> their signal is not really that bad. Help please.
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