Tim,
I have a two meter repeater that outputs 250 watts (approximately +54
dBm) into the duplexer. After the 93 dB of rejection that the duplexer
provides for shear power reduction toward the receiver port, -39 dBm of
transmitter power leaves the duplexer receiver port - headed toward the
receiver. The repeater is a Motorola MICOR VHF equipped with a MICOR
preamp. Bench sensitivity is -125 dBm for 12 dB SINAD. The duplexer is
a stock Wacom WP-641.
This repeater normally uses a tube type power amplifier to achieve the
250 watt level. Using the tube power amp, no desense occurs on the
repeater. I also have a the stock 110 watt MICOR power amplifier (solid
state) that can be switched in. The repeater has a slight amount of
desense when running the solid state PA. Why? It's obviously not the
shear amount of power, as it has been reduced over 3 dB. The culprit is
/transmitter side-band noise/. The solid-state power amplifier provides
no additional protection of the amount of transmitter side-band noise
being produced by the exciter, in fact, being wide-band in nature, the
solid-state unit amplifies the noise. The tube power amplifier,
however, is a High Q (very narrow band - tuned to the exact transmit
frequency) device only providing power gain on the 'desired' repeater
output frequency, and reducing (or at least not amplifying) the
transmitter side-band noise appearing on the receiver frequency.
From your explanation below, I don't believe shear power is ruining
your ability to duplex, but rater the amount of transmitter side-band
noise reaching the receiver. To prove this, install a notch cavity in
the transmit leg tuned to the receiver frequency.
If your duplexer isn't capable of allowing the present radio equipment
to properly duplex, either the duplexer needs replaced, or you can
change the type of transmitter to something else - no, I'm not
suggesting tube-type.
General Electric built a VHF exciter that has 22 dB less transmitter
side band noise (600 kHz from its primary carrier) than most multiplier
counterparts (like your Motorola MICOR). Using it is like installing a
22 dB notch filter between the exciter (transmitter) and receiver where
side-band noise is concerned. This exciter is the MASTR II PLL.
To comment on the folks privately emailing you telling you a MICOR
receiver cannot handle an off frequency carrier of -53 - they are full
of it - period. The answer is HOW CLEAN the adjacent frequency signal
is. If the adjacent carrier is full of noise, and that noise falls on
the frequency the receiver is listening to - then no the receiver can't
handle it, but that's a totally different thing ruining your lunch -
comparing apples to oranges.
Hope this helps...
Kevin Custer
Hi guys,
Took some time away from the project... loosing too many hairs.
Ok, I've gotten another receiver strip, and it has the specified
sensitivity per Mot.
Guess I want to bounce some ideas around with those of you
who are more learned in the arts!
With 250mW (+24dBm) into the transmit port... antenna port is
a quality 50 ohm load, I see -72dBM at the receiver port. (pretty
much what I expect.. 1.8dB loss through the xmit side, & 100dB
notch through the RX side.
With it all hooked up receiving an input signal of about 0.7uV,
application of the 250mw to the transmitter side will cause noise
in the receiver, although not much.
So, it appears that if I add an additional 30dB notch (another 'can'),
the problem at high power may go away.
If I compare what 50 watts with an additional can would be (-130dB)
to what I get with 250mW & current notch (100dB), then it looks like I just
need to add an additional can.
50 w = +47dBm - 130dB future notch = -83dBm at receiver.
250mw = +24 - 100dB current notch = -76dBm
With the current notch @ 50 watts, I see a receive signal of -53dBm.
Some have said that the Micor can handle that, while others (off-line)
have said no way.
This setup appears to support the opinion that -53 is still way too much.
Any comments and thoughts would be most appreciated!
Thanks,
Tim
