[NCARC] 145.115 shutdown

[Chris Kelly]

Hello NCARC:

I am Chris Kelly, K0PF, formerly WD5IBS.

This is in reply to Steve Henry's remarks about 145.115 being heard
on 160.620. I am not on this reflector's mailing list, so somebody
please send me signup info. I was president of NCARC in the early 80s
and on the tech committeee for many years. My remarks are intended to be
as technically useful as possible, and I don't know anything about 
present politics, so please forgive me if I step on somebody's toes.

Regarding intermod:

Virgil Leenerts W0INK is an ARRL Tech consultant who may be of great help
on this topic. Sorry I don't have his email address, but be sure to get
him in on this.

First, make sure the signal is not an image signal or intermod in 
the hearer's receiver, caused by either inadequate image rejection
in the receiver or by receiver intermod caused by strong signals 
near the receiver driving it into overload. Use other makes/models
of receivers to make sure the signal is actually detectable
in other receivers. (which means it is actually on the air, and not 
just in one receiver). Many scanners and even some commercial radios
have poor image rejection.

Second, the repeater as I last knew it (March 1987) on Horsetooth 
(which was on 146.820) had a 6-cavity duplexer and two 
bandpass cavity filters in addition.
I presume these are still used on Horsetooth because that is a very 
hostile RF environment (lots of transmitters). 
They cost the club over $1500 at the time to avoid this very possibility.

Description:
The function of the duplexer is to allow the transmitter and 
receiver to simultaneously use the same antenna.

The function of the two bandpass filters is to protect the receiver
and the transmitter (respectively) from any additional out-of-band signals.

These were Celwave large diameter very high Q cavities, first rate gear. 
We added the bandpass cavities to avoid the possibilities of intermod 
in our receiver or transmitter semiconductors.

Without the bandpass filter, the transmitter duplexer has only moderate
rejection of frequencies ABOVE the transmit frequency. (By 160 MHz 
it is only attenuated maybe 30dB).

Without the bandpass filter, the receiver has very little rejection
BELOW the receive frequency. 

The combination of the duplexer with the bandpass filters yielded 
over 80 dB of isolation for all frequencies outside the ham band.
If these cavities and duplexer are still what's being used, and they
are properly adjusted, the repeater itself should not be the
source of ANY measurable signal 15.5 MHz away from our transmit frequency.

The reason you need to protect the transmitter is that with limited
filtering ABOVE the transmit frequency (eg duplexer only) you can
cause intermod in your transmitter by mixing your desired transmit 
frequency with a strong signal from some other nearby transmitter
and generate undesired signals on a third frequency. With so many transmitters
on that hill, you may have many opportunities to mix various
signals (multiple simultaneously) and have many undesired signals.

Summary:
So my main point is to make sure the Celwave duplexer AND bandpass filters
are in use on the Horsetooth repeater, whatever it's current frequency.
If not, get that setup tuned and back on the horsetooth repeater.

Third: Intermod can occur in many places, including other services' 
transmitters. If our signal is mixing, the receiver should hear not 
only the 145.115, but also some other modulation. Try to see who that
other signal is. The may or may not be the culprit, but try to
identify who it is. 

Fourth: On a high-power site like Horsetooth, many things can mix signals,
including corroded connectors, lightning or corrosion damaged antennas,
etc. Make sure our antennas, connectors, feedlines, etc are in good condition.
This kind of mixing is weak, and usually not heard so far away as 
Loveland is from Horsetooth.

Fifth: There are intermod calculation programs that can identify
sources of two, three, etc sources of intermod. If Horsetooth has
a "site manager" who has a list of transmitters on the site, get this
list and try to find out who might be candidate signals that 145.115
may be mixing with. Make sure NCARC has a clean setup first before
getting third parties involved, however. 

I hope this info is of assistance.
73,
Chris Kelly, K0PF
wd5ibs at verinet.com