[NCARC] Repeater update and a couple requests for help/info

[George Salzmann]

All,

Well, I finally got around to troubleshooting the .115 machine last
night. Here's the scoop so far...

The amplifier is now working again. One of the power control
feed-through capacitors was cracked and shorting against the chassis. I
replaced it with a junk box equivalent and all is good. HOWEVER ...

The exciter is DOA. I haven't gotten too far into it, but I am seeing
about a milliwatt of signal on the spectrum analyzer, which leads me to
believe that the Motorola mystery transistor, M9859 (IF YOU HAVE A
SUPPLY OF THESE, PLEASE LET ME KNOW.) has given up the ghost.

Now, for the intermod. problem that we've been experiencing on both VHF
machines. The 145.115 and 146.625 repeaters are running what I call,
wide open. They're final amplifiers are not buffered from the outside
world, so whatever junk comes down the antenna and feedline, essentially
mixes with the final output stage of the amp. and spits out a whole
bunch of junk. It's more complicated than this, but there is a simple
solution.

Enter the isolator. Often the bane of both the Amateur Radio and paging
worlds because of its "power-robbing properties," for those of you who
do no know, the isolator is a three-port (connector), ferromagnetic
device (round magnet, essentially) that allows RF energy to flow in one
direction and prohibit it from flowing in another. To prevent the energy
from just spinning around and going where we don't want it, the third
port, which is located beneath the in and out ports, has a load attached
to it. The load absorbs the junk coming in to the amplifier, while
allowing the transmitted energy to be radiated. As an unintended
benefit, the isolator will also absorb the RF energy from the amplifier
if something happens to the antenna. One just has to make sure that the
load connected to it will handle the power the amplifier can provide.
Otherwise, things can get real interesting.

A single port isolator, such as I have described, will buffer the
repeater by about 35 dB. A dual-port isolator will theoretically double
this number, but I have yet to see paper match reality. The number
appears to be closer to 50 dB, which is still more than sufficient,
especially at the 100 watt power level we're operating at.

Now, supposedly, an unwanted property of isolators is the creation of
odd-harmonics. This is where my knowledge ends and hopefully someone
else's begins. I have been playing with commercial radio equipment in
various forms for almost a dozen years, and have yet to see significant
3H energy being created by the device when attached to a 50 Ohm load. If
someone can provide me with an explanation of why isolators allegedly
create 3H energy, it would be greatly appreciated.

Back to the intermod. situation. The dynamics of the repeater site has
changed in recent months. There are a number of UHF channels lit up on
the tower we are located on, as well as a culmination of all of the junk
spewing from the microwave radios, and other RF junk, not to mention the
broadcast TV energy floating around. The thought of the calculations
required to figure out where the VHF and UHF noise is coming from just
makes my head hurt (doesn't take much any more.). 

In the commercial world, the FCC requires repeater operators to run a
isolator with a filter designed to knock out second and third harmonics.
The VHF machines we own do not have these devices connected to it. I
think the UHF machine does have a isolator that is integral to the
repeater. Basically what this means to me is if I find an intermod.
source, whether it's a dirty transmitter, leaky cable, whatever, a savy
technician on the other end of the phone should ask the question, "Well,
what type of isolator are you running?" My response would more than
likely cause him to hang up the phone with a much less impressive image
of Ham Radio Operators in his head than before I called him. Bottom line
is this:

      If we want radio peace and quiet on our VHF repeater, we are going
to have to spend the money and buy an isolator/low-pass filter
arrangement. I'm still investigating prices, but a used panel can be
built for about $300 or so. New intermod. panels, tend to sell for
somewhere in the $800-$1200 range. I don't like to spend money just as
much as the next ham, but this is something that needs to be addressed
before we can start chasing down the problem.

Happy Holidays

George, AB0SF