[CoARES-D10] Info on the 145.115

[Steve Henry]

Howdy-

Thanks very much to all who checked into our "repeater interference net"
Some of the findings:

1.	The frequency where the 145.115 repeater output signal is occuring on
appears to be 160.263 MHz  Some hams could pick up the audio most clearly at
160.265 MHz rather than at 160.260 (Great Western Railroad repeater output
and simplex frequency).
2.	The broadcast signal appeared strongest in the N Loveland and West Fort
Collins area.
3.	Different radios seem to have different capabilities to pick up the
interference.
4.	Directional antennas pointed out that the signal was strongest in the
direction of the horsetooth repeater site.
5.	The 100 Hz tone being on or off didn't make a difference in the
interference.


By the way, the GWR repeater is located about a mile South of the Treasure
Mountain site.

Next steps:

1.	George wants to put an isolator and a low pass filter on the output of
the 145.115 repeater.  This is "best practice" at repeater installations to
avoid intermodulation interference.   Evidently lots of transmitted power
from different sources can be picked up by the transmit antenna and coax and
fed back into the output power amplifer whereupon they can mix with the
145.115 output signal and get rebroadcast at mixed frequencies.

Here's a good diagram of what a repeater setup should look like.  We're
lacking the isolator and filters on the transmit side.  See
http://www.anglelinear.com/repeaters/repeaters.html

By putting in the isolator and low pass filter much of this type of intermod
mixing can be prevented.  I found one reference that explains how an
isolator works at  http://www.repeater-builder.com/rbtip/circulator.html
>From several emails I've received, it appears that the 145.115 signal has
historically been broadcast at a number of different output frequencies.
Hopefully use of an isolator followed by a filter will help address a broad
potential class of problems.

2.	If this doesn't solve the issue, then the next step is to do a more
in-depth "fox hunt" at the repeater site.

3.	N7VDR has given us permission to keep the repeater up and will contact me
directly if and when we need to shut it down.  Please go ahead and use it
but at any moment we may need to shut it down due to interference problems.

4.	A couple of good references on intermod are at:
http://www.arrl.org/tis/info/intermod/intermod.html

For our situation, here's a simple calculation for some frequencies of
interest at 2f1 - f2 and 2f2 - f1 where the result is a frequency of 160.263
MHz.

In this case, the likely culprits for 3rd order intermodulation components
would be at:

2 * 145.115 - fx1 = 160.263   fx1 = 129.967 MHz

or

2 * fx2 - 145.115 = 160.263   fx2 = 152.689 MHz

I don't know if either of these are frequencies prevalent up at the repeater
site or not.

Sincerely and '73

Steve Henry, N7GN
NCARC president