[NCARC] 145.115 intermod problem

[Steve Henry]

Hi folks-

As probably most of you know, we're still plagued with intermittent intermod
interference on the 145.115. With the best data we had available, the NCARC
a few months back approved funds to put up a new antenna and jumper cables
as well as to eliminate any potential sources of this problem on the tower.
The other intent of the new antenna was to improve our performance under
windy conditions. This antenna was installed on September 11th.

Since that time, there has been a decrease in the input sensitivity of the
repeater which may or may not be related to the new antenna, and there has
been no difference in the intermod.  I've sought guidance from Virgil
Leenerts, W0INK (ARRL recommended technical specialist) to guide some of the
experiments.  Also George, AB0SF our technical committee officer has had a
lot of good suggestions as well.

This last Sunday Eric Slutz (W0EAS) and I went to the repeater site and
tried out the first of perhaps several experiments to investigate and remove
this noise.   We swapped out a polyphaser lightning protection unit so we
could examine the old one, made sure cables were tight, and improved the
grounding on the 7K to repeater interface box.   Since the intermod usually
happens in the transmitter tail and seems to have 100 Hz tone encode
associated with it,  we also migrated the CTCSS encoder functionality into
the Vertex repeater rather than having this out in the more exposed control
box.   This was to eliminate this device being part of the noise source
since it could be contributing the 100 Hz signal.

Unfortunately, these changes have made little difference in the intermod
problem, nor very much difference in the receiver sensitivity (e.g. hoping
to eliminate a front-end overload problem).

The next experiment will be made this weekend to eliminate the interface box
and replace it with well-shielded cabling between the 7K repeater and the
repeater, and to migrate the CTCSS decoder function into the repeater.  This
will result in probably the best isolation and prevention of intermod that
we know how to do at this time.  The other experiment will be to swap in the
backup 145.115 Vertex repeater to see if the cause of the repeater input
sensitivity loss is due to a problem in the front end of the existing
repeater.  That will allow us to do some bench testing of the current
145.115 repeater also.  We will be taking the repeater down Sunday afternoon
(Oct 1) to make these changes.

If these changes don't make the needed improvements, we will know that the
interference is likely outside of our equipment.  We will need to do some
detailed and time-consuming data gathering to find the issue and will seek
additional guidance from both Virgil and of course George, AB0SF.  Based on
that study we may identify a likely problem transmitter and work with the
owner to make changes, or we may need to install a crystal filter on the
receive input, or may want to try a couple of  bandpass cavities on the RX
side as a less costly option.

'73

Steve, N7GN
NCARC President