[NCARC] 145.115 status report

[Steve Henry]

Hi folks-

George (AB0SF), Eric (K0EUS), and myself (N7GN) ran a few experiments and
setups at the repeater site on Sunday afternoon.

We added in the ability to control the frequency and timing of the CTCSS
encode tone from the 7K repeater controller.  However, yours truely (N7GN)
introduced a design flaw that Eric discovered -- having to do with how the
signals coming out of the receiver/transmitter unit were being interpreted.
We will go back up to the site in the near future with an improved interface
cable with a fix for this problem.

We also added better shielding and torroids on the interface cables, swapped
in the backup Vertex repeater, boosted its output power by 2x, and
programmed the transmitter to put out a fixed 67 Hz tone.

As an aside, I'm not able to pick that up on my Kenwood TS-2000 so it's
possible that there is no tone coming out when transmitting.  I haven't
tried putting my scope on the received audio either to see if the tone is
there.   At any rate there is no 100 Hz encode signal we are generating.
The 100 Hz tone needed to key the transmitter will need to come from
somewhere else if that is part of the problem.

Can you help us by collecting some data?

1)  Is the intermod interference now the same, more often, or less often
than what it has been over the last 6 months?

Our prediction is that it is less often because the CTCSS encode signal
isn't making its way back into the input thereby preventing re-triggering of
the repeater during the transmitter tail.  This was the initial theory
proposed by K0OJ and Virgil Leenerts, W0INK.  So far this seems promising
given the limited data gathered so far.

2)  If you do hear intermod, would you please also quickly tune in and see
if the following transmitters are operating?

	160.263 (Great Western Railroad)
	152.690 (Bob Rule's communications pager)

Why these two?   Intermod typically involves 2 signal components being mixed
via some non-linear amplifier/diode.  These can occur at 2f1 + f2 and 2f1 -
f2.    2 x 152.690 MHz - 160.263 MHz would be the intermod components mixed
in Bob Rule's output tank circuit.  This would be at approximately 145.115
Hz.  Bob Rule's pager antenna beams directly into our antenna from about 200
feet away.  We already know this pager lacks good output filtering and that
our own repeater frequency gets mixed with his output and interferes with
the Great Western Railroad.  It's possible now that the pager is being used
a lot more and that we need to get with Bob to get a filter put on his
output transmitter.   Since good output filtering costs over $1000, we would
like to make sure of our data before moving forward with proposing this
solution to him.  (Bob's very easy to work with and is quite  accomodating
and is likely to go with our recommendation-- probably having us install
this for him.... but again this is a working theory and we want to have data
to support it).

3)  If the 145.115 isn't being used very much and you have the time, could
you try turning off the 100 Hz PL decoder and then see if you hear the
intermod interference?  Simply code in 0* to turn off the PL decoder.
Please make sure to ID with your callsign when you are doing this so that
you stay legal, by the way!  Also note that every 30 minutes the 7K repeater
controller is programmed to turn the 100 Hz tone back on if there has been
no activity.  You may need to shut the PL decoder back off if running these
experiments.  Of course, when you are done, please code in 1* to turn the PL
back on (and identify with your callsign!)

If you do hear the intermod interference, please check these other two
signals (152.690 and 160.263) for activity.

Please send data/ your findings to me directly at n7gn at arrl.net and I'll
publish a summary periodically.


Thanks for your help--

'73

-Steve, N7GN