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Re: [RFI] Mystery 70 cm Repeater Input RFI/EMI?

To: <rfi@contesting.com>
Subject: Re: [RFI] Mystery 70 cm Repeater Input RFI/EMI?
From: "Morse, Earl (E.A.)" <emorse@ford.com>
Date: Fri, 12 Oct 2007 14:31:35 -0400
List-post: <mailto:rfi@contesting.com>
If it is bothering two other 440 machines in the immediate area I would
suspect that it is local to the site and that it is broadband.  It
sounds like something that has gone intermittent and is putting out a
spike train through at least the 440 band.  On-site DFing will most
likely be required.

Most likely difficult to get to and there are probably a couple of
shacks up there run by different radio services.  To be the least
intrusive to those services I would first try to DF the problem to just
one of the buildings then contact that buildings owner and see if they
will work with you.

These sites house all kinds of services, some used often some rarely
used.  Sometimes they notice malfunctions right away, but sometimes on
backup systems or rarely used remote links an issue could go unnoticed
for a long time.  

I wonder if it is bothering any of the public service UHF stuff?  Was it
the repeater transmitter turned off or the other service transmitter
that was turned off in #3 below?

A spectrum analyzer at the site with some Dfing antennas would be the
way to go.

Good luck.  

Hi all,

A local 440 machine here has been plagued with a "popcorn-like" noise on

the input with all but the strongest input signals.  It's got everyone 
baffled.  The clues so far:

1. The noise is less noticable on strong input signals, but still
(Recording posted at URL below.)

2. On weak input signals, the noise is horrific. (Recording posted at

3. The transmitter was turned off at the site, and the noise could
be heard on the input. (!)

4. The repeater is atop a 5500 ft mountain, along with LOTS of other 
services, and I believe a 6 megawatt radar installation is within 20 


 From what I understand, eveything on the repeater was gone over with a 
fine-toothed comb several times over several trips/months, and no 
results.  The trustee says that tapping the "T" connector coming from
duplexer made some _very_ slight difference in the level of the noise,
he didn't think that it could be responsible for the problem.  The
can disappear for days, then come back with a vengance.

I'm posting this for him, as he's swamped with work right now, and I
to take the problem up here, where all the expertise is. :)  He made one

recording, I made another, two files:

"Repeater noise 1" is a QSO that was made with a station transmitting in

the clear, with a beam, at good power, line of sight.

"Repeater worst case popcorn sound" is made by another station, using
power,  no modulation, just keyed down to keep the input open.  Both 
stations are fixed, so that's not mobile popping you'll hear.

The theory right now is that it's some wierd RFI from some new kind of 
radar, but it doesn't seem to have a "sweep" pattern.

As Groucho said,

"Any questions?"
"Any answers?"

Files are at:


Thanks to the group in advance,



Message: 2
Date: Thu, 11 Oct 2007 12:51:42 -0700
From: digital-conjurers@roadrunner.com
Subject: [RFI] Mystery 70 cm Repeater Input RFI/EMI Addendum
To: rfi@contesting.com
Message-ID: <>
Content-Type: text/plain; charset="us-ascii"; format=flowed

Just found out that _two_ other 440 machines on the same site are being 
plagued the same way.


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