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

To: <rfi@contesting.com>, <digital-conjurers@roadrunner.com>
Subject: Re: [RFI] Mystery 70 cm Repeater Input RFI/EMI?
From: "Jim P" <jvpoll@dallas.net>
Date: Sun, 14 Oct 2007 13:13:50 -0700
List-post: <mailto:rfi@contesting.com>
APS-145; observable using a receiver set for AM (the mode, not
'the band') while tuning the 420 - 450 MHz band (a band which
we share with military users.)

I performed an experiment (several years back now) one afternoon
that utilized two AM-mode capable 440 MHz receivers each with
its own Yagi antenna pointing in different directions; I was able to
note that the 'popcorn' noise moved from from one receiver to the
other as the 'platform' flew whatever pattern it was flying

For years several of us in this area (north central Texas) have
noticed the same kind of random, popcorn noise, and had
always chalked it up to intermittant power-line pops and

Occasionally the mode would change and a wideband
whining sound could be heard that would really tear up a
number of UHF repeaters simultaneously in the area, but
most operators thought only *there* machine was being

I might also mention that that 'sound' recording is
exactly the kind of popping that we heard on weak


Jim P // WB5WPA //

----- Original Message -----
From: <digital-conjurers@roadrunner.com>
To: <rfi@contesting.com>
Sent: Thursday, October 11, 2007 12:46 PM
Subject: [RFI] Mystery 70 cm Repeater Input RFI/EMI?

> 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 present.
> (Recording posted at URL below.)
> 2. On weak input signals, the noise is horrific. (Recording posted at URL
> below.)
> 3. The transmitter was turned off at the site, and the noise could _still_
> 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
> line-of-sight-miles.
>  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 the
> duplexer made some _very_ slight difference in the level of the noise, but
> he didn't think that it could be responsible for the problem.  The problem
> 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:
> www.moonlightsys.com/misc/
> Thanks to the group in advance,
> Lin/KJ6EF
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