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[TowerTalk] Noise and WANs - Tracking Interference

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Subject: [TowerTalk] Noise and WANs - Tracking Interference
From: Kevin Hemsley" < (Kevin Hemsley)
Date: Tue, 18 Sep 2001 12:02:11 -0600
Ford Peterson (N0FP) wrote:

> I was completely unable to detect any signals on my HP141T/8555/8552
> analyser.  Unless you put these animals on the bench and control the RF
> path, I doubt you would ever see the signals.  The unit is 50mW spread
> spectrum (both pseudo random and hopping according to the tech that was
> here).

You can watch a spread spectrum signal on a spectrum analyzer; but it does
take some practice, and you have to know what to watch for.  If there is not
much traffic, it can be difficult to spot, but not impossible.  For
monitoring 2.4GHz spread spectrum, It helps to have a 2.4 GHz dish.  Because
it is spread spectrum, the signal is very wide and you won't see a spike on
a center frequency like you are used to seeing with a frequency-modulated

> I could detect nothing on any 2m or 70cm or my HF gear.  This is hardly a
> good test fixture.  I am in a rural area located 3 miles from any
> civilization.  These broadband devices may work fine in a quiet
> but I wonder what kind of mixing products are produced with other signals
> present.

With a spread spectrum signal, there is not much audio to hear at all.  What
you can hear, sounds like static.

> They had been doing modifications to the network yesterday and at 6PM, the
> Hutchinson repeater was falsing about 1 time every 3 - 5 seconds for
> an hour.  The McLeod County Sheriffs department has been unable to use
> handheld radios because of some digital garbage taking out their channel.
> have heard it and it sounds like 9600 baud stuff.  It appears every 50kHz
> for about 1 MHz centered around 154,900.  Each burst lasts abou1 second
> it repeats every 1.5 to 3 seconds.  Very strange.

This sounds similar to an interference case I was recently called on.

A few months ago, I received a call from the superintendent of a school
district who told me that a tech from an area business radio company was in
his office telling him that he had to shut down his wireless spread spectrum
network used to link several schools in his district.  The radio tech told
him that the school district's wireless network had been causing severe
interference to a neighboring county sheriff department's repeater.  He
indicated that the repeater had been inoperable for several months because
of the problem and that they had just now tracked the source of the
interference.  I asked that he put the radio tech on the phone.  The tech
told me that a loud audio signal which sounded like digital signals was
capturing a 450MHz repeater located 60 miles away.  I told him that I
sincerely doubted that the school was the source of their interference.  He
insisted that his $12K spectrum analyzer had "pinpointed" the source of the
interference.  I told him that I would bring my $6K spectrum analyzer and
take a look. :-)  It was a 40-minute drive.  When I arrived he showed me his
analyzer, and a very strong signal on 450MHz.  The signal he showed me was
obviously not a spread spectrum signal, which I pointed out.  He insisted
that it was a harmonic of the 2.4GHz spread spectrum signal.  I disagreed
with him, and to prove my point, I had the district shut off the spread
spectrum radios. To the radio tech's surprise, the signals kept on

The radio tech and his companion left with perplexed looks.  I followed them
out, and he connected his spectrum analyzer to a 450MHz Yagi and
demonstrated that the signal had to be coming from the school because when
he turned the antenna around in a 360-degree circle, it was an evenly strong
signal in all directions.  I pointed out that he was standing 10 feet away
from the school wall that had steel siding.  I recommended that he obtain an
attenuator, and triangulate the source of the signals. (There were 2).  I
offered my step attenuator.  He found a fixed attenuator, and off they went.
On my way out of town, I decided to set up my spectrum analyzer and watch
the signals as I drove.  I had to switch in a good deal of attenuation, and
then started home.  As I passed the town water tower, the signal went off
the scale.  I smiled and drove behind the water tower, where I found a
450MHz Yagi pointing across town.  I followed the signal across town as I
was leaving, and just as I was entering the freeway, I found the other
signal source - a small building housing a data acquisition system.

I contacted the business radio company, and the local county and informed
them of the true interference source.  It turned out that the data
acquisition system was setup on the wrong licensed frequency, and it was
running 50 watts to send the signal a 1/2 mile across a small town.

Kevin Hemsley

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