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Author Topic: Intermitant Rasp at Net Control  (Read 1977 times)
K7KCS
Member

Posts: 27




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« on: May 17, 2005, 07:38:27 PM »

Greetings,

Our Fire district has a Ham Volunteer group with a 2 meter FM rig at the station EOC.  Recently we are experiencing an unknown intermittent interference.  It is a buzzing sound and its duration and occurrence seem to be random.  The on period is around 10 seconds and the repeat cycle around a minute.  We can only hear it when we are receiving a signal, it will not break the squelch but it is strong enough to make us ask for a repeat.  When we drop the squelch we cannot hear the signal, it seems buried in the background hiss.  A mobile parked in the parking lot can hear the same signal, again only when someone is talking.

There is a lot of equipment in the fire station . . . .any ideas where we should start and how we can detect what is causing it?

73�s  Tracy K7KCS
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WB6BYU
Member

Posts: 17795




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« Reply #1 on: May 18, 2005, 01:34:00 PM »

Look for a battery charger, such as those for cordless
drills, radios, flashlights, portable defibrillators, etc.
The on/off rates may vary depending on the state of the
battery connected to it.

I would guess that if you open the squelch, you may hear
the background noise go quite when the interfering signal
comes on (that is, it may be a dead carrier).  But when
there also is a signal being received, the two carriers
can beat against each other and make an audible noise.

Because of the way that most FM squelch circuits are
designed, it is quite possible to have a very strong
signal that doesn't break the squelch.
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