One way to find out where it comes from is by using a horn antenna
because it is very directional. If you place the horn antenna on a turn
table and connected it to a receiver. You should be able to find out
which direction does the noise come from by turning the table. If the
noise is identical from all directions then it must be the cables
connecting to your receiver.
Regards
Vi
----------
From: Scott Douglas [SMTP:[email protected]]
Sent: 26 March 1998 18:15
To: [email protected]
Subject: EMI Site Interference
Hello Group -
Here's a problem for you all to think about and hopefully help
solve.
I have a room in the corner of my building where I perform EMI
pre-scans.
Occasionally throughout the day I have a major interference show
up on my
analyzer. It does not come from the EUT. This is broadband, from
30 MHz to
1 GHz, at levels up to 70 or 80 dBuv. It is not like a flat
carrier but
rather more like PWM driven motor noise. It lasts from 15
seconds to 1
minute in duration. Then ambient returns to normal.
We are a typical ITE manufacture. Our Genrad tester, wave
solder, air
compressors, motor generators, air conditioners, plumbing waste
pumps do
not appear to be the cause although none have been absolutely
ruled out.
The frequency and duration would seem to eliminate them however.
We have swamp on one side, the interstate highway on another,
fields on
the third and the town on the last side. The only potential
source I can
find is a medical clinic about 350 meters away from us. They are
the
nearest building (the next being twice that far) and have an
X-ray
machine. Yesterday I went there at lunch and asked was the X-ray
in use
that day and they said yes but could not tell me a specific time
that it
was used.
A software engineer here tells me that he has experienced radio
problems
in the parking lot outside my corner of the building. When he
parks in a
certain spot, his FM radio (the entire band) gets wiped out by a
zhhzhhzhhh kind of noise. Move the car several feet any
direction the
problem goes away. This closely resembles what I see on the
analyzer. Our
analog guru suggests it is not the x-ray system as x-rays only
last a few
seconds. A one minute x-ray would burn you up he says.
I never noticed this interference in another corner room of my
building
diagonally opposite my present location nor have I noticed the
FM radio
problem myself.
Questions:
1. What are the possible sources of such interference?
2. Is the problem radiated or conducted? I suspect radiated
as the signal
goes away when you disconnect the antenna from the analyzer.
4. Why does this problem show up in only a small specific
location on our
property?
4. What are the potential solutions?
I will be happy to answer any other questions you think of (if I
can) and
I look forward to the interesting comments I will receive from
you all.
Thanks in advance.
Scott Douglas
[email protected]
