Subject: [NEXTEL-1] RE: IM

From: [email-masked]
Date: Fri, 23 Apr 2004 13:12:54 -0400

-------- Original Message --------
From: Eierman David-CFED01 <[email-masked]>
Subject: RE:  IM

Is it nuisance interference or is it disruptive interference?  
If they are operating receiver carrier squelch, nuisance interference
becomes disruptive to the listener.
Sounds like you need to go measure level going into their receiver.  
Is their yagi antenna looking right at you?
 
Without details of both systems and both paths, all I can do is
postulate
the worse case scenario, which shows that it is possible for a 0 dBm IM
signal radiating from your transmit antenna to be received at the victim
receiver well above receiver threshold.  
 
Radiated IM products should be measured in the microwatt range (20-30 dB
lower), not the milliwatt range.  
 
Your question about Low Pass Filter.  Filter would need to go between a
source of harmonic generation and the IM mix point to be effective. 
After
IM mix point, you would need to filter the IM freq.
 
Isolators are ferro-magnetic devices, and produce IM products about
50-70 db
below the carrier power passing thru the isolator.  The more stages, the
higher the harmonic level.  I have seen these harmonics pass thru
certain
bandpass cavities with less than 30 dB attenuation (if cavity passes
fundamental freq, why shouldn't it pass harmonics?), and then mix with
other
harmonics in the antenna network to produce IM products.  Low pass
filter
between isolator and cavity attenuates these harmonics, preventing them
from
getting thru transmit combiner and into antenna network.  This is
usually an
on-site issue, because you would expect the IM levels to be lower.  For
100
watt transmitter, harmonics would be below 0 dBm, and coming out of
cavity
below -30 dBm, and then mixing into IM at even lower levels.  Not likely
the
source here.
 
Historically UHF antennas have not been tested for IM generation, like
800/900 antennas are.  With 5 transmitters into antenna, at 20 watts
each
after combining, total power at 100 watts.  Antenna 3rd order IM usually
measured using two 20 watt transmitters.  5th order IM should be about
10-15
dB below 3rd order.  Antenna would need to be very bad and produce 3rd
order
IM at about 40 dB down to get radiated 5th order level near 0 dBm.  
 
I haven't looked at crossband combiners in over 20 years, so I don't
know
enough about them to comment on their ability to generate harmonics
and/or
IM.  I don't know if they contain any ferro-magnetic material.  But, if
they
would need to produce 5th order IM at less than 55 dB down from single
carrier level to produce radiated 5th order product at 0 dBm.  Could be
additive effect from coupler at bottom of tower and coupler at top (3
dB?).

IM level is often related to total power (actually current) flowing thru
the
IM source junction.  More transmitters that are on, besides the 2 source
freq's, could also raise the IM level slightly.  
 
Regards,
 
David  
 
David W. Eierman
Principal Staff Engineer
Motorola, CGISS, Spectrum Strategy
Phone: 410-712-6242

-----Original Message-----
From: Steve Bosshard [mailto:[email-masked]] 
Sent: Friday, April 23, 2004 10:51 AM
Subject: RE: IM

David,

Does the unwanted signal also have the advantage of the system antenna
gain?  For 30 Watts (+44.7 dBm.) after combining, and 10 dB (+54.7 dBm.)
antenna gain minus 150 ft of 7/8 cable (-1.2 db.) for +53.57 dBm. erp
(227.5 Watts erp).  Looking at a sample off the air, and the offending
signal being 50 db down would place the offending signal at +3.57 dBm.
or 2.2 milliwatts erp.  This 2.2 milliwatts travels through space 4
miles and clobbers a 30 watt with yagi remote broadcast signal that is
also about 5 miles out, albeit, not on a tall tower, but rather either
roof mounted or ground mounted on a tripod.

This is where I am having trouble understanding - seems like the 30 Watt
transmitter would capture over the 2.2 milliwatt signal.

Will keep you posted as things develope - also these measurements were
before the 22 db notch.

Steve

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