Subject: [NEXTEL-1] RE: Third Order Intermod

From: [email-masked]
Date: Thu, 22 Apr 2004 10:36:03 -0400

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

This is one of those oddities of the UHF band, where adjacent service is
using what would be base transmit sub-band under Part 90, as a base
receive
band.  We normally only see this type of issue between Part 90 systems
across 465 MHz boundary.  
 
If the radiated level of the 5th order IM product on 451.925 is only
about
50 dB below the radiated level of your signals on 451.425 & 451.675,
then a
4 miles, the IM level is at about -100 dBm, assuning that your net
antenna
network gain is only 3 dB.  Assuming that BAS receiver is 100 kHz wide
with
10 dB noise figure, its internal noise threshold is about -115 dBm. 
Even
without adding in BAS antenna gain and line loss, your IM signal is
about 15
dB above receiver noise floor.  
 
I am assuming levels of your 451 signals (free space loss) at BAS
receiver
site are about -50 dBm, so unless they are using no filtering, have an
extremely bad receiver IM rejection spec, and are using a very high gain
antenna network, the likelihood of this 5th order IM mix occuring in BAS
receiver is very low.  
 
Possible sources for a strong 5th order mix location are:
1) harmonics generated in isolators are getting thru cavities and mixing
in
N-way connectors, other connectors in line or antenna, and in cross-band
coupler 
2) you are using connectors containing ferro-magnetic material (nickel)
or
connectors have other issues that make them prone to generating
harmonics
and/or IM
3) cross-band coupler
4) very bad antenna (probably not IM rated, like 800 antennas are)
5) additive effect from any or all of above sources
 
If issue is isolator, then harmonic filter placed between isolator and
cavity will solve problem.  Make sure harmonic filter suppresses both
2nd
and 3rd harmonics.
 
Other issues, your looking at solving each one separately.  

Regards,
 
David  
 
David W. Eierman
Principal Staff Engineer
Motorola, CGISS, Spectrum Strategy
Phone: 410-712-6242
Fax:     410-712-6208
Cellular: 443-421-0659
 
New Address:
7031 Columbia Gateway Drive
3rd Floor
Columbia, MD  21046-2289

-----Original Message-----
From: Steve Bosshard [mailto:[email-masked]] 
Sent: Wednesday, April 21, 2004 9:49 AM
Subject: Third Order Intermod

Have a 5 channel trunking system.  5 channel combiner.  Dual Juntion
isolator plus 10" cavity plus double shielded cables to a junction, then
one to the antenna.  I do have cross band couplers for 450/800, and
there is an 800 antenna along with an ASP700 antenna at the top.

For years all is well.  Yesterday, a broadcast station using 450.925 for
a remote pickup called telling me I was causing interference.  My tower
is about 4 miles from the broadcaster's remote pickup site.  Don't know
if the problem just started, or if the broadcaster just started doing
remotes.  Don't find any licenses for 450.925 around Killeen, Texas.

There is a direct 3rd order hit. 3X 451.425 minus 2X 451.675 = 450.925

Stations look clean and off the air at the tower, the product is just
above the noise floor and better than 50 db down from peak carrier
signals.

My plan is to place a 22 db. Notch cavity in the combined transmit
output line.

Any suggestions from the great vast knowledge base?

Steve Bosshard

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