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Re: [TenTec] Omni VI, opt.3 UHF Intermod

To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Subject: Re: [TenTec] Omni VI, opt.3 UHF Intermod
From: "Bob McGraw - K4TAX" <RMcGraw@Blomand.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Thu, 13 Dec 2007 15:47:24 -0600
List-post: <mailto:tentec@contesting.com>
Having had a Butternut vertical when we were in S. FL, all the mechanical 
parts and clamps and SS screws lend way for rectification to take place. 
Likely it's time to clean it up mechanically.

Intermod is two or more signals which mix to produce products which are the 
sum and difference of the two plus the two original signals (4 signals 
present) and should 3 or more external signals get involved then things get 
worse mathematically.  There's software on the web that determines the 
probability of IM if certain signals are know.

In a factual case when we lived a few miles east of Nashville, TN,  the sum 
of channel 4 TV {67.25 MHz}plus channel 5 TV {77.25 MHz} mixed together 
produces a signal at 144.500 MHz. and the difference comes out spot on 10.0 
MHz.  These two signals with their respective video modulation created havoc 
with the EME receiver and a big 2 meter antenna when pointed west. 
Solution: add an electrical 1/2 wave of coax resonant about the frequencies 
across the front end of the receiver.  Problem solved.

So the LP filter is doing it's job in removing the offending RF from the TV 
transmitter.  Oh, these UHF transmitters and their gain antennas often make 
for several megawatts of ERP.  Other words, lot'a  RF.

Bob, K4TAX

----- Original Message ----- 
From: "Phil" <flor0045@metnet.edu>
To: <tentec@contesting.com>
Sent: Thursday, December 13, 2007 2:12 PM
Subject: [TenTec] Omni VI, opt.3 UHF Intermod

>I have the subject rig.  The intermod was intermittant.  It took a looong
> time to figure out it was being caused by channel 23 or 29 (I can't
> remember) about 7 miles from me.  My antenna is a Butternut vertical.  The
> problem is now gone thanks to an old Drake LP filter.
> Phil, K0UBC
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