Guys,
First, those are currently used IFs, and N4ZR had it right about the truly
"old" 21 MHz IFs. Believe it or not, I ran across someone using a TV that
had a 21 MHz IF as recently as 15 years ago. I was operating portable and
using 15m at the time, and I very quickly was contacted by an irate viewer
whose black and white set got blown away when I got active on 15m. Several
hours of previous operating on 20m and 40m had no effect upon that viewer.
(The 40m harmonics must have been well-surpressed on my rig!)
There is also a not-so-obvious problem with the TV IFs given below: image
issues. Current analog TV Ch 2 uses 54 to 60 MHz. Take 58.50 MHz (the
sound carrier frequency for Ch 2) and add TWICE the audio IF (45.75 x 2 =
you get 91.5), with the result being 150 MHz. TV sets being what they
are, and IF strips being less than wonderfully selective, you can have
image problems with people watching CH 2 and you are using a 2m
transmitter. This took awhile to figure out, as our local 2m repeater
would get complaints of audio TVI from viewers located within about 1/2
mile of the transmitter site (the repeater has about 75 to 100 watts of
output). Use of a spectrum analyzer showed that our crystal controlled
transmitter was very clean, so when I found out that the only time we got
complaints was when someone was watching the local Ch 2 station, the
reasonable explanation is image. I have subsequently found that I can zap
my own TVs if I set any of them to Ch 2 and operate at any power more than
5 watts on 2m. It's nice to know that in my part of Iowa, analog Ch 2
goes away permanently next year. It is also interesting to note that
because of the inverse mixing that occurs with an image (and where the
bandpass is located within the TV set IFs), our repeater causes problems,
but the county sheriff's radio, located on the same tower as our repeater
antenna and operating about as many MHz above 150 as we are below, causes
no problems to viewers. That transmitter is also around 100 watts.
I might ad that I cobbled up a band reject filter tuned to our repeater's
output frequency that could be inserted into the coax input of a TV or
VCR. This filter produced about 18 dB of reduction (measured on my
network analyzer) with negligible loss at any TV channel frequency. I put
the filter into a small aluminum box with type F coax fittings and made it
available to anyone who would like to try it. Weird - no takers. With
time and care, an even better filter could be made. However, I guess that
TVI due to image from the repeater isn't bad enough to spend 30 seconds
installing an in-line filter provided for free.
73, Dale
WA9ENA
"Jim P" <jvpoll@dallas.net>
Sent by: rfi-bounces@contesting.com
02/14/2008 08:00 PM
To
"rick darwicki" <n6pe@yahoo.com>, <rfi@contesting.com>
cc
Subject
Re: [RFI] Evils
Old TV IF freqs::
Video 41.25 MHz
Audio 45.75 MHZ
2nd IF Audio 4.5 MHz
Ham bands?
???
Jim P // WB5WPA //
----- Original Message -----
From: "rick darwicki" <n6pe@yahoo.com>
To: <rfi@contesting.com>
Sent: Thursday, February 14, 2008 7:40 PM
Subject: [RFI] Evils
> So why do so many devices like VCR heads, TV color ocsillators etc and
the
old TV IFs fall in ham bands?
>
> Was this a commie plot to kill ham radio?
> _______________________________________________
> RFI mailing list
> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>
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