It appears that a lot of people missed the point in all this.
Original poster posited:
> > 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?
Had 'they' wanted to kill off HR 'they' would have
stuck with the 21 MHz freq, but, such has not been
the case (witness the 'move' to 41.25/45.75 MHz).
The choice of 21 MHz was probably 'opportunistic' at
the time, and the choice of something higher (like 41 MHz)
later on had benefits including image rejection (esp. when
tunig the UHF CHs) and the ability to obtain a flatter frequency
response (necessary for good fidelity while amplifying
'luminance' and 'chrominance' info while still at an IF frequency
and before detection) across the smaller percentage bandwidth
(of 45 MHz vs 21 MHz) and requiring fewer stagger-
tuned IF transformers than at 21 MHz to achive that 4.5 MHz
(or so) BW.
The choice of 3.58 (3.579545 MHz for the purists) by
the National Television System Committee (NTSC) was a
compromise not the least of which was some to achieve
backward compatibility with monochrome television; there
are also certain integer relationships the 3.58 frequency
has with the vertical and horizontal sweeprates as well as
being the 'carrier' frequency for the DSB surpressed-carrier
color (chromonance) information; choosing 3.58 for the
chrominance information also put the 'color' information
within the IF bandwidth of the existing IF 'footprint'
and RF chansel width of the existing B&W broadcasting
channel allocations.
As sets have become smaller, there is far less radiation
from three-five-eight oscillators (one could usually hear
several in hte 80M band in the evenings in a residential
area) and there is far less trash acroess the AM broadcast
through SW bands from the Hoizontal 'sweep' circuits
that used to utilize a catergory of vacuum tubes known
as a 'sweep' tube.
I think I mentioned in one e-mail that today there is a
far graeter predominance of 28.635 MHz 'cariers' emanating
from PCs and CRT computer monitors than 3.58 nowadays;
one can hear a great many of these carriers in residential
areas on a 10 M radio.
Jim P // WB5WPA //
----- Original Message -----
From: <dgsvetan@rockwellcollins.com>
To: "rick darwicki" <n6pe@yahoo.com>; "Jim P" <jvpoll@dallas.net>
Cc: <rfi@contesting.com>
Sent: Friday, February 15, 2008 10:40 AM
Subject: Re: [RFI] Evils
> 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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>
>
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