[RFI] Evils

Kelly Johnson n6kj.kelly at gmail.com
Sat Feb 16 11:11:32 EST 2008


Of course ingress at 3.58MHz can be a problem.  That appears to be the
case with one of my TVs.  All the TVI shows up as "color variations"
and is most prevalent on 80m and a little bit on 40m.  It appears to
be very susceptable to ingress RFI at 3.58Mhz.


On 2/15/08, Jim P <jvpoll at dallas.net> wrote:
> 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 at rockwellcollins.com>
> To: "rick darwicki" <n6pe at yahoo.com>; "Jim P" <jvpoll at dallas.net>
> Cc: <rfi at 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 at dallas.net>
> > Sent by: rfi-bounces at contesting.com
> > 02/14/2008 08:00 PM
> >
> > To
> > "rick darwicki" <n6pe at yahoo.com>, <rfi at 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 at yahoo.com>
> > To: <rfi at 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?
> > > _______________________________________________
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> > >
> >
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> >
> >
>
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