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Re: [RFI] RFI Susceptibility of DTV?

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] RFI Susceptibility of DTV?
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Fri, 23 Jan 2009 15:18:33 -0800
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On Fri, 23 Jan 2009 17:32:54 -0500, Roger (K8RI) wrote:

>There's a lot of channel switching around, but I think you'll find the 
>low VHF including channel 2 will still be widely used and are not going 
>away. They certainly are not being reassigned to other services.  The 
>only reassignments to other services are channels 56 and up.

That's in conflict with what I'm seeing. One piece of info that came to 
me in response to my initial question was a report showing DTV to be 
quite susceptible to interference from impulse noise and documentation 
showing that impulse noise is much stronger on low-band VHF channels (2-
6). Broadcasters were given the option to stick with their low-band 
channel for DTV or switch to the higher channel assigned them following 
the transition. Most low-band stations have chosen to move to higher 
channels, and most mfrs of TV antennas are now building VHF/UHF models 
that cover ch 7-52. 

To learn what is happening in your area, go to the link below and enter 
either your lat/lon coordinates and search for DTV stations within a 
radius, or search for the call letters of your local station. 

http://www.fcc.gov/fcc-bin/audio/tvq.html

For each station, you'll see listings for LIC (licensed), STA (Special 
Temporary Authority), or CP (Construction Permit).  TV = Analog, DT = 
Digital 

I just did a quick check of SF, LAX, NYC, Chicago, Detroit, Kalamazoo. 
All of the low-band VHF stations are moving to either high-band VHF or 
UHF channels. 

>From my study of the FCC website, it looks like some DTV stations will 
be using a bit more power post-transition than they were initially. 
However -- it has been shown that DTV makes a perfect picture at a 
signal to noise ratio of only about 16 dB (and then drops off the cliff 
with QSB). That's at least 30 dB better than analog, and is one reason 
that broadcasters are typically licensed for about 10dB lower power on 
their DTV rigs than an analog TX on the same channel. Also, remember 
that VHF and UHF broadcast licenses are ERP, which is TX power plus 
antenna gain minus losses in transmission line and antenna combiners. 

BTW -- many thanks for all the info folks have sent me. For those 
interested in this matter, I'm getting the impression that DTV is less 
susceptible than analog on the same channel, and the fact that ch 2-6 
are going away will help even more. 

By the time the DTV transition is complete, TVI coupled into the input 
stage of TV receivers is likely to be almost a thing of the past, and 
we'll be left with RFI to the audio/video systems (including those the 
TV sets are hooked up to), RFI to telephones, computer speakers, etc. 
most of it coupled by pin 1 problems. 

73,

Jim K9YC




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