HDTV Antenna

Brandon Stout bms at mscis.org
Mon Jan 8 14:32:01 MST 2007


plug.org at 2nerds.com wrote:

> In the case of the DTV-Utah facility on Farnsworth peak, each station
> has its own transmitter, but they're all fed into a big combiner which
> then feeds a common antenna. (which may be a distinction appreciated
> only by TV-transmission nerds. :-)
>
> Anyway, at both my home and office I've observed a similar problem in
> that some OTA ATSC stations come in like gangbusters while others are
> poorly received, if at all.  Knowing that the signals are literally
> coming out of the same antenna, I was initially baffled by this.  The
> best explanation I've heard is that the problem isn't a matter of a
> weak signal, it's a matter of a poor signal-to-noise (S/N) ratio,
> which is greatly affected by destructive interference due to multipath
> which, in turn, varies highly with both frequency as well as the
> geographic locations of the transmitter and receiver.  In other words,
> in any given spot, a receiver might have fabulous S/N ratios on
> several stations but lousy S/N ratios on other stations.  Here along
> the Wasatch front, where most DTV signals come out of the same
> antenna, the only difference for any given receiver location is the
> frequency.  Different frequencies at any given receiver location will
> have different patterns of destructive interference.  On analog TV,
> multipath is often "seen" as ghosting in the picture.  On digital TV,
> multipath doesn't cause ghosting in the picture; it reduces the
> quality of the digital data stream being received--sometimes to the
> point that the data can't be recovered at all.  Also keep in mind that
> multipath and destructive interference can be worse on UHF than VHF
> because of the higher frequencies; higher frequencies mean that
> smaller differences in path length can add up to bigger destructive
> interference problems.  The mountains all around us don't help; I was
> surprised when I heard that "dirt" can reflect a UHF TV signal, but
> I'm told it does--very effectively. :-)  Anyway, if your ATSC receiver has
> a S/N meter (mine does), check it out and see if it correlates with
> the poorly-received OTA ATSC signals.
>
> If you're in a position where you can "see" Farnsworth peak, then it's
> highly unlikely that a "weak signal" is the cause of any reception
> problem; it's much more likely that your antenna is receiving several
> "copies" (reflections) of the problem signal--each "copy" of which is
> separated in time based on the differing path lengths.  Your antenna
> will be receiving the strong "direct" signal as well as some number of
> slightly weaker reflections, and those reflections tend to
> destructively interfere with the direct-path signal.  About the only
> thing you can do in such circumstances is use an antenna with a more
> "directional" pattern; it's not overall "gain" that is of interest,
> but the degree to which side and back lobes have significantly lower
> gain than the (hopefully narrow) forward lobe which is pointed towards
> the transmitting antenna.
>
> Chris

Chris,

You have some great information here, though I "recommend" fewer quote
marks :) .  Just for the fun, might I recommend reading the gallery of
"misused" quote marks?

http://www.juvalamu.com/qmarks/

Brandon Stout
http://mscis.org




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