HDTV Antenna

plug.org at 2nerds.com plug.org at 2nerds.com
Mon Jan 8 10:31:09 MST 2007


On Sun, 7 Jan 2007, Barry Roberts wrote:
> It looks like several stations come from exactly the same spot, so I'm
> wondering how I could be getting KTVX-HD and KUTV-DT perfectly and not
> KSL, especially if they are all coming from the same transmitter.

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



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