Re: Do receiver antennas need matching or not?
- From: Art Unwin <ArthurUnwin@xxxxxxxxx>
- Date: Wed, 19 Mar 2008 10:25:31 -0700 (PDT)
On Mar 19, 11:44 am, Jim Lux <james.p....@xxxxxxxxxxxx> wrote:
Jeff Liebermann wrote:
Ghosts are a big problem where I live. Or multipath and the resulting
dropouts the case of DTV broadcasts. That's why I like highly
directional designs (both horizontally as well as vertically). Other
than that, I can get all of our local stations with rabbit ears and a
UHF loop.
I live too far away from the local digital TV xmitters to get reliable
reception. My rule of thumb is that if OTA analog TV reception is
marginal, digital TV will be worse.
Highly directional antennas are the right way to eliminate ghosts
(reflections). However, I keep running into problems with f/b (front
to back) ratio problems, where the ghosts are reflected from behind
the antenna. That's where the lower gain, but higher f/b ratio
antennas, such as a barbeque grill backed bowtie array, makes more
sense. My preference is to use single channel narrowband yagi's for
maximum gain, but that gets really ugly as one per channel is
required.
Actually, with modern high quality receivers (which may not include the
$40 DTV to NTSC converters), they use a rake receiver to coherently
combine the multiple paths, so it's very likely that you'd get a better
signal with digital (i.e. the receiver would lock and you'd get ANY
output) than with analog in a multipathy, but not faint signal environment.
One of the design goals for DTV was to be multipath immune.
Note that in Europe, they actually transmit the identical signal from
multiple transmitter sites, guaranteeing multiple arriving signals with
different timing. Different modulation scheme, but the same multipath
issues. The same has been proposed for use in the US
However, with DTV, signal strength is more of an issue,mostly because
there isn't a "degraded mode" like there is with analog. Most viewers
are willing to tolerate remarkably degraded signals on an analog
channel, so folks are used to being able to receive a TV signal well
beyond the nominal service boundary. With digital, either you got it or
you don't. Some gain in the antenna helps..
I am hoping that because of the narrow filter requirements of ham
radio
a fast acring multipath antenna switch would allow the dominant
polarisation to over rule all otherssince the ear can only accept a
few samples at a time.
Art
.
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