In article ,
Bill Horne wrote:
I know I need a rooftop antenna, but the available commercial literature
is more hype than help. I'm hoping that someone out there has expertise
on the new antennas being offered for HDTV.
I know this depends a lot on what the signal strengths are at my QTH,
..... and on whether the channels in your area are all going to be on
UHF, or whether there are some which will still be on VHF high-band,
or even one or more on VHF low-band.
but assuming that all-other-things-are-equal:
* Are flat-panel units worth the price?
As to price I can't say (although there's an open-source design or two
floating around which can be home-fabricated).
Flat-panel designs (bowtie-and-reflector, usually, I think) are
probably decent solutions for UHF-only areas. I don't expect that
their performance on VHF is terribly good, unless they're very large.
* Can I get by with an old VHF-UHF combination antenna?
If you've got a decent ghost-free picture on NTSC with a combo
antenna, chances are probably fairly good that it'll work well enough
for ATSC digital.
If all of your ATSC stations are going to be UHF, you might get better
results with a multi-bay UHF-only antenna... the combo antennas don't
always have a very clean pattern (good front-to-back ratio)
* What's the tradeoff between feedline length and antenna gain?
Well, a longer feedline leads to greater losses, and if your signal is
already at the lower limit of what's acceptable then you'll need to
make up the lack somehow else... higher antenna gain, or a preamp, or
increasing the signal "seen" by the antenna.
There's a definite tradeoff between feedline length and antenna
*height* - raising the antenna will increase the feedline length (and
losses), but may give the antenna a better "view" of the transmitter
antenna and thus a stronger/cleaner signal.
If you're a good distance from the transmitter (more than a few
miles), raising the antenna is often a very good tradeoff...
if you don't, the transmitter antenna may be below the horizon (from
your antenna's point of view) and you'll be depended on diffracted
signal rather than line-of-sight signal. An extra 10' of antenna
height may increase the signal into the antenna by quite a bit more
than the amount you'll lose in 10' of feedline.
* Do masthead preamps perform well,
A lot of cheaper preamp designs are prone to overload if there's a
strong signal nearby (e.g. fire, police, ham radio), and this is
Ungood. Some are prone to instability - they can start oscillating
without warning - and this is Really Ungood.
and is the cost and maintenance
worth it?
That depends very much on your situation.
If many of the stations you're receiving are quite distant, and if you
need a long feedline, and if you've already got a good antenna, and if
you buy a high-quality masthead preamp which is resistant to
strong-signal overload, and if there are no nearby strong-signal
transmitters, then such a preamp may well be worthwhile.
I tend to think that preamps are a decent way to compensate for losses
in the feedline. Unless they're quite good, though (low noise figure
and high overload resistance) they're not a good substitute for
getting enough of a clean signal *into* the antenna - positioning the
antenna better/higher, and using an antenna with more directional gain
are better bets. Remember, a masthead preamp will amplify ambient RF
noise, and multipath reflections just as much as it'll amplify the
desired input signal!
--
Dave Platt AE6EO
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