On Fri, 25 Apr 2008 10:48:17 -0700, (Dave Platt)
wrote:

In article ,
Cecil Moore wrote:

Are there any omnidirectional HDTV antennas?
My LPDA is too directional.

Well, considering that there really is no such thing as an HDTV
antenna per se, yes. HDTV in the U.S. (ATSC) uses a subset of the
same frequencies used by ATSC television, and so all the same
considerations apply.

With 2 to 4 stations on the same frequency allotment as the old analog
in some instances. At any rate some squeeze 4 in the space that used
to be used by one. They do this at the expense of bandwidth (IE
resolution)


The commonest commercial solution to your need seems to take the form
of a saucer-shaped radome, containing an amplified
near-omnidirectional UHF/VHF antenna. One source is

http://www.starkelectronic.com/allant.htm

Internally, I believe that these consist of a standard or folded
dipole, with the elements "swept" into an S shape. They're probably
not terribly good antennas, as far as matching or sensitivity or

They're not! I had one. Also the amp "up there" makes them more
susceptible to overload in interference. They aren't worth the effort
to put them up let alone the price, at least the ones I tried weren't.

pattern is concerned, which is likely why an onboard amplifier is
included.

Just a plain old pair of crossed, folded dipoles work just fine. I
have such a set up just below my AV-640 and run about 600 watts on
6-meters.


If you wanted to tune up only one (or a few closely-spaced) HDTV
stations with an omni, you could probably build yourself a turnstile
antenna... a crossed pair of bow-tie elements, and a 90-degree phasing
harness cut for the middle of the band would probably work. I doubt
that'll work as a general-purpose solution for the full band, though.

It does here at least out to about 40 or 50 miles from 40 feet.


The specific sort of antenna design you might need to work well in
your area, will depend a lot on which frequencies are going to be in
use for DTV in your area after The Big Day Next Year. In many areas
of the country, all TV signals will be in the UHF band - this
simplifies matters for residents of those areas, as only a UHF antenna
will be needed.


On the big tower I have some long (longest I could get) UHF antennas
with remote mounted amps at 90 feet. One points WNW while the other
points South. They do well for close in stations any where near the
direction they are pointed. I have used them with a combiner but that
results in ghosts on analog and drop outs on digital.

BTW these antennas are about 30 feet below a pair of 12L, vertically
polarized yagis and about 12 feet above the tops of a pair of Diamond
repeater antennas. The tribander is about 8 to 10 feet above the TV
antennas and the 7L 6-meter yagi is about 15 feet above the tribander.
I run the legal limit to the tribander and a 1000 to 1200 PEP on
6-meters. So far no problems.

In more crowded areas of the country (and in some locations where the
broadcasters don't want to re-fit their main transmitters) DTV will
include stations operating in the VHF high band (channels 7-13) and
you'll need an antenna which works on these frequencies as well.

I receive them just fine on the UHF antennas again out to about 40 to
50 miles.


In a very few areas, there will still be stations operating DTV in the
VHF low band (channels 2-6), and residents of those areas will still
need full-coverage antennas.

Only the one in real close (under 30 miles) come in OK on the UHF
antennas. (Ch 5 is about 20 miles)


Also, is TVI from ham transmitters reduced with
HDTV?

It depends.

The worst ham TVI problems seem to have affected Channel 2, due to its
proximity to the 6-meter band and its second-harmonic relationship to
the 10-meter band. There will be *very* few TV stations using this
frequency after The Day, and this may reduce TVI complaints for hams
operating on 2 and 6 meters.
Only problem I ever had was one neighbor having a foot ball party out
in his garage using rabbit ears for an antenna about 120 feet from the
base of my tower. "Hello Test" with 50 watts to the 12L pair at 130
feet would give him a blank screen on Ch 12. I gave him a directional
TV antenna and from then on I was invited over to the parties and free
beer.



Strong-signal overload/desense problems of TVs (or antenna/CATV distribution
amplifiers) will probably still exist, especially for TV stations
operating in the VHF low and high bands. I imagine that high-power
ham transmissions in the 70 cm band may cause overload problems for
UHF TV stations in some cases.

The actual behavior of the TV sets, in the face of ham TVI, will
definitely be different. No more herringbones in the video, no more
desense-related snow. Instead, the picture and audio will be
unaffected, until the TVI is severe enough that the DTV tuner and
demodulator can no longer get enough of a signal to allow the
digital error-correction process to work - at that point, the image
will start freezing, breaking up into rectangular "macroblock"
artifacts, or disappearing entirely. TVI is likely to present as much
more of an "all-or-nothing" thing that it did in the past.
Roger (K8RI) ARRL Life Member
N833R (World's oldest Debonair)
www.rogerhalstead.com