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.

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

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.

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.

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.

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.

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.

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.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!