Re: How does digital TV broadcast prevent ghosting effects?

Jeff Liebermann wrote:
Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> hath wroth:

A rather huge log-periodic on a mast.
Bingo. Are you *SURE* it's really a log periodic antenna or some
kludge that only looks like one? Duz it look like one of these?
<http://en.wikipedia.org/wiki/Log-periodic_antenna>

It does, just a whole lot bigger.

Which one of these? I believe you mentioned it was a Channel Master:
<http://www.pctinternational.com/channelmaster/0612/antennas_outdoor.html>
If it's one of the 36xx series, they're similar to the Radio Shock
monstrocity that I modeled. (I'm still looking for where I buried the
NEC2 files).


None of those, AFAIK it's a commercial grade version for CATV heads. Very old and not a spot of wear (other than the occasional bird droppings), pretty impressive.


It is much sharper. I spend hours on the ladder turning it. You can completely lose a station into the noise at those 30-50 degrees.

Ladder? I thought you had a tower or pole. If you're that close to
the ground or roof line, you're going to get a quite a few local
reflections.


The mast is bolted to the side of the house and you have to get on a ladder to untighten it, turn the whole mast, then retighten.


Well, I don't wnat to go to that length ;-)

It wasn't really all that bad. What I should have done was contact
the local CATV company and ask them if they had any unused single
channel antennas at the local headend. I know they have these because
they're moving away from over the air feeds, and switching to fiber
and satellite distribution. However, I built my own. The first
prototype was a piece of PVC pipe, with welding rod for elements. Dual
T-match and balun on the driven element. Once I got one channel
working, and determined that the others would probably work just as
well, I built 3 more antennas and amps. Figure on one weekend per
antenna.

TV isn't too important and nowadays monitors have PC connectors. So eventually people might be able to cut it all loose and obtain their teevee from the web. Some day.

Nope. The future is "personal TV", where you wear a 3D headset and
are literally immersed in the story. Anything less than terrestrial
fiber doesn't have the bandwidth. You read it here first.


So all pedestrians are now schlepping fibers behind them?


Well, I guess it is too late now. Whatever happens, when this leads to unpleasant consumer feedback in 2009 someone will dig into the matter of "who dunnit".

Notice how the deadline on Feb 17, 2009 is AFTER the election in Nov
2008. Congress and the FCC know a problem, especially when they
create it. If there's every going to be a "popular uprising", it will
be over something like the loss of analog TV.


And that one could be huge. There is stuff that voters forget, and then there is stuff they don't forget. And you can be sure that this will be mined in the election after that one, along the lines of "these are the guys who dunnit".


Out here there are echoes with very long delays. Tens of microseconds.

Then, they are NOT coming from bouncing around the indicent path.
They're coming from the sides, or more likely, from the back. Since
your antenna is apparently within easy reach (on a ladder), you get to
try a really dumb experiment. Find a 20ft fiber glass window washer
or painting telescoping pole. Mount the biggest wire mesh you can
find to the top to block the signal. Move it around the sides and
back of the antenna until the reflections (ghosts) disapper. If you
have multiple reflections, this has the potential of making things
worse, but at least you'll get a clue as to from which direction the
reflections are coming from. My guess(tm) is from the back. If the
wire mesh makes it worse, try again with something that absorbs RF
(wet towel works nicely) and doesn't reflect RF.


I've tried those tricks. Thing is, reflections (bounces) seem to happen multiple times. Meaning a back reflection comes bouncing back from the front again. That's why it gets really bad when a 747 freighter glides in, probably because it's a nearly perfect reflector.


If you don't want to have the neighborhood suspect your sanity by
running around your roof with a giant fly swatter, find a bow tie
antenna (with a flat reflector) and try it on one of the UHF channels.
This won't solve the VHF problem, but for UHF, it has a highly
desireable high f/b ratio. If the reflection is from the back, as I
suspect, then it should eliminate it.

Also, please try not to fall off the roof. You have but one life to
sacrifice to the HDTV gods.

There's also an antenna selector web site at:
<http://antennaweb.org>
It just declared that there are no TV stations that I can see at my
location. Oh well.


Quote "These large, multi-element rooftop antennas are used in weak signal areas for maximum possible TV reception. These antennas can be used in ANY LOCATION, but require an amplifier and roof mounting to receive blue and purple channels. Amplifiers are not recommended for yellow channels."

Hmm, not all that helpful ...

--
Regards, Joerg

http://www.analogconsultants.com/
.

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