HDTV Forum

This is related to users in the Atlanta area with reception problems on windy days. While the following is technically interesting it should be noted that the source of the problem is a marginal antenna system.

> >Also wind moving the transmission towers...

Yes all 500,000 tons

--- In [email protected], wagadt@b... wrote:
> Modern towers such as the one FOX 5 uses, and other new ones in
Atlanta, move very little in the wind due to the anchoring of the
antenna support arms. However the actual antennas mounted on the
arms are free standing and will sway slightly in the wind. When we
have high winds on the ground it is often much windier at the top
where the antennas are. While the movement should be minor and have
very little effect on the transmision characteristics, I know of no
real reaserch to show what happens. And under ice loads there may be
further movement of antennas and even the tower and guy lines.

Actually a tower will move approximately the width of the tower face in a 70 mph wind. The tower that supports WATL-TV and DT, WTBS-DT and WUVG-DT moves about 12 feet in a 70 mph wind. A 70 mph wind is very common at 1000 feet above the ground.

This movement equates to about 0.75 degrees of movement. Most TV stations use 0.75 degrees of beam tilt which will place the main lobe of the radiation pattern slightly below the horizon for best coverage. When the tower moves another 0.75 degrees in the wind the radiation pattern will move accordingly which will cause varying signal strength because the apparent beam tilt changes from 0.75 degrees to as little as 0 degrees or as much as 1.5 degrees. If one has a marginal signal this can cause a problem. The phenomena also will cause multipath which is the major problem with the wind moving the antennas and towers.

In addition the wind will bend the antenna itself and this can cause a varying gain in the antenna system and even phase problems in the transmitted signal which would look like a close in ghost in analog or a degradation in S/N in digital.

Ed Johnson - WATL

Actually electrical beam tilt, which is a constant change in the
electrical phase along the antenna to tilt the pattern down, is done
on most antennae to place the main lobe on the ground at the horizon
for maximum coverage. In special cases where the majority of audience
is close in (less than 20 miles away) or the antenna is extreme high
in relation to the city of coverage the beam tilt may be more to
place the main lobe into the population center. Null fill, which is a
variable manipulation of the electrical phase along the antenna, is
used to fill in lower signal areas in the pattern in close to the
stations (within 15 miles).

Depending on the gain of the transmitting antenna and the 3 dB
beamwidth of the lobe, a 0.75-degree shift could make as much as 6 dB
difference in the signal strength depending on location. If one has a
good signal and a good receiver then this is not a problem. If one
does not have a good signal level and/or the receiver cannot handle
the change in receive level then reception problems occur.

All of the antennae used in Atlanta for analog and digital have at
least 20% null fill or a smooth pattern to help eliminate the in
close problems. Most antennae in Atlanta run 0.5 to 0.8 degrees of
beam tilt depending on the antenna HAAT (Height Above Average
Terrain). At 1000 feet HAAT the horizon is 0.58 degrees below
horizontal, at 1250 feet it has increased to about 0.64 degrees. The
0.75 degrees we use, which is now the most common places the maximum
signal on the ground all the way to the horizon with the in close
(closer than 40 miles) favored just a little.

One must remember that the wind will move the buildings, trees, water
tanks and the receive antennae also. It is the complex change in all
of the variables in motion that causes the change in signal and
apparent multipath at the receive location. Complicating the mix is
that the wind is not constant but is constantly changing so none of
the apparent motion is relative to the other objects.

The best solution is a good antenna system that is in the clear and
is adequately supported to limit sway in the wind.

Ed - WATL

Zyg,

That is true in relation to the maximum level at the peak of the main lobe. But at distances of less
than 10 miles the moving antenna will cause a much greater signal change. Example:

Our DTV antenna has a beam tilt of 0.75

I disagreed with Ed; and not only did I disagree with him,
but I did so very strongly. Why, because I felt that his
comments would mislead many newcomers to OTA digital TV,
who already have to wade through enough misleading
information as it is.

The truth is that the difficulties with digital and HDTV
reception are primarily due to problems with the viewers
equipment and or location. It has been my experience that
some people will simply not admit to themselves that their
incorrectly installed antenna system is the cause of their
inability to watch HDTV on their very expensive receivers;
and when they see or hear comments suggesting that it's the
transmitting antennas blowing in the wind that is the source
of their problem, then it becomes a tough road in trying to
explain to them the real situation.

-Zyg-

.... As TV signals pretty much travel in a staight direct line
of sight, it is easy to see that the shortest signal path,
and the one with the least attenuation, is when the receive
antenna is mounted at the same height as the transmit antenna.
It is somewhat cumbersome to have a 1000ft tower in the
subdivision, so the next best place is to put the antenna
at the highest point you can go. For people who have to
fight the covenant police, the FCC has allowed TV antennas
to be installed 12 ft. above the highest point of the building.
This information is also provided on the FCC website.

... Now let's get to the wind. If we were back at our
hypothetical 1000ft tower, the wind wouldn't cause a problem.
However, we have to be back at more practical heights. At my
location I have no problems when the wind blows. The lower
your antenna goes the more the surroundings start to affect
it. The problem is not the wind, it is that the antenna
is not in a good location, and other signal paths start
bouncing of buildings, the ground, etc., etc. and cause
difficulties. That is another reason to install the antenna
as high as you can.

Just in case someone else brings up the movement of transmit
antennas in the wind. The FCC has dismissed the beam tilt
wind problem stating that the problems are not as bad as some
predict, and only transient in nature. I copied the exact
wording in my previous post, and all of that is to be found
on the FCC web site too.

To conclude, I realize that there are going to be many cases
where people will have problems with poor locations and
inadequate sites. Your main moderator CB, happens to have
a very nice and picturesque home, but as far as TV signals
go it is definitely a poor site - yet CB has managed to make
most of the sitution, and is able to receive all the stations.

For those who just can't get OTA, please wait; Directv is
going to provide the Atlanta HDTV stations soon!

-Zyg-