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From: Neal McLain (nmclain@compuserve.com)
Subject: Re: Digital TV Towers
View: Complete Thread (14 articles)
Original Format
Newsgroups: comp.dcom.telecom
Date: 1997/11/29
In Volume 17 Issue 330, Roy Smith <roy@mchip00.med.nyu.edu>
wrote:

> There has been a lot of talk in the aviation mass media about
> digital TV towers.  The problem is that with the advent of
> digital TV, we are going to see (if you can believe the
> aviation press) an explosion of new TV transmission towers, 
> and not only that, but taller ones.  These towers present a
> hazard to air safety, especially when built near airports.  
> My question is why?  What is do different about digital TV that
> requires the building of new towers?  I would think it would be
> fairly straight forward to just add additional transmitter
> antennas to the same tower structures that exist today for
> conventional TV transmission, with no net increase in the
> number of towers (and thus, no net increase in the air
> navigation hazard).  Why would this not be the case?

Two reasons:

REASON #1: Mechanical loading on the tower.

During the so-called "transition" period, each broadcast station has
the right to use two channels, one for its new DTV service and one for
its current analog service.  In most cases, that means it needs two
antennas, one for each channel.

A typical VHF broadcast antenna is maybe 100 feet long, two or three
feet in diameter, and weighs several tons.  UHF antennas are smaller
(the higher the frequency, the shorter the wavelength; consequently,
the shorter the antenna requirement).

But even a UHF antenna adds a big load to a tower.

The real problem is dynamic (wind) loading.  Tower design codes
require that the entire tower structure, including all antennas,
withstand various combinations of ice and wind; in northern states, a
typical requirement is three inches of radial ice on all surfaces with
a 100-mph wind.

Most existing towers were originally designed to support one antenna,
and cannot safely support the additional load of a second antenna.

REASON #2: Pattern distortion.

The "pattern" of an antenna is a graph of its signal strength plotted
against distance from the antenna.  The ideal pattern for an
omnidirectional antenna is a circle with the antenna at the center.
With careful design, it's possible to intentionally alter the pattern
to direct the signal in specific directions; for example, antennas in
Chicago are designed with a kidney-shaped pattern to force the signal
north, south, and west, but not east (there aren't many viewers out in
Lake Michigan).

If a broadcast antenna is installed on the side of a tower, the
tower itself distorts the pattern.  If two or more antennas are
installed side-by-side, each antenna distorts the pattern of the
others.  These types of pattern distortions aren't intentional:
they can cause all sorts of undesired anomalies such as ghosts
and no-signal gaps.  Consequently, every broadcaster wants to
have his antenna placed on the top of the tower, with no other
antennas anywhere around.

The usual result is that each station has its own tower.  In many
cities, the towers are grouped in the same general geographic area
("antenna farm"), but the individual towers are still several hundred
feet apart to minimize pattern distortion.

With the advent of DTV, the broadcaster faces a question: where does
he put the new DTV antenna?  Even if the existing tower can support
the mechanical load of a second antenna, it's usually not possible to
place it so that the two antennas don't distort each other's patterns.

The obvious solution to both of these problems is to build a second
tower, as many broadcasters are now attempting to do.  But this leads
to further problems:

  - FAA Approval.  As Smith noted in his original posting, towers
    present a hazard to air safety, especially when built near
    airports.  The FAA has an elaborate notification and approval
    procedure; in theory, if the FAA approves a new tower, it
    meets all FAA safety requirements and does not pose a hazard
    to aviation.  But getting FAA approval for a new tower near a
    major city is difficult, and it's becoming more so as air
    traffic congestion increases.

  - Local Zoning Approval.  Local zoning authorities typically
    oppose new towers on a variety of grounds including
    aesthetics, environmental concerns, and RF radiation hazard.

  - Land Acquisition.  When the original analog-TV towers were
    built 30 or 40 years ago, they were usually placed in rural
    areas.  By now, many of these towers are surrounded by urban
    sprawl, and land prices have soared.  So new antenna farms
    must be found, often many miles from the original ones.

    This, of course, exacerbates the FAA and zoning approval
    problems.

One way to alleviate these problems is to construct "community
towers".  A community tower is single large tower designed from the
outset to support multiple antennas.  Even broadcasters who compete
ferociously for viewers sometimes jointly own community towers.
Non-broadcast entities, such as paging, cellular, and PCS companies,
also can participate.

There are compelling reasons for this: a community tower for all
antennas requires less land than multiple towers.  The FAA generally
encourages community towers as an alternative to multiple single-use
towers, especially if it means getting rid of some existing towers.
Even local zoning authorities can accept a new community tower on the
promise that existing towers will be removed.

So how do you put several television broadcast antennas on one tower
if everybody wants to be on the top of the tower?  One way is vertical
stacking: placing one antenna on top of another in a sort of totem
pole.  Another way is to place the antennas side-by-side, but spaced
far enough apart that pattern distortion is negligible.  A horizontal
spacing of 100 feet is generally accepted as sufficient.

By combining these two approaches, it's possible to put a dozen or so
antennas on one tower.  The end result looks something like this:

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If there are only two antenna stacks, a horizontal support, perhaps
100 to 150 feet long, supports the stacks (for obvious reasons, this
is called a "T-top" design).  If more antennas are required, a
triangular platform can be used ("candelabra" design).

Of course, community towers aren't new: residents of San
Francisco will recognize this design:


                  |     |

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                  |     |
                  \     /
                   \   /
                   /   \
                  /     \

But the advent of DTV has made this whole issue far more complicated
because every television broadcast station is now going to have to
find a place for a second antenna.  In view of all the problems
associated with constructing a second tower for each broadcaster,
we're likely to see more community towers in the future.


Neal McLain
nmclain@compuserve.com


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