Microwave Towers and Satellite Antennas

15 views
Skip to first unread message

Neal McLain

unread,
Feb 21, 2003, 10:15:59 PM2/21/03
to
Daryl R Gibson <d...@bluediamond.byu.edu> wrote:

> Sad to see those towers lose their horns, or go away
> altogether, but the bandwidth that microwave provided just
> can't compete with fiber -- and the current glut of fiber means
> that in most cases, there's a cheaper, better way to do it.

A further reason for the demise of microwave towers relates to the rise
of C-Band satellite antennas for television signal distribution.

Two frequency bands commonly used for terrestrial microwave transmission
were (and still are):

4-GHz band (3.7-4.2 GHz)
6-GHz band (5.9-6.4 GHz)

When the FCC first authorized the use of geostationary satellites for
communications, it apparently perceived the service as a sort of
long-distance substitute for terrestrial point-to-point microwave
systems. Consequently, it assigned those same two frequency bands to
the satellite service: 6-GHz for uplink and 4-GHz for downlink.

Consistent with this perception, the FCC imposed technical requirements
on all ground-based satellite antennas, both transmit and receive, to
prevent interference to or from existing terrestrial microwave antennas:

- Satellite antennas had to meet technical specifications
concerning beamwidth, gain, and sidelobe performance.

- They had to be licensed by the FCC, even if they were
used for receive-only service.

- As part of the licensing procedure, they were subject to the
"frequency coordination" process. Under this process, the
license application had to include a technical study
showing that the proposed antenna would not cause
interference to, or receive interference from, any existing
terrestrial microwave antenna using the same frequencies.

Several companies went into the business of performing satellite
frequency-coordination studies. Some of the early coordination studies
look quaintly primitive today: an "interference map" was printed on a
large sheet of semi-transparent paper so that it could be laid over a
USGS 7.5-minute quad map. Since the printers of the day could only
print text, interference data was shown as text numerals scattered
across the map. Terrestrial microwave beams stood out clearly as
strings of numerals shooting across the map.

One of the largest frequency-coordination firms was ComSearch,
originally in Dallas, now in Virginia's DC suburbs. ComSearch still
does frequency coordination studies for satellite ground stations; see
<http://www.comsearch.com/eng_svc/svc-sat-freq.jsp>.

In 1975, HBO launched its signal on a geostationary communications
satellite for cable TV operators nationwide. Within a year or so, two
other companies followed suit:

- Ted Turner's Turner Communications Group launched the signal
of Atlanta UHF station WTCG (now TBS Superstation).

- Pat Robertson's Christian Broadcasting Network launched its
CBN network feed (which, after morphing several times, is now
ABC Family).

Both of these services were quickly accepted by cable operators. They
offered operators something not available from any other source:
basic-tier programming aimed at a national audience.

Of course, cable operators had to build receiving antennas to get these
new services. Under FCC rules in force at the time, they had to go
through the whole frequency-coordination and licensing procedure. In
order to meet the technical requirements, these antennas generally used
10-meter reflectors installed on large concrete foundations. Cable
operators installed a lot of them during 1975-77.

Because of the cost of 10-meter antennas, cable operators petitioned the
FCC to have the rules changed to allow smaller antennas, and within a
few years, the FCC relaxed the technical requirements so that antennas
as small as five meters could be used. But FCC still had to protect the
existing terrestrial microwave service, so it didn't eliminate the
frequency-coordination and licensing requirements.

Other companies soon began constructing satellite antennas for
television reception. Holiday Inn constructed the "HI-Net" network to
distribute HBO (or Showtime) and a proprietary videoconferencing service
to its franchisees. The broadcast industry began using satellites to
distribute syndicated programming.

All of these antennas were constructed under FCC rules in force at the
time, and all were licensed. Although the licensing procedure was an
expensive hassle, licenses are valuable insurance: a license gives the
owner of a receive-only satellite antenna the same legal standing as a
telephone company. Even a small cable operator or a Holiday Inn with a
receive-only antenna could block AT&T's attempt to build a new
terrestrial microwave link.

Meanwhile, a new phenomenon was beginning to take shape: the backyard
dish industry. Many entrepreneurs started making and selling small
C-band antennas to private individuals. These antennas didn't meet FCC
requirements, but they worked well enough for most buyers (except for
the unlucky few who found themselves in a 4-GHz terrestrial microwave
path). Besides, the programming was free!

At first, the FCC didn't pay much attention to all this. After all,
since these antennas weren't legal in the first place, the FCC didn't
much care if they didn't work because of terrestrial interference.

But Congress was paying attention: backyard dish owners were becoming a
noisy voting block. Under pressure from Congress (and, probably,
foreseeing the inevitable), the FCC rescinded the frequency-coordination
and licensing requirements, essentially opening the field for anybody to
own a backyard dish legally.

While all this was going on, the telephone industry was also growing,
with ever-increasing need for more capacity. But getting more capacity
by microwave was becoming more difficult because of all those
receive-only satellite antennas. Licensed antennas were popping up
everywhere. Unlicensed backyard dishes were an even bigger problem:
because they weren't documented, nobody even knew where they were. Even
though the telcos were under no legal obligation to respect backyard
dishes, they were well aware of the PR problems that dish-owning voters
could cause in Congress.

The obvious solution was for the telcos to abandon microwave altogether,
and switch to fiber optics. Once they started laying fiber, they could
get all the capacity they needed, and they could replace their existing
terrestrial microwave capacity at the same time. During the late 80s
and early 90s, many terrestrial microwave paths were abandoned.

A personal case in point. In a previous life, I used to conduct
microwave-interference studies for companies contemplating constructing
new licensed receive-only antennas. On one particular study in the
early 90s, taken from the roof of a downtown-Chicago skyscraper, I
couldn't get an accurate reading: the telco microwave signals were so
strong that they swamped the AGC circuits in the receiver. A few years
later, from that same point, telco microwave signals had almost
completely disappeared: all I got were a few blips from towers many
miles away.

The history of the shrinking satellite antenna can be seen in the
antenna farms at many cable TV headends today. Most newer satellite
antennas are small (5- to 6-meter reflectors) on small foundations (I've
even seen antennas on wood bases!). But sometimes I still run across a
big 10-meter Andrew monster, or at least the massive concrete foundation
where one once stood.

Commander Riker <b...@easylinkgroup.zzn.com> wrote:

> A random thought. Why would AT&T kill the Microwave Towers? In
> the case of a nuclear attack or terrorist attack, wouldn't it
> be wise to have the Microwave towers as a backup?

Or as decoys.

According to an old legend popular among cable TV engineers, a certain
cable TV operator was having trouble with hunters shooting at
tower-mounted microwave antennas at a remote headend site. He solved
the problem by placing a large decoy antenna, complete with a painted
target, low on the tower.


Neal McLain
nmc...@annsgarden.com

Reply all
Reply to author
Forward
0 new messages