Point-to-point microwave links (was 1964 World's Fair) [Telecom]

I'd like to know -
> 1. What [point-to-point microwave] band(s) is/are available
The two frequency bands most commonly used by AT&T Long
Lines for terrestrial microwave transmission were:
4-GHz band (3.7-4.2 GHz)
6-GHz band (5.925-6.425 GHz)
In a typical two-way link, different bands would be used in
opposite directions.
Each band encompasses 500 MHz and can be subdivided into
various frequency blocks ("channels") depending on the
application. The most common channel plan provides 24
channels, each 40 MHz wide. The 4-GHz frequency plan is at
formatting link
-- the 6-GHz frequency plan is
identical, but shifted up 2.225 GHz.
My guess is that the microwave link mentioned by Fred
Goldstein in V27:189 ["main hop (probably 6 GHz) was an
established Bell route"] was one of those 40-MHz channels.
The video signal was probably frequency-modulated, with an
aural subcarrier at 4.5 MHz.
When the FCC began authorizing point-to-multipoint
satellite service, it envisioned it as an extension of the
existing point-to-point microwave service. It authorized
the use of the 6-GHz band for uplinks, and the 4-GHz band
for downlinks. Furthermore, it imposed the same licensing
requirements on satellite antennas that were already in
place for microwave stations.
The FCC apparently didn't anticipate the rise of consumer
satellite antennas. Conflicts between existing 4-GHz
point-to-point microwave links and consumer downlink
antennas (legal or otherwise) soon became a hot political
formatting link
Another important point-to-point microwave band is the
12.7- to 13.2-GHz block, often called the 12-GHz band. Like
the 4- and 6-GHz bands, the 12-GHz band can be subdivided
into various frequency blocks for specific applications.
This band is available for use by several industries
(telephone, television broadcast, common carrier, cable TV,
among others) on a "co-equal" (first-come-first-served)
basis. Several channel plans exist:
Group A - (20) 25-MHz channels.
Group B - (19) 25-MHz channels.
Group K - (40) 12.5-MHz channels.
Channels in these groups can be used for point-to-point FM
video transmission, with an aural subcarrier at 4.5 MHz.
Other industries can use them for other purposes (e.g.
blocks of TDM or FDM voice circuits), but I'll leave it for
others to fill in the details.
Groups C, D, E, and F - (up to 43) channels of various
widths, were designed to match the cable television
distribution spectrum. These blocks were intended for
local point-to-multipoint cable TV signal distribution
systems using standard NTSC analog video signals.
formatting link

Back in the 1970s, the cable TV industry (and the common
carriers that served them) made extensive use this band.
Groups A, B, and K were used to import distant television
broadcast stations to cable TV headends. A case in point:
Midwestern Relay Company (a microwave common carrier owned
by the Journal Company) carried the signals of WVTV
(Milwaukee) and WGN-TV (Chicago) to cable TV systems
throughout Wisconsin and parts of Minnesota.
Groups C, D, E, and F were used for "AML" (Amplitude
Modulated Link) local signal distribution. Cable companies
built numerous AML systems to interconnect central headends
with nearby communities up to 30 miles away.
Making equipment for this purpose was such a lucrative
business that no less a company than Hughes Aircraft
Company was the principal supplier. When I first
encountered AML equipment, I thought it strange that a huge
defense contractor like Hughes would be in the cable TV
equipment business. But it made sense: only a company with
Hughes' enormous technical expertise and financial
resources could have tackled such a project. Furthermore,
Hughes didn't have to worry about quarterly earnings: it
served only one stockholder, the Howard Hughes Medical
But after cable companies began using fiber for local
distribution, most AML microwave systems were dismantled.
Numerous old antenna towers were abandoned, removed, or
recycled into cell towers.
As for Hughes Aircraft, the bottom fell out of its market,
and it left the business. But AML equipment is still in
use today in developing countries, and several smaller
companies still manufacture and sell new or refurbished
I'll skip over questions 2-6. Perhaps someone else will
tackle these questions.
7. If you ever bought "air rights" for any of your paths,
> and if you think they're worthwhile.
I assume by "air rights," you mean an FCC license. Back in
the time period that started this thread ("early 1970s"),
you didn't "buy" a license. You applied to the FCC for a
construction permit, and after spending a great deal of
money on site surveys, frequency coordination, and legal
expenses, you (hopefully) got your permit. Once
constructed, then you spent more money getting it licensed.
It was good for a fixed period, and you had to renew it
every few years.
Was it worthwhile? Compared to the cost of getting a
signal from A to B by any other (then existing) means, it
was the only feasible way for cable TV companies to do
it. Of course, the rise of satellite communications and
fiber optics since the 1970s has dramatically lowered
costs and increased capacity. So today, it probably
wouldn't be worth it except for unusual situations
such as river crossings.
On the other hand, it appears that cellular companies
still think it's worthwhile for some purposes. A cell
tower can support a microwave antenna at a relatively
small incremental cost, so running a couple of T-1s
over a microwave link probably costs substantially
less than leasing the circuits.
Neal McLain
***** Moderator's Note *****
Thanks for a very informative post. I changed the channel width
numbers to "MHz", instead of "GHz".
When I say "Air Rights", I'm not talking about an FCC license:
everyone has to have a license, and of course people didn't use to pay
anything but a nominal fee for them (although the bands might be
allocated by auction these days), the rights I was thinking of are,
literally, rights to the air above the properties between two
miccrowave sights. In other words, they are contracts between
microwave licensees and property owners, which obligate the land owner
to refrain from building above an agreed-upon height, so that the
structures never cut off the microwave path.
I thought such agreements were common, but I guess not: I know that
the phone company sometimes entered into them, but I don't know how
much such rights are worth and what the tradeoffs are if a microwave
licensee chooses to forego them and risk an obstruction.
Bill Horne
Temporary Moderator
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Neal McLain
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