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

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-- 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 issue.

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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.

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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 Institute.

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 equipment.

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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