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

> Thanks for a very informative post. I changed the channel
> width numbers to "MHz", instead of "GHz".
Oops ... I'm suitably embarrassed.
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.
I've heard of such agreements, but I've never been involved
in such a case. I assume that such an agreement would be a
permanent recorded easement on the land so that it would be
binding on all future landowners if the land were sold or
I suppose the same type of agreement could apply to
satellite antennas. I was once involved in a situation
where a new building blocked the view from existing
antennas to several satellites near the center of the
geostationary orbit. Since no easement existed, the
antenna owner had to install new antennas.
In this particular case, the antenna owner was KSDK-TV (NBC
Channel 5, St. Louis), and the new building was the Thomas
F. Eagleton Federal Courthouse immediately to the south.
The existing antennas were located on the roof of the KSDK
studio, a two-story building. Fortunately, it was
possible to install the new antennas on the roof of an
office tower located next to the studio building. Although
the office tower wasn't as tall as the new courthouse
building, studies showed that the new antennas could be
placed so that they would just "peek" over the courthouse
formatting link

Neal McLain
***** Moderator's Note *****
IANAL, so I won't speculate on how the details are worked out, but
some companies do think air rights are important and are willing to
pay for them, either because they're doing something with the links
that's too profitable to risk interruption (as in the case of a tv
station), or where the anticipated costs of moving the path (which
might include negotiating leases under time pressure) are so high that
they're unthinkable.
If the federal government bought some land that was encumbered by air
rights, I wonder if they'd be obligated to honor them: they're not the
sovereign that encoumbered the land, so that's another factor. I
suppose that insurace would be available, so if any of the readers are
in that business, feel free to chime in.
BTW, speaking of "peeking" over another building, can you explain why
the fresnel zone around a dish is so large? I know it exists and that
I have to allow for it, but I've never understood the reasons.
Bill Horne
Temporary Moderator
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Reply to
Neal McLain
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In Los Angeles there must be such an agreement, there are building around the Grand office of at&t, because if you look around the area you can see the microwave is clear.
Reply to
Steven Lichter
Bill, it's basically just straightforward diffraction of electromagnetic fields, the same at radio or microwave frequencies as at optical frequencies, combined with scattering effects produced by metal or dielectric structures or boundaries.
The general rule of thumb is that the diffraction effects become stronger, and the scattering calculations messier, when either the dimensions of the structures or the distances involved become closer to the wavelength of the radiation. Diffraction effects for optical beams (where wavelengths are generally much, much smaller than lens or aperture diameters) are generally "clean" and simple to calculate. Diffraction and scattering effects get messier and harder to calculate for microwave or especially radio frequencies.
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