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Re: Boston FMs
<<On Mon, 20 Aug 2001 19:58:26 -0700 (PDT), Matthew Osborne <firstname.lastname@example.org> said:
> Whats the technical term I've heard used on this list
> for that before, the tropo?
``Tropo'' is short for ``troposphere'', which is the lowest layer of
the atmosphere. The most common form of VHF DX is what's known as
tropospheric ducting (or ``trop'' for short), because it's the only
mechanism that operates entirely at that layer of the atmosphere.
Recall that radio waves, like all electromagnetic radiation, are a
form of light. Other modes of VHF DX operate reflectively, high up in
the atmosphere, and span very large distances. (Consider drawing a
straight line perpendicular to the transmitter tower straight out into
the upper atmosphere, and then reflecting back -- at the same angle,
just as with light -- to a another point on the ground. That's how
E-skip and meteor scatter work.)
Trop, by contrast, operates in a refractive mode. Just as with a
lens, the angle of refraction depends on the speed of light in the
material, which in the atmosphere depends entirely on the density of
the air. Here in the Boston area, such conditions are provided
regularly thanks to the influence of the ocean. At night, the
55-degree waters of Cape Cod Bay cool the air above it, which makes
that air more dense. The built-up areas retain heat and radiate it
back into the atmosphere above them, making that air warmer and thus
less dense. The result is an almost perfect ``lens'' directing radio
signals from the Cape into Boston, so I can hear WFCC, WKPE-FM, WOCN,
WQRC, WPXC, WRZE, and WXTK most summer nights.
This phenomenon is common throughout the cool-water regions on both
coasts. In other places, a more specific (and usually active) weather
phenomenon -- such as a tropical storm -- can cause similar effects.
(Recall that one of the hallmarks of a hurricane is low barometric
pressure. Low pressure implies low density.) In more unusual
conditions people in Boston have heard stations on tropo ranging from
Atlantic City and DelMarVa all the way up to Nova Scotia.