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Tropo - tropospheric bending
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Tropo is the other major form of DX propagation; as the name implies,
it's dependent on conditions in the troposphere where
weather takes place. In contrast to Es, tropo is best on higher
frequencies --- though there is no downward
progression of "minimum usable frequency". As a rule tropo
is best on UHF, very good on the high VHF band and FM, but definitely
inferior on the low VHF band. However, unless
you have a top-notch UHF receiving installation,
it may seem to you that tropo is best on high VHF and FM bands.
Tropo occurs along temperature inversions, often associated
with frontal passage. It often happens over a large, stable
high pressure area ahead of a cold front, especially where there is an
influx of warm air from the Gulf mixing with colder
air from the north. By correlating your tropo
DX with weather maps, you should eventually be
able to recognize the conditions likely to produce tropo
in your area. Pay special attention to areas of the same atmospheric pressure
(connected by isobars).
Extremely
long distances (up to 1500 miles on UHF) may apply when,
as rarely happens, the front is a straight line between
you and the station. Tropo is legendary along the Gulf Coast --- where
it's known as Gulf tropo. This has been known to blanket the
entire coast up to 250 miles inland for a week
at a time. This usually happens in non-frigid portions
of the winter, and in the fall and spring.
Arid high elevations and mountainous areas form an
effective barrier to tropo. Thus there are no instances
known of tropo across the Rocky Mountains. Colorado and New
Mexico stations east of the mountains do occasionally
get tropo. Gulf tropo extends as far inland as
Monterrey, Mexico, and as far south as Veracruz and
other points along Campeche Bay. The entire island of Cuba
can make it to the US on tropo. Other Caribbean islands have never
reached the US on tropo; but easterners should
be on the lookout for Bermuda, which
has. Eastern mountain ranges are neither high nor dry
enough to block out tropo. The midwest and Great Plains are perhaps
second only to the Gulf Coast as prime areas of tropo
activity. Areas around the Great Lakes are also excellent.
DXers in cold northern climes may expect
little if any tropo during the winter months, except
during abnormal warm spells. The spring and fall months seem to
be the best, when there is a fairly wide temperature variation between
day and night.
Ordinary tropo builds up quickly after sunrise but tends to "burn off"
during the hot afternoon hours; it may fade back
in after sunset from the same area seen in the morning.
Tropo may link up with other propagation
modes, making it difficult to ascertain just how the
signal gets from one place to another. Transequatorial scatter
reaching the latitude of the Tropic of Cancer may be
spread further by simultaneous tropo; instances of Es in the 1500-1900
mile range may be explained by a tropo link-up at one or both ends.
There is no minimum distance for tropo. Depending
on your equipment, you may notice tropo improvement
on stations as close as 50 miles; with a reasonable setup
east of the Rockies, distances in the range up to 600 miles are
not uncommon. UHF distances may at times surpass 1000 miles.
Tropo ducting is a condition which seems to behave rather like "skip",
in that a nearer station in the same direction,
on the same channel, may not necessarily block
out a more distant one. The signal
is actually ducted between air masses
at different heights. As a result, the duct may pass
over a closer station. Ducts are often frequency
selective and may, for example, "carry"
a few UHF channels and not affect
others. Ducting may appear at any time of the day or night,
and is the cause of most tropo over 400 miles.
A duct may appear and vanish in little over an
hour, or last for days. Tropo is the
"steadiest" of any propagation; it seldom has rapid
fading, but may fade slowly in and out.
Weak tropo in the range slightly beyond that
normally received is often
called extended groundwave.
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