Tropo (tropospheric bending)
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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