Propagation: Tropo (tropospheric bending)

   
 
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27 June 2017, 21:56 UTC 


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Propagation:
  Sporadic E skip (Es)
  Backscatter
  Tropo
  Meteor Scatter (MS)
  F2 Skip
  Aurora
  Trans-equatorial scatter (TE)
  Lightning scatter (LS)

Propagation
by Glenn Hauser 
News

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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