.




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.   

 

 

 

Last modified:
02 nov 2001

© millennium
1999 - 2002
http://www.qsl.net / s57onw /