=  NOW 40 WPM  =  
ON SUNDAY EVENING OF THE ARRL VHF CONTEST ON SEPTEMBER 13.  BUT THE TROPO
CAME IN THE FOLLOWING 2 DAYS AFTER THE CONTEST.  THE TROPO SETTLED INTO THE
MIDWEST ON SEPTEMBER 14 SEE FIGURE 1.  KF0M EM17 WORKED STATIONS INTO
TEXAS.  ON SEPTEMBER 15, N0JK EM28 WORKED NORTHEAST TO K9MU EN44 ON FT8.
THE TROPO DRIFTED EAST, AND K1HTV FM18 SAID THE BAND WAS OPEN CONTINUOUSLY
FROM 0130Z ON SEPTEMBER 16 TO 0300Z ON THE 17TH.  HE WORKED 66 STATIONS IN
14 STATES ON FT8.  THE BEST DX CONTACT F

JON JONES, N0JK, N0JK ARRL.  ORG THE WORLD ABOVE 50 MHZ AUTUMN WEATHER
BRINGS TROPO DX CRISP SEPTEMBER MORNINGS AND SUNNY DAYS BROUGHT SEVERAL
OUTSTANDING TROPOSPHERIC OPENINGS IN SEPTEMBER.  ON SEPTEMBER 15 AND 16, A
HIGH PRESSURE SYSTEM SETTLED OVER THE EASTERN HALF OF NORTH AMERICA.
STATIONS INCLUDING KR1ST FN21, WZ1V FN31, AND K1HTV FM18 WORKED AS FAR WEST
AS WA0I EM39 IN CENTRAL MISSOURI ON 2 METERS.  RON, WZ1V, CALLED IT THE
BEST TROPO OPENING IN 10 YEARS.  TROPO APPEARED AGAIN ON SEPTEMBER 25 AND
26.  THAT EVENING, STATIONS IN THE DALLAS FORT WORTH METROPLEX WORKED TO
NEW ENGLAND.  USING TROPOSPHERIC PROPAGATION FOR DX TROPOSPHERIC
PROPAGATION OCCURS WHEN THE DIFFERENCES IN AIR TEMPERATURE AND HUMIDITY
CAUSE A GRADIENT THAT CAN REFRACT RADIO SIGNALS BACK TO THE EARTH BEYOND
THE LINE OF SIGHT HORIZON.  IT IS QUITE DIFFERENT THAN SKIP ON HF RADIO,
WHERE THE IONOSPHERE BENDS AND REFLECTS RADIO SIGNALS.  THIS TAKES PLACE 50
250 MILES HIGH.  TROPOSPHERIC PROPAGATION, OR TROPO, TAKES PLACE ONLY A
MILE OR SO ABOVE THE EARTHS SURFACE.  IT IS CAUSED BY A TEMPERATURE
INVERSION.  THIS IS A BOUNDARY LAYER WHERE A SHARP CHANGE TAKES PLACE IN
TEMPERATURE AND HUMIDITY, USUALLY CAUSED BY A LAYER OF WARM, DRY AIR OVER
COOL, MOIST AIR.  NORMALLY THE MOISTURE CONTENT AND TEMPERATURE OF THE
LOWER ATMOSPHERE DECREASE WITH ALTITUDE.  IN AN INVERSION, THE TEMPERATURE

SUDDENLY RISES, AND THE MOISTURE CONTENT DECREASES SIGNIFICANTLY.  RADIO
SIGNALS GOING THROUGH THE INVERSION ARE BENT AND REFRACTED BACK TOWARDS THE
EARTH.  SOME INVERSIONS BEGIN AT THE GROUND WITH FOG.  THE INVERSIONS THAT
ARE MOST USEFUL FOR VHF/UHF DX TAKE PLACE 1,500 6,000 FEET HIGH.  WHEN A
STRONG INVERSION OCCURS, THE TEMPERATURE RISES, AND THE HUMIDITY OF THE AIR
DECREASES IN A LAYER.  THIS CAN CAUSE SUPER REFRACTION OR DUCTING.  THE
ABILITY OF AIR TO REFRACT RADIO SIGNALS IS DEFINED AS N, WHICH INVOLVES
TEMPERATURE, MOISTURE CONTENT, AND AIR PRESSURE.  COOL, MOIST AIR HAS A
HIGH N VALUE.  WHAT WE ARE INTERESTED IN IS THE CHANGE IN N, OR DN.
TROPOSPHERIC PROPAGATION IS STRONGEST WHERE THE INDEX OF REFRACTION, OR DN,
CHANGES SHARPLY WITH ALTITUDE.  NOT ALL RADIO SIGNALS ARE REFRACTED TO THE
SAME DEGREE.  IT TAKES A HIGHER DN GREATER THAN 19 TO REFRACT 2 METER
SIGNALS THAN 70 CENTIMETER SIGNALS.  THE INVERSION LAYER DEPTH ALSO
DETERMINES THE FREQUENCY OF SIGNALS THAT MAY BE REFRACTED.  UHF SIGNALS CAN
BE REFRACTED BY SHALLOW LAYERS, LOWER FREQUENCIES NEED A DEEPER LAYER.  FOR
50 MHZ SIGNALS TO REFRACT BY TROPO, THE LAYER NEEDS TO BE OVER 1,000 FEET
=  END OF 40 WPM TEXT  =  QST DE W1AW  <