Tropospheric Ducting

    Most of the time on the VHF/UHF bands to be able to contact other stations you must have line-of-sight with another amateur station to be able to contact them.  The VHF/UHF radio-path horizon distance exceeds the geometric horizon by approximately 15% so there is actually a small provisions that enables you to contact stations where your antennas cannot see each other.  Nonetheless, 15% is not much and in most applications this will only allow you to contact stations around 10 miles further on simplex.

    In conditions of tropospheric ducting however, it is possible to contact stations that you do not have line of sight with.   Tropospheric ducting is quite a rare happening in the Chicagoland area.  In costal areas it is much more common to experience strong levels of ducting.  When ducting happens you will hear interference on your local repeaters and possibly even voices coming through on distant repeaters that happen use the same frequencies as your local repeaters.  The distant repeaters will most likely use different PL tones because they are often located in different states.  If you do hear distant repeaters on your radio, try to key them up and hold a QSO with someone in a different state.  From my experiences, I think tropospheric ducting can be quite exhilarating. 

    The upper atmosphere is made up of mainly nitrogen and oxygen with a little hydrogen, helium, and other gases mixed in. All the atoms making up these gases are neutral which means they have no charge. As the atoms absorb ultraviolet radiation from the sun a process begins and ends with atoms being positively charged. These positively charged atoms are known as Ion's. This process I have mentioned is called Ionization.  This is where we get the term Ionosphere. The sun goes through cycles (11 years to a full cycle) where sunspots are high or give off more radioactive energy. This causes the Ionosphere to intensify and allows for a radio signal to more easily bounce back to the earth.

    Ducting occurs when humidity and temperature create another "wall" of ionized particles in the troposphere.   This creates a tunnel or duct. Our signals get "sucked up" into this duct and bounces between the two layers until it finds a path back to earth. This can be several hundred miles away from its point of origin. Signals that are normally too weak to cause interference become quite strong and can drown out weaker signals on our band.

Typical meteorological conditions which can be favorable for ducting are:

   1.Warm dry air over a cooler surface, especially a cool sea

   2.Surface cooling under clear skies above

   3.Anticyclone (high pressure) or developing high pressure ridges with a cold surface

   4.Sea breezes undercutting warm air above

   5.At fronts with a strong thermal contrast