If you are using mobiles or portables that utilize extended coverage means, (i.e. Repeaters, Cellular Towers, etc.), then you have had periods when your communications seemed less than satisfactory. Excessive amounts of static, intermittent operations, squealing or whining and we don’t mean from the radio users. One significant cause of this problem is a phenomenon known as "Tropospheric Ducting".

Ducting of radio waves can influence signal levels at all distances from purely local to beyond 2500 miles. There are 2 modes of ducting, Extended Local and Long Distance. The Extended Local is more common and ordinarily effects spectrum within 200 miles. This can have an incredible impact on frequencies in both the VHF and UHF range.

Weather fronts are usually the cause of this phenomenon and the radio waves are introduced between the inversion layers and the ground. The inversion layer is where cold and warm air layers meet. This forms a duct or tube in the atmosphere where radio signals enter and are bent. Signals then follow this duct and drop out at the other end. Tropospheric bending is more noticeable in regions where the climate is influenced by large bodies of water such as "the Great Lakes Region" or a "coastal region" is influenced by large bodies of water and enjoy the greatest degree of tropospheric bending. Hot & dry desert areas see little of it.

Tropospheric effects can show up at any time, in any season. However, late spring and early fall are the most favored periods. Signals from far outside normal communication range can suddenly appear causing interference with local repeaters. When this happens mobiles and hand helds will have problems trying to overcome these signals or their communication can be blocked altogether, depending on the strength of the interfering signals. "Ducting" may last anywhere from a few minutes to hours. It may linger for weeks, and cause periodic interruptions.

In most radio applications there is usually a single transmitter or repeater used to give radio coverage in a given area. The output of these transmitters can often be as much as 150 watts (500 watts in paging). Which is why "ducting" is more extensive in land mobile operations. Although higher power transmitters have a greater probability of "ducting", even low power transmitters can be "ducted". As in the cellular field where there are multiple lower power transmitters (Cell Sites). Have you have ever applied for a license in the "Great Lakes Region" and have had it held up or denied because of Canadian objections? This is what they call " Could cause harmful interference!!" And is their primary concern. When these (ducting) conditions occur there is nothing humanly possible we can do to prevent them. We can only hope that the weather front or conditions move off quickly and take the ducting inversions with them.