Practical_Antenna_Handbook_0071639586

Space wave
40 p a r t I I : F u n d a m e n t a l s
Tropospheric wave
Temperature
inversion
zone
h
D 1
D 2
Earth's surface
Figure 2.20 Ducting phenomenon.
Ducting allows long-distance communications from lower VHF through microwave
frequencies, with 50 MHz being a practical lower limit and 10 GHz being an illdefined
upper limit. Airborne operators of radio, radar, and other electronic equipment
sometimes report ducting at even higher microwave frequencies.
Antenna placement is critical for successfully using ducting as a propagation medium.
Both the receiving and the transmitting antennas must be either (1) inside the
duct physically (as in airborne cases) or (2) able to propagate at an angle such that the
signal gets trapped inside the duct. The latter is a function of antenna radiation angle.
Distances up to 2500 mi or so are possible through ducting. Certain paths where frequent
ducting occurs have been identified:
• Great Lakes to Atlantic seaboard
• Newfoundland to Canary Islands
• Florida to Texas across the Gulf of Mexico
• Newfoundland to the Carolinas
• California to Hawaii
• Ascension Island to Brazil
Another condition is noted in polar regions, where colder air from the land mass
flows out over warmer seas (Fig. 2.21). Called subrefraction, this phenomenon bends EM
waves away from the earth’s surface—thereby reducing the radio horizon by about 30 to
40 percent.