Estimation of HF Skywave Fieldstrength in Tropical Region Using Air Method (Revised for E-Mode Propagation)

Abstract

The ‘AIR Method’ for the estimation of H.F. Skywave field strength laid by broadcast transmitters in the Tropical Zone, was developed by India (Rao 1969) as a result of long-term ionospheric absorption measurements on 5-MHz frequency at New Delhi. The method is simple in application with a set of easily readable nomographs. The applicability of this method has been put to test on a large number of broadcast circuits over the years and it has been found that the method is quite suitable when transmissions are via F-layer of the ionosphere. However, when transmissions are via E-layer, the method did not apply well. In this paper, an attempt has been made to modify this method suitably so that it can be applied satisfactorily when transmissions are via E-layer of the ionosphere. The comparisons of the measured values of field strength with those estimated using the modified formula for transmissions via E-layer, indicate good correlation.

Indexing Terms:

• H.F. Fieldstrength
• E-layer Propagation
• Ionospheric Absorption