Abstract :
Propagation tests indicate that the skywave field strength from a medium-frequency broadcasting transmitter can be reduced by 16dB on paths to the north at midlatitudes in the southern hemisphere, when vertically polarised transmission is replaced by orthogonal transmission. The reduction is observed to vary with time and path bearing, the variation with time being such that, during any one night, the median reduction is a maximum when the field strength is high, and a minimum when the field strength is low. Short-term variations are considered to be consistent with a small variation in the limiting polarisation of the ionosphere, and with ray deviations from the most direct path. Long-term variations are considered to be due to changes in the nondeviative absorption from night to night. The observed change in improvement factor with path bearing is in agreement with that expected for the transmitting-aerial configuration. Implementation of this system awaits the development of suitable transmitting aerials.
