Two aspects of propagating radiowaves causing deterioration in the quality of received signal are studied. Estimated values of absorption obtained from the two CCIR formulae (CCIR, 1978, Second CCIR computer based method for estimating skywave field strength and transmission loss, Report 252-2 (supplement), International Telecommunication Union, Geneva; and CCIR, 1990, Third computer based method for estimation of field strength (2-30 MHz), signal to noise ratio, LUF and basic circuit reliability, Recommendations and Reports of the CCIR, 1990, Vol.VI Report No.894-2, International Telecommunication Union, Geneva) have been compared with measured values and it was observed that CCIR (2) has an advantage over CCIR (1) both in accuracy and simplicity. The differences between the measured and estimated absorption are within 2 to 3 dB, in case of CCIR (2) method. From the analysis of recorded data of field strength during the daytime it is observed that the fading is mostly due to multipath propagation of radiowaves causing selective fading, resulting in linear distortion of the received programme. It is also found that the fading range tends to be higher at higher frequencies and also during equinoxes in comparison to other seasons. Based on the study a fading allowance of 9 dB is proposed on a short-term basis to ensure stable conditions of the signal for optimum signal to noise ratio for communication purposes.
展开▼
