Fadings of VHF Signals at 70.3 MHz and 160.6 MHz in a Metropolitan City

It is observed that when the vehicle mounted radio receiving systems move through the roads of a metropolitan city, the rapid fluctuations of the received signals occur. This paper deals with the theoretical explanation of such a behaviour for the systems operating at 70.3 MHz and 160.6 MHz. The theoretical model developed in this paper accounts for the signal losses from the diffraction phenomenon occuring over and around the physical obstructions(like buildings, hillocks), and reflections from the structures, it has been assumed that 2 the diffraction losses depends on the log h or W2/d', where d' is the distance between the measurement spot and the physical structure, h is height and w is width of the obstacle. The field strengths of the received signals at various locations in the city areas have also been measured. The resemblance of the experimental and theoreti­ cal behaviour has been discussed and analysed in this paper. It has been ooserved that vehicle mounted radio receiving systems moving through the streets of a metropolitan city like Bombay, experience large and rapid fluctuations of the signal amplitude over the transmitted signals (1,2,3). A close observation of the received signal waveform reveals, number of interferences minima in the signal magnitude distribution. The type of interferences lead to a most commonly used term 'fading'. Fading may be considered as the result of the diffraction and reflection of VHF waves from the physical obstructions such as high rise buildings, hillocks etc. The prediction area of coverage with respect to 'fading' and the potential interference of an existing radio signals is of vital importance in the spectrum planning and management. The aim of this paper is to develop a theoritical model to study the behaviour of incoming VHF signals at various locations in the city areas. This model accounts for the losses in the transmitted signal strength due to diffractions and reflections from the physical structures. The theoretically calculated received field strengths have been compared with the experimentally observed field strengths at the various locations in the Bombay city, and a close resemblance between the two has been found. Field strengths on two land mobile radio systems operating at 70.3 MHz and 160.5 MHz have been measured at various locations in the city. At these locations weak signal strengths have been received. 2.0 Description_of_the_s^stems The characterstics of the two systems are, 1. Transmittersjoower-20W (for 160.6 MHz system link) -15W (for 70.3 MHz system link) Antenna gain is zero due to omni directional type and height is approx. 20 m. 3. Tyge of mobile_antenna