On the properties of elevated layers over Calcutta and its effect on VHF propagation

With a view to studying the tropospheric irregularities, a multistation observation facility was installed by the Indian Statistical Institute (ISI), Institute of Radiophysics & Electronics (INRAPHEL) and Bengal Engineering College (B.E. College) around Calcutta. In all these three stations necessary provision have been made for simultaneous recording of TV transmission (in the frequency band of 189 MHz) from Satkhira centre in Bangladesh 75 kms from Calcutta. Over Calcutta the authors have detected the presence of elevated structures in different seasons by means of a sodar operating round-the-clock at the ISI. These layers are generally associated with local stability of a large degree. The presence of any type of elevated layers (horizontally, ground-based, or titled) tend to increase the field strength by about 10-25 dB. Titled layers, probably as a result of focussing effects due to layer tilt, appear to produce the greatest enhancement of signal strength. Also they act as a phase changing screen in the propagation path of the VHF signal. As a result, a diffraction pattern is produced at the receiving point of the VHF signal