A v.h.f. field-strength survey on 90 Mc/s

Abstract

The paper discusses the results of a field-strength survey on 90 Mc/s from a site at Wrotham, Kent. The effects of the height of the site and of the transmitting aerial and the profile of the transmission path on height gain are discussed in Section 1. Measurements in built-up and hilly districts are discussed in Section 2. Those in flat built-up areas indicate that buildings of normal height in London reduce the average field at a receiving-aerial height of 20 ft by about 10 to 12 db, and that an average for one street will be the average for the district to an accuracy of about ± 4 db. The variation of field strength along the street is less with horizontal than with vertical polarization. In hilly country, the minimum field strength occurs on the near-side slope of a valley and not at the lowest point. The minimum is slightly lower with horizontal than with vertical polarization, and in this case the latter is slightly to be preferred. In Section 3, graphs of measurements along ten radials using vertical polarization and three using horizontal polarization are discussed. These measurements show that, in general, signals of both polarizations are propagated equally well. A field-strength contour map of south-east England has been prepared from the radial measurements, but the values taken from these have been extrapolated to allow for the use of a high-gain (8 db) transmitting aerial, a 500-ft mast and a power of 25 kW.