ts per metre, Z is the imped- The primary influence on the atmosphere is radiation from the ance of free space (377 ohms), P is power density in watts per sun, which causes the ionised layers to form and creates global metre, and Pd is power density in watts per metre². climatic and regional weather patterns. Solar radiation follows an 11 year sun spot cycle, which has a direct correlation with ionisation and is recorded on a scale from 0 to about 200. This is a smoothed E2 = ZoPa (27.4) average of the number of sunspots, which are observed disturbances on the sun's surface. A sunspot number is issued by the Sunspot Data Equation 27.5 can be used to convert directly from dBW to dBuV, Centre in Brussels and also the Telecommunications Services where E is in dB. Centre in Boulder, Colorado, U.S.A. and is a smoothed 12 month average of solar activity indicating the degree of ionisation that can E = Pd dB (1W/m2) 145.6 (27.5) be expected. High sunspot numbers are an indication of better conditions for long range h.f. communication and this may also lead to unwanted long distance propagation in the v.h.f. bands creating On the left hand side of this equation, the dB represents a voltage interference in mobile communications and FM and TV broadcast- and not a power ratio. ing services. The voltage at the centre of an un-terminated half wave dipole in The sunspot number being a running average may not be an the path of a radiowave and aligned along the axis of the electric accurate indication of daily or hourly conditions. The solar flux is a vector (i.e. no polarisation loss) is given by Equation 27.6, where V measure of solar activity taken at 2.8GHz and is a better indication is the voltage at the centre of the dipole, E is the field strength in of real time ionospheric conditions. It is measured continuously and volts per metre, and a is the wavelength in metres. quoted on a scale usually in the range from 60 to 260. The US The propagation medium 27/5 100.0 FREQUENCY (GHz) Figure 27.2 Free space loss versus distance