1: Calculation of power densities around three TV towers

Calculation method	We calculated the power density of the electromagnetic fields using the effective radiated broadcasting powers of the four TV channels in Box 1.1. The maximum power is directed just below the horizon to ensure signals are received over a wide area. Though large antennas produce "nulls" (or regions of low signal) close to the towers, we assumed that they had been designed to ensure that the signal in these areas would be at least 10% of maximum. We calculated the power density from the formula of the National Council on Radiation Protection and Measurement. 6 Synchronising pulses, which lock the picture to the correct horizontal and vertical positions and ensure correct colours, give overlaying amplitude modulation frequencies of 50 Hz and 15.6 kHz.
Distance calculations	The three towers are situated in a triangle with sides of about 1.5 km (Figure 1). As they are at slightly different heights, a mean height of 130 m has been used in the calculations. The distances given in Figure 2 are from the geographic centre of the towers. For distances up to 2 km, the distance from each individual tower was used in calculating power densities. Beyond 2 km the radiation was considered to effectively emanate from the geographic centre.
Power densities	The calculated power densities (Figure 2) are the power sum of the four TV signals, the power density of each channel being calculated individually, taking into account distance from the relevant antenna and the gain reduction at the calculated angle below the horizon. No account was taken of ground reflections, which vary the signal over short distances of some half a wavelength (2.5 m at 60 MHz to 0.7 m at 215 MHz), nor of signal reduction by buildings, vegetation or ground undulations. The power density at the centre was calculated to be approximately 1 µW/cm 2 , with the maximum in the area being approximately 8 µW/cm 2 and reducing to 0.2 µW/cm 2 at 4 km. Some measurements made in the region of Tower 1 by the Commonwealth Department of Communications found actual levels to be five times less than those calculated (see Discussion). Figure 2: Logarithm of calculated power densities (in uW/cm squared) for TV signals from the three TV towers against distance in kilometres from the centre of the towers.