 a value of a quantity SO that it can be 15 Directive gain of an antenna converted to a specific value of that quantity. This is a The ratio of the field strength at a point in the direction practice of convenience which has developed, SO it is best of maximum radiation to that which would be obtained to start with the basic role of the decibel as a dimen- at the same point from an isotropic antenna, both sionless number. The bel itself is not normally used in radio work. antennas radiating the same total power. 16 Antenna beamwidth Decibels and power The angular width of the major lobe of the antenna ra- diation pattern in a specified plane. The usual criterion If we wish to compare two powers, P1 and P2 then we for beamwidth is to measure between points either can do SO by dividing one by the other. The resulting ratio side of the beam axis where the power density has fallen is P1/P2 and is a pure number. To express this in decibels to half (3 dB down) of that on the axis. This is usually the form is: referred to as the '3 dB beamwidth'. Ratio (dB) = 0 log(P1/P2) 17 Equivalent radiated power (ERP) If P1 = 1600 W and P2 = 2W then the simple ratio is: The product of the power into the antenna and the gain referred to a dipole. It is often used to specify the power of UHF/VHF broadcast transmitters. The ratio in decibels is 10 log 800 = 29.03 dB. 18 Equivalent isotropic radiated power (EIRP) Since the decibel is based on logarithms, a number of The product of the power into the antenna and the gain simplifications follow. The basic rules for ratios which are referred to an isotropic antenna. pure numbers are therefore: 1. Multiplying numbers merely requires the addition of 19 RF machines and RF plant the decibel values. RF energy is now increasingly used to undertake 2. Dividing numbers requires the subtraction of one manufacturing operations which use heating and these decibel value from the other. terms are used here to refer to machines generally. In As powers can be in kilowatts or megawatts, it can practice t