tude modulation Figure 2.5-5 RF signal, amplitude modulated by a low frequency signal. intelligence may be speech, music, and television form of a simple audio frequency sine wave. It can be pictures. Other professional work includes voice and data seen that the peak instantaneous amplitude of the 50% transmission by a variety of methods, radar transmitters modulated wave is against A for the carrier. Clearly the transmit RF signals in a series of pulses and SO on. total power is greater when the carrier is modulated and The process of sending intelligence is referred to as hence any field measurements made will need to be the process of modulation and the technical methods of related to the modulation state. For amplitude modula- doing so are wide ranging and outside the scope of this tion, Figure 2.5-6 shows the relationship between sine book. It is however useful to illustrate the general nature wave amplitude modulation depth versus transmitted RF of amplitude modulation which has some significance power and RF current. when carrying out radiation measurements, and also to Figure 2.5-7 shows a pulse transmission where the illustrate the principle of pulse transmission. carrier is transmitted for time tp (the pulse duration) and Figure 2.5-5 illustrates the waveform of a carrier and with a pulse repetition rate of n Hz (pulses per second). It of the same carrier with 50% modulation applied in the is, of course, not possible to show this to scale since there Power Current Modulation index Figure 2.5-6 Sine wave amplitude modulation depth versus RF power and RF current. CHAPTER 2.5 Introduction to RF and microwave radiation Pulsed carrier transmission Figure 2.5-7 Pulse modulation. will be too many cycles of carrier in each pulse to matter for the purposes of this comparison. An energy actually illustrate them. For example a radar working of 12 eV corresponds to a wavelength of 1.03 X 10-7 at 1 GHz and with 2 us pulses will have 2000 cycles of metres (103 nm) which can be seen from Figure 2.5-8 carrier in each p