enna) and this convention is followed in this book. If the diagram is rotated until the electric field is horizontal then the wave polarisation is Examples: horizontal. Apart from this 'linear polarisation', other forms such as circular or elliptical polarisation are also 1. f = 200 kHz a = 1500 metres used for specific purposes. 2. f = 10 MHz a = 30 metres WAVELENGTH Direction of wave Figure 2.5-4 Representation of a plane wave. CHAPTER 2.5 Introduction to RF and microwave radiation Table 2.5-2 Nominal quarter wave antenna length for a number of Table 2.5-4 Microwave band letters (IEEE). frequencies. Frequency (GHz) Band letter Frequency (MHz) Length - one-quarter wavelength (m/cm) 750 m 8-12.5 7.5 m 12.5-18 0.75 m (75 cm) 18-26.5 0.075 m (7.5 cm) 26.5-40 10 000 0.0075 m (0.75 cm) When f is in MHz, the division simplifies to: 1(m) = spectrum. The term 'microwave', mentioned earlier, 300/f. This lends itself to easy mental arithmetic! does not appear in the listing although with the advent of Wavelength is an important parameter in considering microwave ovens it has become widely used and misused antenna systems and propagation since it is a factor in in the public domain. There is no generally agreed determining the physical dimensions of antennas. definition but it is often used to apply to frequencies Without going into antenna detail at this stage, some from several hundred MHz upwards. idea of the physical comparison of wavelengths can be It should be noted that the term RF is used here across obtained from the examples of the length dimension the whole spectrum as a generic term and the term of a N/4 (one-quarter wavelength) antenna for a few 'microwaves' merely refers to a portion of the RF frequencies shown in Table 2.5-2. Practical antennas will spectrum. be a little shorter than the theoretical calculations of The abbreviated band identifiers in Table 2.5-3 from Table 2.5-2. VLF to UHF are in frequent use but the abbreviations Radio waves can therefore be referred to either by the SHF and EHF are less u