e in some parts of the world fall into the low frequency or LF portion of the spectrum along with navigational beacons and many other types of transmission. Moving up in frequency, the medium frequency or MF section of the spectrum can be found. The medium wave broadcast band can be found here. Above this are the lowest frequencies of what may be considered to be the short wave bands. A number of users including maritime communications and a 'tropical' broadcast band can be found. Between 3 and 30 MHz is the high frequency or HF portion. Within this frequency range lie the real short wave bands. Signals from all over the world can be heard. Broadcasters, maritime, military, weather informa- tion, radio amateurs, news links and a host of other general point to point communications fill the bands. Moving up further the very high frequency or VHF part of the spectrum is encountered. This contains a large number of mobile users. 'Radio Taxis' and the like have allocations here, as do the familiar VHF FM broadcasts. In the ultra high frequency or UHF part of the spectrum most of the terrestrial television stations are located. In addition to these there are more mobile users including cellular telephones generally around 850 and 900 MHz as well as 1800 and 1900 MHz dependent upon the country. Above this in the super high frequency or SHF and extremely high frequency or EHF portions of the spectrum there are many uses for the radio spectrum. They are being used increasingly for commercial satellite and point to point communications. 24 Newnes Guide to Radio and Communications Technology Polarization There are many characteristics of electromagnetic waves. One is polariza- tion. Broadly speaking the polarization indicates the plane in which the wave is vibrating. In view of the fact that electromagnetic waves consist of electric and magnetic components in different planes, it is necessary to define a convention. Accordingly the polarization plane is taken to be that of the electric component. The polarization of a radio