rotrudes into waveguide Hollow centre Figure 4.14 A waveguide launcher applications it is more convenient to use the open circuit probe. These launchers can be used for transmitting signals into the waveguide as well as receiving them from the waveguide. The dimensions of a waveguide are very important. It is found that below a given frequency called the critical frequency a waveguide will not operate. Also if the waveguide is made too large its cost will be higher than necessary and there is the possibility of higher order modes travelling along it which may introduce problems. As a result there are a variety of standard sizes of waveguide, and the correct one can be chosen dependent upon the frequency in use. They are allocated numbers and as an example waveguide WG10 is used for frequencies between 2.60 and 3.95 GHz. The advantage of a waveguide is that it offers a very low degree of attenuation or loss at these frequencies. At these frequencies the loss for a 30 metre length would only be about 1 dB dependent upon the exact frequency and whether the waveguide walls were made from aluminium or copper. This particular waveguide has internal dimensions of 72 X 34 mm and has a cutoff frequency of 2.08 GHz. Signals can be transmitted directly out of a waveguide into free space. Although the directional properties are not very good there is no need to terminate them in an antenna for them to be able to radiate a signal. As a result it is very important NEVER to look down a waveguide as it is possible that power could be radiated from it if it is connected to a source of radio frequency power. High levels of power can quickly damage the eye, as people have found to their cost in the past. Types of antenna There is a wide variety of different types of antenna that can be used and each has its own advantages and may be used for specific applications. Often an antenna may be envisaged in a format similar to a television Antenna systems 95 array, alternatively it may be a wire antenna, or it may just be a short length o