There are lots of different types of coherent antenna. All sorts of sizes, shapes, designs, etc. Trying to learn about them all would be like setting out to paint the Forth Railway Bridge with a artist's fine paintbrush! Fortunately, we can divide antennas into three rough classes based on how their dimensions compare with the wavelength of the fields they're designed to work with.
7.1. Antennas with dimensions comparable to the wavelength.
The simplest type
of practical antenna in this group is the halfwave dipole. The length
of this is chosen so that the dipole resonates at the operating
wavelength. The resulting current and voltage pattern this produces is
illustrated in figure 7.1.
The current distribution along the dipole is
and the voltage distribution is
provided that f is approximately equal to the dipole's resonant frequency, . Power is coupled into or out of a dipole via a pair of connections placed on either side of a small central gap. The ratio of the magnitudes, to , determines the antenna's effective radiation resistance which is the impedance this gap presents to anything connected to it. For a halfwave dipole . This is sometimes called the Driving Impedance of the antenna since it is the impedance power has to be ‘driven into’ when the dipole is used as a signal transmitter. As was mentioned in the previous lecture, the peak gain of the halfwave dipole is 2·15 dBi.
Halfwave dipoles are used a great deal at SW-VHF-UHF frequencies because they are so easy to make. They're so common that they're usually just called ‘a dipole’ and it's taken for granted that they are about a half wavelength long unless their length is specifically mentioned. Although convenient, the dipole has a relatively low gain, however we can obtain much higher antenna gain values by using an array of dipoles. The behaviour of arrays can be analysed using the principle of field superposition. This can be illustrated using the examples shown in figure 7.2.
Figure 7.2a shows an array of ‘point sources’, . Each one of these would, taken alone, produce a field, , at some other point, R. The principle of superposition tells us that the total field produced when all the sources are radiating will just be the sum of their individual fields. Hence the total field produced by the point source array will be
where, in the illustration since there are just five sources.
The same principle can be used to calculate the combined effect of an array of dipole sources. Figure 7.2b shows the example of an End Fire array of dipoles. All of the dipoles are driven by power flowing along a common ‘feeder’ or waveguiding arrangement. (A pair of wires in this case.) The dipoles are spaced apart. This means that the i'th dipole receives its driving voltage ‘earlier’ in phase than the front, , dipole.
Consider the field radiated in the direction at an angle, , to the plane normal to the dipole lines. The front dipole will radiate a field
in this direction. The i'th dipole will be earlier by a phase but the field it radiates must cover an extra distance
to ‘catch up’ with the field from the front dipole moving off in that direction. This means that the total field radiated in the direction will be
(This assumes that all the dipole elements in the array are driven with the same amplitude, . In practice this may not be correct and we'd have to alter the above expression. However, we're just using this example to illustrate a general method, so we can ignore this as an embarrassing detail!)
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