The Monopole Antenna
A monopole antenna is one half of a dipole antenna, almost always mounted above some sort of ground plane. The case of a monopole antenna of length L mounted above an infinite ground plane is shown in Figure 1(a).
Figure 1. Monopole above a PEC (a), and the equivalent source in free space (b).
Using image theory, the fields above the ground plane can be found by using the equivalent source (antenna) in free space as shown in Figure 1(b). This is simply a dipole antenna of twice the length. The fields above the ground plane in Figure 1(a) are identical to the fields in Figure 1(b), which are known and presented in the dipole antenna section. The monopole antenna fields below the ground plane in Figure 1(a) are zero.
The radiation pattern of monopole antennas above a ground plane are also known from the dipole result. The only change that needs to be noted is that the impedance of a monopole antenna is one half of that of a full dipole antenna. For a quarter-wave monopole (L=0.25* ), the impedance is half of that of a half-wave dipole, so Zin = 36.5 + j21.25 Ohms. This can be understood since only half the voltage is required to drive a monopole antenna to the same current as a dipole (think of a dipole as having +V/2 and -V/2 applied to its ends, whereas a monopole antenna only needs to apply +V/2 between the monopole antenna and the ground to drive the same current). Since Zin = V/I, the impedance of the monopole antenna is halved.
The directivity of a monopole antenna is directly related to that of a dipole antenna. If the directivity of a dipole of length 2L has a directivity of D1 [decibels], then the directivity of a monopole antenna of length L will have a directivity of D1+3 [decibels]. That is, the directivity (in linear units) of a monopole antenna is twice the directivity of a dipole antenna of twice the length. The reason for this is simply because no radiation occurs below the ground plane; hence, the antenna is effectively twice as "directive".
Monopole antennas are half the size of their dipole counterparts, and hence are attractive when a smaller antenna is needed. Antennas on older cell phones were typically monopole antennas, with an infinite ground plane approximated by the shell (casing) of the phone.
Effects of a Finite Size Ground Plane on the Monopole Antenna
In practice, monopole antennas are used on finite-sized ground planes. This affects the properties of the monopole antennas, particularly the radiation pattern. The impedance of a monopole antenna is minimally affected by a finite-sized ground plane for ground planes of at least a few wavelengths in size around the monopole. However, the radiation pattern for the monopole antenna is strongly affected by a finite sized ground plane. The resulting radiation pattern radiates in a "skewed" direction, away from the horizontal plane. An example of the radiation pattern for a quarter-wavelength monopole antenna (oriented in the +z-direction) on a ground plane with a diameter of 3 wavelengths is shown in the following Figure:
Note that the resulting radiation pattern for this monopole antenna is still omnidirectional. However, the direction of peak-radiation has changed from the x-y plane to an angle elevated from that plane. In general, the large the ground plane is, the lower this direction of maximum radiation; as the ground plane approaches infinite size, the radiation pattern approaches a maximum in the x-y plane.
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