10.4 Quarter wavelength monopole antenna − ground-plane antenna

Basic theory

The quarter wave antenna is a single element antenna fed at one end that behaves as a dipole antenna. It is formed by a conductor in length λ/4. It is fed in the lower end, which is near a conductive surface which works as a reflector. The current in the reflected image has the same direction and phase that the current in the real antenna. The set quarter-wave plus image forms a half-wave dipole that radiates only in the upper half of space.

In this upper side of space the emitted field has the same amplitude of the field radiated by a half-wave dipole fed with the same current. Therefore, the total emitted power is one-half the emitted power of a half-wave dipole fed with the same current. As the current is the same, the radiation resistance (real part of series impedance) will be one-half of the series impedance of a half-wave dipole. As the reactive part is also divided by 2, the impedance of a quarter wave antenna is 36 + j21 . The gain is the same as that for a half-wave dipole (λ/2) that is 2.14 dBi.

Fig. 10.4A.1 Wire model of ground plane antenna

Far from the antenna and near the ground, electromagnetic fields and radiation patterns are the same as for a half-wave dipole. The impedance is not the same a with a good conductor ground plane. Conductivity of earth surface can be improved with an expensive copper wire mesh.

When ground is not available, as in a vehicle, other metallic surfaces can serve a ground plane, for example the roof of the vehicle. In other situations, radial wires placed at the foot of the quarter-wave wire can simulate a ground plane.

Fig. 10.4A.2 Frequency response of standing wave ratio of ground plane antenna (the length l = 10.257 m)

Fig. 10.4A.3 Frequency response of input impedance of the ground plane antenna (the length l = 10.257 m)

Fig. 10.4A.4 Radiation pattern of ground plane antenna at 7.1 MHz (the length l = 10.257 m)