Quarter Wave Monopole Antenna:
A quarter wave monopole antenna is half of a dipole antenna placed over a grounded plane. The geometry of such antennas is shown in Fig 7.9(a) and equivalent half wave dipole is shown in fig 7.9(b).
If the ground plane is perfectly conducting, the monopole antenna shown in Fig 7.9(a) will be equivalent to a half wave dipole shown in Fig 7.9(b) taking image into account.
The radiation pattern above the grounded plane ( in the upper hemisphere) will be same as that of a half wave dipole, however, the total radiated power will be half of that of a dipole since the field will be radiated only in the upper hemisphere.
An ideal quarter wave antenna mounted over a perfectly conducting ground plane has radiation resistance 36.56
, half that of a dipole antenna, radiating in free space. The directivity of such antennas become double of that of dipole antennas.
| Fundamentals of Antennas and Radiating systems |
Thus directivity of such dipole antenna is 1.64 as compared to 1.5 for an elementary dipole. The half power beam width in the E-plane can be found to be 780 as compared to 900 for a Hertzian dipole. |