Question 8.9: Find the radiation resistance of a short monopole antenna th...

The short electric monopole antenna near the ground is equivalent to a short dipole antenna with a length that is twice as long, as shown in the following figure. The additional contribution is from the “image antenna.” In order to ensure that there is an excellent ground plane beneath the monopole antenna, conducting wires are typically implanted underneath the antenna, and they emanate radially away from the monopole for a distance that is approximately equal to the height of the antenna.

It is possible to estimate the validity of the approximation for a short dipole that is λ/4 in length. From the previous example, the radiation resistance is computed to be R_{rad}  ≈ 12.3  Ω , while the actual value obtained from the theory of the linear antenna is R_{rad} = 6. 7  Ω. Hence, even for this small length, the short dipole approximation is not a good estimate.