Effective length/height of the antenna:
When a receiving antenna intercepts incident electromagnetic waves, a voltage is induced across the antenna terminals. The effective length he of a receiving antenna is defined as the ratio of the open circuit terminal voltage to the incident electric field strength in the direction of antennas polarization.
.............................................(7.36)
where Voc = open circuit voltage
E = electric field strength
Effective length he is also referred to as effective height.
Antenna Equivalent Circuit:
To a generator feeding a transmitting antenna, the antenna appears as a load. In the same manner, the receiver circuitry connected to a receiving antenna's output terminal will apear as a load impedance. Both transmitting and receiving antennas can be represented by equivalent circuits as shown by figure 7.6(a) and figure 7.6(b).