quantity EoAplate. Hence, the output of the antenna is characterized by the source and
load impedance.
Typically, the source impedance of a flat-plate antenna is considered to be purely
capacitive and any resistive and inductive components are ignored. Therefore, the
source impedance becomes
Z, (3-11)
jOICant
Cant is the capacitance of the flat-plate antenna that can either be estimated
theoretically or determined experimentally. The antenna capacitance is a function of
the plate area and the height of the plate above a reference plate buried in the ground.
It may be possible to estimate the antenna capacitance using the relation C = oA/d,
where d is the distance between the antenna plate and the grounded reference plate
beneath it, but it is best to measure it. The load impedance is usually specifically
designed to provide the appropriate system response for the quantity which is to be
measured. In other words, the load impedance will be different depending on whether
one wishes to measure the electric field or its time-derivative, as well as the bandwidth
desired.
The generalized load impedance will be considered to be a capacitor in parallel
with a resistor; any inductive components are ignored. The reason for this will become
apparent when the antenna implementation is discussed. The capacitor, Cit, is typically
known as an integrating capacitance and the resistor, R, is known as the load resistance.
Therefore, the load impedance becomes
1 Rc R
ZL= -1 R j- (3-12)
jmCint R- 1 + jRCint
Since the source and load impedances are both in parallel with the ideal current
source, they can be combined. The total shunt impedance, Ztotal, is
(Zt = ZZ ( R R3-13)
Ztotal = Z IZL jCat I + jcoRCint 1+ joR (Cat +Cint)3)