The capacitance of the 0.16 m2 flat-plate antennas was measured to be about
80 pF. Therefore, using Equation 3-18, the -3 dB bandwidth of the response is
1 1
C = 2.5 x 108 s1 (3-66)
RC,,nt (50 Q) (80 pF)
This corresponds to a frequency of fo a 40 MHz. Since the dE/dt measurements
were to be band-limited to 20 MHz (-3 dB) via anti-aliasing filters at the digitizers
inputs, the response of the antenna can be considered uniform over the complete
frequency range of interest. Therefore, Equation 3-24 can be used to approximate the
voltage output of the dE/dt antenna. The expression for the voltage at the input of the
fiber-optic transmitter is
dE,/A n nor ) (3-67
VFOT (t) = FoAplateRGPIc dEnrmt (3-67)
The attenuation for the dE/dt measurements (Gpic) was varied over the course of
the 2002 season. A value of 0.199 (-14 dB) was originally used, however this resulted
in an output voltage that was too low. Therefore the attenuation was changed to 0.316
(-10 dB) but this too resulted in a low output voltage. Finally, the attenuation setting
was changed to 0.501 (-6 dB) to obtain an acceptable output voltage.
Using this final value for Gpic along with Aplate = 0.16 m2 and R = 50 Q yields
the following expression for vFOT (t) when dE/dt is expressed in units of V m1 s1
VFOT(t) =(3.55 x 1011) dErm (3-68)
If dE/dt is expressed in units of kV m 1 s 1, the above expression becomes
VFOT(t) = (0.0355) dEO(t) (3-69)
dt
The voltage present at the input of the digitizing oscilloscope is the voltage
present at the input of the fiber-optic transmitter modified by the fiber-optic link. If the
effect of the link is assumed to be a frequency independent gain or attenuation, Glink,