ESCO Technologies ETS Lindgren 3301B, User Manual

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Theory of Operation | 29
9.0 Theory of Operation
9.1 The Rod
There are three factors of primary interest in understanding a rod antenna. These
factors are its effective electrical length, the impedance it presents to the
measurement system, and the interaction of the rod with the ground. The
discussion here will contain itself to discussing an electrically short antenna
element. The Model 3301B is designed to be electrically short over its
measurement range. Electrically short means an antenna that is designed to
have the physical antenna element be short when compared to the wavelength of
the highest frequency in its measurement range. The current distribution on any
rod antenna will be sinusoidal. If a rod is short enough with respect to the
wavelength being measured, that distribution will approach a linear distribution
and may be assumed to be linear for all practical purposes. The linear current
distribution will allow for linear scaling of the rod length. If the rod length is
reduced by half, then the received voltage will be reduced by half. The 41” rod is
approximately a meter long. Electrically, then, it is a half-meter long. The
assumption of linearity will remain accurate at least to one-eighth wavelength.
At this point some resonance behavior is possible. A rod that is electrically a
half-meter is one-eighth wavelength at 75 MHz. To keep the response of the unit
linear and avoid resonance behavior, the Model 3301B pre-amplifier is limited to
roll off before this frequency has been reached. However, this avoidance of
possible resonance carries the penalty of 6 dB on the antenna factor. Because
field strength is measured as volts per meter, the measurement must be
normalized to 1 meter. This means that 6 dB must be added to a reading taken
with a half-meter rod.
The second factor of concern is the impedance the rod presents to the
measurement system. The resistive component is non-significant in this situation.
The controlling impedance is the capacitance of the rod to ground. This
capacitance may be calculated by the formula:
C = [55.63 * h] / [ln (h/a)]
h = length of the rod, in meters
a = radius of the rod, in meters
The natural logarithm of h over a is represented by ln . For the Model 3301B rod,
the capacitance is 10 picofarads (pF). This capacitance is significant because it
combines with the total input capacitance of the amplifier to form a voltage
divider. It can quickly be seen that if the FET input capacitance and circuit stray
total 10 pF, there will be a 6 dB loss through this stray capacitance. Even if the
total input capacitance where reduced to 4 pF there would be a 2.5 dB loss.