ETS-Lindgren 3301B Active Rod & Field Antenna (Archived) User Manual
Page 37
Antenna Impulse Response
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begins to discharge through its resistance to ground. If the PRF is slow enough
so that the capacitor is fully discharged before the next pulse arrives, it may
never trigger the indicator. The antenna amplifier, on the other hand, must satisfy
the instantaneous energy demand. It must respond to the peak demand without
any integrating effect. If the product of the pulse width and PRF is less than
0.003, the saturation indicator will fail to indicate properly. The time constant of
the saturation indicating capacitor also sets the persistence of the saturation
circuit. To have the saturation circuit cease to indicate saturation within a
reasonable time after the antenna moves out of saturation, a reasonable RC time
constant must be maintained. This choice of RC time constant also creates this
limit for impulsive signals. In this region, the user must closely monitor the
antenna output for saturation.
11.3 Pulse Charging
The second criterion that must be met is that the PRF must fall below the rate
listed in Impulse Characteristics on page 35 for the appropriate duty cycle. This
requirement is created by the AC coupling in the Model 3301B circuitry. To allow
maximum dynamic range through the Model 3301B, the various stages are AC
coupled. Furthermore, to allow the extremely broad bandwidth of the unit, large
value, low impedance capacitors are used to provide the AC coupling. When a
pulse passes through these capacitors, a charge is developed. After the pulse
passes, this charge drains through the associated resistance to ground at an
established RC constant. However, if the next pulse occurs before the residual
charge has fully discharged, then a residual DC bias is created.
This bias artificially alters the DC level on the saturation indicator comparison
capacitor, causing it to fail to properly indicate saturation. Again, in this region,
the user must closely monitor the antenna output for saturation.
Archived 3/18/10