ETS-Lindgren 3301B Active Rod & Field Antenna (Archived) - Manual (Page 36)

36 | Antenna Impulse Response
In the same way, the antenna will not efficiently pass spectral components that
fall below its bandwidth. For the Model 3301B, the selected low frequency roll off
will determine how wideband the response is. The very low frequency
components basically describe the flat top and bottom of the time domain pulse.
So, when the waveform is band limited by the antenna, the resulting waveform
will have a decay constant returning the output to ground rather than maintaining
a flat topped pulse.
A third phenomenon that occurs when measuring impulse type waveforms is that
saturation of the amplifier is harder to detect. By definition, impulsive type signals
spread their energy over many spectral components. So, when viewed in the
frequency domain, the energy demanded of the antenna amplifier may appear
much lower than it actually is. The energy demanded of the antenna amplifier to
properly pass a pulse is not just the peak pulse in the frequency domain
waveform, but rather the sum of the energy contained in all the spectral lines. By
looking in the time domain, it is easy to see that the amplifier must provide the
energy to go from ground to the pulse peak almost instantaneously. The impulse
delivers all its frequency domain spectral components in phase. So the antenna
must provide the vector sum of all the frequency domain spectral lines.
Therefore, extra care must be taken to protect the antenna amplifier from
saturation when making impulse measurements in the frequency domain. The
internal attenuators in the Model 3301B allow it to overcome many of the barriers
which earlier units presented to such measurements. These attenuators allow the
user to safely keep the amplifier in its linear region.
The saturation indicator in the Model 3301B will provide an accurate warning of
impending saturation, provided two conditions are met. First, the product of the
pulse width to pulse repetition rate must be greater than 0.003. Second, the
pulse repetition rate must be less than the value listed in Table of Acceptable
Pulse Repetition Rates on page 16.
If an impulse falls outside of these two parameters, the saturation indicator will
not accurately warn of saturation. However, the antenna will accurately handle
the impulse signal within its bandwidth and dynamic range limitations.
The reasons for these requirements are two engineering trade-offs which must
be made, and are explained in the next two sections.
11.2 Pulse Desensitization
The Model 3301B saturation indicator operates by creating a DC level on a
capacitor feed through a rectifying diode. This DC level is then compared to a
second DC level which is set at the amplifier 1 dB compression point. This circuit
works extremely well for CQ type signals. However, for fast transients the
capacitor, like all capacitors, has an integrating effect. The DC level established
by a fast transient is spread out in time. After the pulse passes, the capacitor
Archived 3/18/10