ETS-Lindgren 3301B Active Rod & Field Antenna (Archived) - Manual (Page 31)

Theory of Operation | 31
It is the output impedance which determines the effective low frequency cutoff of
the unit. The Model 3301B is still usable at an extremely low 30 Hz. In fact,
artificial limiting was introduced to protect the unit in situations where overloading
from power frequencies would be a problem. The low frequency roll off switches
in to unit allow the user to set the low frequency cutoff according to the particular
testing needs.
The second stage is made up of three exceptionally high quality transistors. The
first is configured common emitter for voltage gain and to give a precise
180 degrees phase reversal for the output. The second two transistors for a
Darlington pair to drive the 50-ohm output. The output of this stage is then DC
isolated, matched to 50 ohm and passed through a common mode choke. The
result is that cable VSWR and common mode noise problems are minimized.
The dynamic limits of the Model 3301B are set by the amplifier. Sensitivity is
determined by the noise developed in the first stage. The ultimate limit on
sensitivity is determined by the thermal noise generated by the input impedance.
The impedance presented to the amplifier by the rod is almost entirely capacitive.
A 41” rod typically presents an impedance of about 12 pF. Therefore, the input
impedance presented to the amplifier will decline with increasing frequency. At
the low frequencies, where the input impedance is high, the thermal noise
generated by this impedance will also be high. If the input impedance of the
amplifier is lowered to reduce thermal noise then the rod will be loaded down and
lose sensitivity. This combination of thermal noise and rod impedance sets an
ultimate limit on the sensitivity achievable by active rod antennas. The sensitivity
of the Model 3301B improves with frequency in direct relation to the declining
input impedance.
A common mistake made in regard to sensitivity is to ask what the noise figure of
an active rod is. Noise figure is defined as: The ratio of output noise of a unit over
the output noise expected solely due to the thermal noise of the resistance of the
input impedance. Since the input in the case of a rod is primarily reactive, the
definition literally has no meaning. What is useful is to determine what the
minimum discernible signal of a unit is. That is, what is the smallest signal which
can be seen in the presence of the amplifier noise? There is a trick here. The
smallest signal seen above the noise in a rod antenna is not determined by the
noise of the amplifier. Signal is drained away through capacitive loss before it
reaches the amplifier. An amplifier with more noise may actually be more
sensitive if it also loses less signal to capacitance at the input.
The upper limit of the Model 3301B is determined by the ability of the gain stage
to amplify a signal. The gain stages capable of handing field strengths of 0.7 V/M
are available. Impulsive type signals produce the same effect but in such a way
as may not be immediately obvious. An impulse signal presents the amplifier with
signals at a number of frequencies all in phase with each other so that
measurement of the field strength at any one frequency will not appear to be very
Archived 3/18/10