ETS-Lindgren 3301B Active Rod & Field Antenna (Archived) - Manual (Page 35)

Antenna Impulse Response | 35
11.0 Antenna Impulse Response
The Model 3301B is designed and optimized for the measurement of CW type
signals. However, this unit is fully capable of dealing accurately with impulsive
type signals, with some special considerations. This section guides the user who
intends to use the Model 3301B to measure impulsive signals. By following the
provided guidance, accurate measurements of impulsive signals may be
performed. The Model 3301B may be used to measure the important
characteristics of an impulse signal that fall within its bandwidth and dynamic
range. However, the measurement of impulsive signals requires some special
cautions to avoid saturation of the antenna amplifier.
11.1 Impulse Characteristics
Before discussing the Model 3301B response to impulse signals, a brief review of
the nature of impulse signals is appropriate. For our purposes we will consider a
specific type of impulse signal, the periodic, rectangular pulse. The pulse may be
described by three parameters: the pulse width, t; the pulse repetition rate, PRF;
and the pulse amplitude, A. For a diagram, see Periodic Rectangular Pulse Train
on page 47. By Fourier analysis, we may convert this time domain representation
into its frequency domain equivalent. In the frequency domain, the impulse
becomes an infinite series of discrete spectral lines whose envelope is described
by the formula:
Y=K*sin(x)/X
K = A*t*PRF
The nulls occur at regular intervals spaced n/t apart, where n is an integer. The
discrete spectral lines are spaced evenly, at internal multiples of the PRF. For an
illustration, see Spectrum of Rectangular Pulse on page 47. Two characteristics
should be noted. First, increasing the pulse width, t, narrows the separation of
the nulls. So wider pulses tend to concentrate their energy in a narrower
frequency span. Second, decreasing the PRF decreases the frequency
separation of the individual spectral lines. For an illustration, see Altering Pulse
Width/Pulse Repetition Rate on page 48.
By considering the frequency domain representation, any antenna response to
an impulse may be understood. The antenna will only pass those spectral
components that fall within its bandwidth. In the case of the Model 3301B,
components above 30 MHz will be attenuated or not passed at all. Remember, to
fully describe a rectangular pulse, an infinite bandwidth is required. The lack of
high frequency components shows up on the time domain as a rounding of sharp
corners and a slowing of the rise and fall.
Archived 3/18/10