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Fig. 6. Maxima and minima for different kh values.
due to the discontinuity at the junction between the
linear antenna and the coaxial line has to be taken
into account. However, no rigorous theoretical
formula is available for the type and amount of end
correction to be used if a scattering (or receiving)
dipole is loaded by a coaxial line with finite
line-spacing. For short dipoles (kh < n/2) the
disagreement between experimentally measured position of
the minimum as a function of the reactive loading
and the theoretically determined position can be
accounted for by a small positive capacitance of
0.1 pF, assumed to be shunted in parallel to the
input terminal of the line section, for which
conventional line equations are used for the input
impedance. The addition of this capacitance mainly
affects the region around infinite load (kd = n/2)
leaving the region around kd = 0 and n unchanged
and yields good agreement between theory and
experiment.
The losses in the transmission line and in the
movable short-circuit will affect the magnitude of
the minimum to a large extent. This can be studied
by means of the pole-and-zero diagram. The theory
assumed purely reactive values of the load
impedance. As was pointed out before and is easily seen
from a p-z-diagram, the magnitude of o/X" in the
minimum is always larger if Z contains a
positive real part, since the minimum of an
analytical function is necessarily obtained on the
boundary — i.e., in this case the imaginary axis. There
is also a slight shift in the position of the minimum
due to losses in the coaxial line, which in principle
can also be evaluated from a p-z-diagram.
Practically, however, this shift is rather small.
Since the actual standing-wave ratio was unknown,
no attempt has been made to correct the experimental
results with respect to the magnitude of the
minimum.
For comparison the magnitude of the minima and
maxima as predicted from theory and the
experimentally obtained values are plotted as a function
of the antenna length (kh) in fig. 6. As pointed out
before, measured values below o/X2 = 10~3 are
uncertain and the measured points plotted as 10"4 in fig.
6 mean that these readings are at or below the noise
and disturbance level of the experimental
equipment. It is quite obvious that the largest reduction
in back-scattering is achieved for short dipoles
(A-/i<4). The back-scattering of longer dipoles is
rather uncritical with respect to a centre load,
although a slight reduction still can be achieved for
kh > 2 n. The agreement between theory and
experiment with respect to the magnitude of o/X2 is seen
to be fair.
Further Experimental Investigations
Off-centre loading
From the foregoing results it can be seen that no
great reduction of the back-scattering cross section
is possible for antennas longer than one wavelength
due to the small voltage induced across a centre gap8.
In order to investigate the possibility of reduction
for longer metallic cylinders, off-centre loading may
be considered, as shown schematically in fig. 7. In
this case, the outer conductor of the coaxial line
used as the reactive load in the experiments with
the Vs" antennas was used as the lower part of a
cylindrical antenna, length hlt and the upper part
was made of solid %" brass rod, length 7i„. This
gives an antenna with a diameter of %" for wyhich
the total length and point of loading could be varied
by changing the solid top part and allowing the
outer conductor of the coaxial line to protrude more
or less above the ground screen. This configuration
is equivalent to an antenna loaded at two points
symmetrically situated around the centre of the
antenna.
With several combinations of kht in the range 0
to 4 and kh2 ranging from 1 to 6, o/X" was measured
Fig. 7. Off-centre loaded antenna.
.74 ELTEKNIK 1959
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