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Fig. 8. o/Xamjn — f (cx) for various kh.
for various positions of the short circuit. The curves
thus obtained were very similar to those shown for
centre-loading, fig. 5. From these curves the
measured minima are shown in fig. 8 as a function of
the position of the loading point, cx = hj (ht + h2),
with the total length, kh, as parameter. oc. — 0
corresponds to centre-loading and oc = \ to an unloaded
antenna. It can be seen that material reduction of
the back-scattering is possible with ocpa 0.5 up to kh
about 6. For longer cylinders even an off-centre
loading appears to give no noteworthy reduction.
The experiments were made for kh up to 9.
Although not extremely deep, the minima of o/X2 for
finite oc. are rather sharp as a function of the load
reactance.
Multiply loaded antennas
In an effort to reduce the back-scattering cross
section for longer antennas, the next logical step
was to try reactance loading at more than one point
along the antenna. The result of these experiments
was, that a reduction of o by a factor 10 is possible
for antennas as long as kh = 9. However, the number
of variables now became so great, that no complete
study could be made.
Frequency dependence
In a practical application in order to reduce
back-scattering from cylindrical objects, the dependence
of o/X2 on frequency is of interest. In the case of a
metallic cylinder loaded by a section of transmission
line, obviously the frequency dependence of the
input impedance of the line will almost exclusively
determine the useful bandwidth. As is easily seen,
the characteristic impedance of the loading line
should be as large as possible in order to achieve
greater bandwidth. This was verified by measuring
o/X2 as a function of frequency for the same length
of antenna, kh = 2, loaded by two different coaxial
lines of characteristic impedances 55 Q and 115 Q
respectively. The low impedance line gave a
reduction of o by a factor 100 over a 7 % frequency band,
while the high impedance line maintained the same
reduction over a 13 % frequency band.
Complete mapping of the behavior of centre-loaded
antennas
In order to study more completely the influence of
centre-loading of antennas of different lengths, the
curves of a/X3 shown in fig. 5 were supplemented
by similar measurements for 15 additional lengths
of antennas, ranging from kh = 1.25 to kh = 9, for
the same diameter, ka = 0.066. The same set of
measurements was also made for an antenna thickness
of Yé", ka = 0.132, for 21 different lengths in the
range kh = 1.0 ... 9.5. The results of these
measurements for ka = 0.066 are shown in condensed form
in fig. 9. A similar figure was obtained for the
thicker rod. The figure shows in the form of
"altitude chart" a/X2 as a function of the reciprocal load
reactance and antenna length. The various curves
are lines of constant o/X2. The value is indicated
beside each curve. Dotted lines indicate estimated
values.
The general behavior of the back-scattering cross
section of the thicker geometry is the same as for
the thinner wire, the major difference being that
the maxima are more rounded off, the minima are
slightly broader, and shifted to lower impedance
values. Also the magnitude of o/X2 in the minima
is in general somewhat higher. For a cylinder of
length near two wavelengths, kh = 2 ct, there is again
almost no variation in o as a function of the load.
Conclusions
The investigation shows that the back-scattering
cross section of slim cylindrical objects shorter than
about one wavelength can be reduced to a very great
extent by a suitable reactance load in the centre
of the antenna. When this reactance is realized by
a section of transmission line, the reduction is
narrow-banded. If lumped loads are used, the
bandwidth is likely to be improved. It is also conceivable
to use a reactive load formed by the input
impedance of a band filter of some kind.
For longer objects, a reduction is possible if the
Fig. 9. Chart of o/X2 for centre-loaded linear antenna.
ELTEKNIK 1959 1 75
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