Full resolution (JPEG) - On this page / på denna sida - 1959, H. 4 - Some Aspects on the Design and Use of the Helical Antenna, by göran Svennérus
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quantities are given in the case of a conical
counterpoise antenna. — The beam widths are nearly the
same for different polarizations (disregarding a
minor level difference at maximum). — The phase
difference between the field components E& and
Efp is 90 degrees throughout the investigated angle
region. The phase is nearly constant for •—60° < 0
< 60° indicating that this modified helical antenna
can be regarded as a point source. The centre of
phase of the antenna is located on the helix axis
in the plane of the cone aperture. — The result of
the measurements of the axial ratio in different
directions is in good agreement with the amplitude
and phase conditions. The antenna produces nearly
circularly polarized radiation in all directions in
which a conventional paraboloid reflector placed
in front of the antenna is being illuminated.
From both the mechanical and the electrical point
of view it is of great value to be able to use a feed,
the only support of which is a metallic cylindrical
rod coinciding with the axis of the paraboloid. In
the case of a helical antenna this supporting rod
will also be coincident with the helix axis. The
advantages of the funnel-shaped counter-poise apply
both with and without the supporting rod. The
influence of the supporting rod on the illumination
field can be reduced to a minimum by means of a
Fig. 7. Primary source radiation properties of a two-turn
helix in xz-plane at the design frequency. Plane
counter-poise to the left, conical counter-poise to
the right.
- field component Eq
–-field component Eq>
Fig. S. Illumination phase error as a function of
frequency for a two-turn helix.
- conical counter-poise,
–-plane counter-poise.
proper choice of the rod diameter in relation to
the diameter of the helix conductor as well as by
the use of a quarter wave choke placed on the rod
just in front of the helix.
In fig. 8 the phase error is plotted as a function of
frequency. Maximum phase difference considering
all polarizations and all planes through the antenna
is shown, to the left limited to angles 6 within ±
30 degrees and to the right within ± 60 degrees.
The conical counter-poise helical antenna seems to
be rather acceptable as a point source within a
frequency band of about 30 %.
Some practical performances
It is quite clear from a mechanical point of view
that it is an advantage to place the entire helix
inside the conical counter-poise. In this case the
helix has to be shortened to about one and a half
turns. As has been already pointed out in the
litte-rature2 such a short helix cannot produce circularly
polarized radiation without some kind of
end-loading. Fig. 9 gives an idea of how this can be
successfully arranged.
In absence of supporting rod the helix may be
terminated by an extension of the conductor directed
from the end of the helix to the axis and parallel
with the beginning part of the helical antenna.
There are, however, many other ways to arrange
an acceptable end-loading.
In the presence of a coaxial supporting rod the
end-loading can be arranged according to the
illustration to the right in fig. 9. In the aperture plane the
helix is continued bv a conductor arc of about 200
ELTEKNIK 1959 1 56
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