Full resolution (JPEG) - On this page / på denna sida - 1958, H. 7 - The Quarter-Wave Dipole, by Bengt Josephson
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Fig. 9.
Sketch of
micro-strip aircraft
antenna.
fed through a 600 ohms twin cable. The dashed
curves in figure 14 a show the resistance and
reactance within the frequency range 2—16 MHz as given in
the reference for the dipole length 20 m and the
distance 30 cm between the antenna wires. The
efficiency is assumed to be within 50—65 %.
The properties of this antenna have been calculated
with the formulae (20) — (22) assuming a wire
diameter of 3 mm and the result is shown in figures
14 a and b, solid curves. In order to verify these
calculations both impedance and efficiency were
determined by scale model measurements. These were
carried out on half an antenna over a conducting
plane. Scale factors of 1 : 30 and 1 : 60 were used,
i.e. the wire diameters were 0.1 mm and 0.05 mm
respectively. The measured points shown in the
figures are in good agreement with the theoretical
curves.
It is concluded from these curves, particularly from
the very poor efficiency, that such a resistance
loading of a folded dipole is of little value for
broad-banding purposes.
APPENDIX 2
Impedance of the Open Folded Unipole with
Displaced Feed Point
The approximate expression (6) was shown for
the radiation resistance of the antenna in figure 3.
We shall now discuss methods for a calculation of
both the reactance and the resistance components
of the antenna impedance.
Figure 15 a shows the antenna proper with the
applied voltage V, open end voltage V and input
impedance Z = V/I, and in 15 b an equivalent
arrangement is shown. The total current I in the feed
point may thus be calculated as the vector sum of
an unbalanced current A7X and a balanced current /,
which are calculated according to 15 c and d
respectively with due regard to the images.
From transmission line equations the current I..
is obtained as function of the voltages V and V and
the dimensions.
Fig. 10. Outside and inside views of the antenna in
figure 9.
Fig. 11. Impedance of the antenna in figure 9, mounted
on large groundplane a) without, and b) with
plastic covering.
El TEKNIK 1 95ß 99
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