Full resolution (JPEG) - On this page / på denna sida - 1958, H. 7 - The Quarter-Wave Dipole, by Bengt Josephson
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Fig. 15.
Open folded iinipole with
displaced feed point and
equivalent circuits.
In the non-resonant case we are here considering
(s <A/4), the impedance Z’oc can be calculated
neglecting that part of the antenna which is above the
height sL. Thus again the use of equation (12) with
the insertion of Zb = O and Zs = —j Z0 cot ß s. gives
with a closc approximation
Y’o
Y,i (s.)
Igßx
(26)
This formula for Y’oc has been verified by
measurements on the actual configuration within a wide
frequency range (1,5:1). With practical dimensions
given earlier for this antenna type, Y’oc is almost
purely reactive.
It remains to calculate the impedance Zsc and this
can be done as was explained with reference to
figures 15 and 16. However, now V = O which makes
a considerable simplification.
Hallén’s equations for the impedance of an
unsym-metrically fed antenna are very complex. Zuhrt5 has
shown that this impedance approximately is equal
to the arithmetical mean value of the impedances of
two centre-fed dipoles corresponding" to each part of
the unsymmetrically fed antenna. This
approximation is valid only when the lengths of the antenna
parts are not too unequal.
For our purpose it is of interest to consider the
impedance of a non-resonant unipole (s < A/4)
driven at the mid point between the ground and the top,
figure 17 a. This impedance may be derived with
close approximation with the aid of equation (23)
as follows. The wanted antenna impedance is
ZV
Z’oc
(27)
Z’sc is the impedance of a unipole with the length s.
The open-circuit impedances Zoc and Z’oc are seen
in figure 17 b and c. In this non-resonant case
the influence of the image dipole in 17 b and the
parasitic elements in 17 c on respective impedances
are very small. Therefore
Zo
2 Z’fl
and from (27)
Zsc 2 Z’sc
(28)
(29)
Fig. 16.
Antenna equivalent to the unipole and
its image in figure 15 c.
Fig. 17.
Explanation of equations
(27)—(29).
ELTEKN I K 1958 ] () 1
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