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The Quarter-Wave Dipole
Bengt Josephson, Chief Research Engineer,
Research Institute of National Defence, Stockholm
Summary
A new type of dipole antenna has been developed,
which at a length of 0.25 to 0.3 wavelengths is
matched to a coaxial line or a twin line. In principle
Ihc antenna may be considered as a special form of
a folded dipole (or unipole). The theory of this
antenna is discussed, leading to the impedance
properties of unsymmetrically fed antennas.
The method has been particularly advantageous in
designing a low height, broadband aircraft antenna,
in which case the radiating and feeding elements
form parts of microstrip system. Also other
applications have been considered, such as shortened
antennas for YHF and UHF TV and broadcast receivers.
In an appendix general formulas for
impedance-loaded folded dipoles are developed and applied to
a special case.
Theoretical Considerations
The notations used in this part are explained in
Appendix 1.
An antenna according to figure 1 lias the impedance
Z = Z,i + j . Zu t g ßs
4
(0
This can be seen from equation (12) in Appendix 1
by insertion of Zi, = oo and Zs = jZ0tg ßs, where
ß = 2n/X. If s < A/4 the first term Zd in (1) has a
eapacitive component while the second term is
inductive.
Thus the impedance of this antenna, which may
be named an "open" folded unipole as distinct from
the conventional (closed) type, is real at a certain
length sr < A/4. If both branches of the antenna have
d
Fig. 1.
Open folded unipole.
621.396.674..1
the same diameter, then k = 1 and the resonance
length sr is approximately 0.15 A with rather small
dependence on dimensions. The resonance length
will be still shorter if the "open" arm is made
thicker than the driven arm. With this arrangement the
unbalanced current is larger in the thicker branch,
i.e. k > 1 in equation (1).
However, the resonance resistance of the open
folded unipole is very low, of the order of 10 ohms
or less, which makes it unsuitable for matching to
normal cables.
Along a thin antenna the current is approximately
distributed according to parts of sine curves.
Feeding in a low-current point gives a high impedance.
Fig. 2. Current distributions at an open folded unipole
(k = 1.2).
The distribution of currents at an open folded
unipole is demonstrated by figure 2. At the driven arm
the balanced and the unbalanced (radiating)
currents add, so that the total current is almost constant.
At the other arm the two currents have opposite
phases, and their amplitudes arc such that the total
current is zero at the end.
Thus it is clear, that a feed point according to
figure 3 gives a higher impedance than that of
figure 1* We shall now assume that approximately the
same current distributions exist in these two cases.
If x is the distance from the groundplane, the
balanced current at this point is
/2(*)= M 0)
eos fi (s — .r)
eos ß s
(2
ELTEKNIK 1958 1 1 9
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