Full resolution (JPEG) - On this page / på denna sida - 1958, H. 7 - The Quarter-Wave Dipole, by Bengt Josephson
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Fig. 5.
Coaxial type
quarter-wave dipole.
Twin Cable Arrangement
In figure 7 a symmetrically driven antenna
according to the shown principle is illustrated. It is made
of and fed by an ordinary polythene insulated 300
ohms twin cable. With suitable dimensions a real
impedance of 300 ohms was measured at the
frequency where the antenna length is 0.27 I. With the
measures 2 s = 135 cm, 2 x = 38 cm, d = 5 cm and
impedance compensation through a small
capacitance across the feeding cable, a larger bandwidth
was obtained. A measured impedance curve is shown
in figure 8. The length of this dipole is 0.28 I at the
mean frequency 62 MHz.
Micro-strip Arrangement
The new antenna type in a micro-strip design has
provided an improved solution to some aircraft
antenna problems within the VHF and UHF ranges.
The mechanical and aerodynamical as well as the
electrical requirements on external antennas for
small high-speed aeroplanes are very severe. Built-in
cavity type antennas are in many cases not feasible,
because of large space requirements, small
bandwidth and the poor radiation pattern partly caused
by the limited choice of locations.
Regarding external antennas all of their main
disadvantages, such as drag and other aerodynamical
disturbances, risk of damage and ice formation,
nec-cessity of reinforcement at the antenna base, are
strongly increasing with the height of the antenna.
In view of this a low height is a primary
requirement and also facilitates the choice of antenna
locations.
On the other hand, the breadth of the antenna may
be large, in so far as one from aerodynamic reasons
wants a large aspect ratio of the profile, generally of
the order of 10 to 20.
The aircraft antenna in figures 8 and 10 functions
basicly as that in figure 3. The broader conductors
of a micro-strip system form the radiating elements
and these electrically thick radiators greatly increase
the bandwidth compared with that of the antenna
in figure 3.
With this design impedance correcting elements
may also be built in along with the feeding line, as
shown. The antenna is mechanically supported by
a fiber-glass covering, which is given a proper
aerodynamic shape.
The impedance properties of this antenna type are
exemplified in figure 11, which also illustrates the
influence of the plastic covering on the impedance.
The bandwidth is about 25 % at a VSWR less than 2.
The height of the antenna is about 0.16 I at the
mean frequency.
Another variant has been developed, which when
mounted on the fuselage of an airplane has the
relative bandwidth 2.5: 1 at VSWR less than 3.3. No
parallell stub is used in this case. The height is about
1/8 at the low frequency limit.
Mounted on a circular ground plane the radiation
patterns of these antennas are wholly
omnidirectional.
APPENDIX 1
Impedance-loaded Folded Dipole
We shall calculate the impedance and the efficiency
of the antenna in figure 12. An equivalent circuit of
half this antenna is shown in figure 13. The total
current / in the antenna is composed of the balanced
current I.2 from the sources (2) and the unbalanced
current 2 from the source (1). This latter current
may be arbitrarily divided between the two
branches, with the amount (2-A-) I^ in the driven arm and
k I, in the other. It can be shown that
k,
. , 60 «,
1 + .. • In –
Z0 fli
(?)
where Z0 is the characteristic impedance of the
two-wire line forming the antenna, and at and a,
are the radii of the driven arm and the other arm
respectively. The impedance of the antenna is
Z
V
V
I (2 - A) /, + h
For the currents we have the following relations
V + V’
(8)
2 /, =
Za
CO
Fig. 6. Quarter-wave dipole.
ELTEKN I K 1958 7 ]
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