Full resolution (JPEG) - On this page / på denna sida - 1958, H. 7 - The Quarter-Wave Dipole, by Bengt Josephson
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Fig. Open folded unipole with displaced feed point.
and the unbalanced current
/,(*)=/! (0)
sin ß (s — x)
sin ß s
(3)
The total current at the point x of the open arm
will be
/(*)= k /.(x)- h(x)
(4)
We denote by R and Rx the radiation resistances of
the arrangements in figures 1 and 3 respectively and
get from the condition of equal radiated power in
both cases the transformation ratio
Rx _ I (2 — k) U (0) + U (0)
R k 11 (x) — h(x)
(5)
Using equations (2) and (3) and the condition
/„(0) = k 1^(0) for Zf, = oo in equation (11),
Appendix 1, we can write
Rx
R
1 Isin 2ßsV<
k- \ sin fix!
(ß)
This simple formula, though very approximately
deduced, has been in good agreement with
measurements near resonance. The antenna height sr at
resonance shows a very small dependence on the
location x of the feed point, which seems to justify the
assumptions made about the current distributions.
The antenna is matched to the feeding cable by a
suitable choice of x.
In summing up, the elements of the antenna in
figure 3 have a triple function as radiating elements,
as inductive series reactance element and as
transformer.
A more detailed discussion on the impedance of
this antenna type is made in Appendix 2.
Measurements and Applications
The major part of the antenna consists of a
transmission line, and different line types can be used, the
choice being determined of the application. The
design of antennas based on coaxial line, unscreened
twin line and strip line has been considered.
Coaxial Arrangement
In this case the test antennas were made of a 50
ohms coaxial line with outer conductor of soft
copper tubing, which could be bent to the desired
shape.
Unipoles. Some measured and calculated data
which were made on the unipoles in figures 1 and 3
in order to check the formulas, will be reported.
With the dimensions s = 210 nun, 2 a = 3.5 111111 and
</ = 21 111111 (i.e. Z0 = 300 ohms) the height of the
base fed antenna was 0.15 A and the radiation
resistance 10 ohms at resonance. These values were
obtained both from measurements and calculations,
equation (1).
For the antenna in figure 3, having the same
dimensions as above and the feed point x = 10 cm
above the groundplane, a resonance Rx = 40 ohms
(at the feed point) was measured, and the resonance
height was also here 0.15 A. Calculated from formula
(C) Rx =4.3 R = 43 ohms. The bandwidth when
fed from a 50 ohms cable was 3.5 % for a VSWR
<2.
It may be pointed out that the top of the antenna
is D.C. grounded, which is advantageous in some
applications.
Dipoles. A dipole has been made as shown in figure
4. With a suitable choice of feed point the resonance
impedance was 50 ohms and the length was then
2 s = A/3. It was checked that outside currents on
the feeding cable at right angle to the antenna were
negligible.
This antenna type can be further shortened, as
mentioned earlier, by a nonsymmetrical
arrangement, corresponding to A* > 1 in equations (1) and
(6).
Experimentally this has been made by use of a
metal foil soldered to the antenna conductors as
shown schematically in figure 5. Being accurately
matched to the feeding cable at a length of A/4, the
antenna may be called a "quarter-wave dipole"
(figure 6).
Fig. A.
Coaxial type shortened
dipole.
1 96 ELTEKN I K 1958
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