Practical_Antenna_Handbook_0071639586

200 p a r t I I I : h i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
The shorter the dipole is, relative to a half-wavelength, the greater the amount of
loading required and the narrower the bandwidth of the combined assembly. The
loaded dipole is a very sharply tuned antenna. Because of this, you must either confine
operation to one segment of the band or provide an antenna tuner to compensate for
the sharpness of the bandwidth characteristic. However, efficiency drops markedly far
from resonance even with a transmission line tuner.
Figure 6.14D and E shows two methods for constructing a coil-loaded dipole antenna.
Figure 6.14D shows a pair of commercially available loading coils especially designed
for this purpose. Unless you have a way to measure their inductance, they must
be used with the wire segment lengths specified by the manufacturer. The ones shown
are for 40 m, but other models are also available. The inductor shown in Fig. 6.14E is a
section of commercial coil stock connected to a conventional end insulator or center
insulator. No structural stress is absorbed by the coil—all forces are applied to the insulator,
which is designed for normal dipole tensions.
Inductance values for other lengths of antennas can be approximated from the
graph in Fig. 6.15. This graph contains three curves for coil-loaded short dipoles that are
10, 50, and 90 percent of the normal half-wavelength size. Find the proposed location of
the coil, as a percentage of the wire element length, along the horizontal axis. The intersection
of a vertical line from that point and one of the three curves yields the required
inductive reactance (see along vertical axis). Inductances for other overall lengths can
be rough-guessed by interpolating between the three available curves, and then validated
by cut and try.
Two points should be made with respect to Fig. 6.15:
• Loading coils are most effective (in terms of how much inductance is necessary
to “replace” a specified length of the dipole) at the center (or feedpoint) of the
dipole.
• Once again, there’s no “free lunch”. The shorter the total wire length of the dipole
is, the less the overall efficiency of the antenna structure as a whole will be.
The Bent Dipole
In Chap. 3 we pointed out that the bulk of the radiated field from a l/2 dipole originates
in the center half of its total length. That is, the outer l/8 section on each end of the dipole
is important for establishing resonance but it contributes little to the total signal received
at a distant point. The bent dipole allows the use of a full l/2 dipole in limited space by
bending the outer portions of the dipole. The simplest approach is to let them dangle
(perhaps with a slight weight or pull-down cord at each end to keep them from swinging
in strong winds), but the ends can also be bent horizontally, instead. In fact, the ends can
be bent just about any way you want! Just keep them away from the horizontal center
portion of the dipole and avoid doubling back at any points along the wires.
Figure 6.16 shows a bent dipole with dangling ends. The main lobe of the horizontal
ly polarized field is down about 0.5 dB in free space and 0.6 dB when the center portion
is 0.3 l above typical ground. The feedpoint impedance will be lower than for the conventional
(totally horizontal) l/2 dipole but still potentially a decent match to 52-Ω
coaxial cable. Of course, open-wire line is a great way to feed it. The author had great
success on the 80- and 40-m bands for a number of years with a low 80-m bent dipole
like that of Fig. 6.16; the antenna was looped over the branches of a series of maple trees
separating his apartment from the cemetery out back!