Practical_Antenna_Handbook_0071639586

394 P a r t V : H i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
Selecting the Right Polarization
The final component of improving the HF signal from an emergency or portable station
setup is to match antenna polarization to the intended coverage distance(s) and the surrounding
ground characteristics. Although there is occasionally a reason (most often for
low-band operation by DXpeditions in major amateur radio competitive events) to use
vertical polarization for portable operation on the lower HF bands, by far the most
likely scenarios dictate the use of a horizontally polarized antenna.
Putting up a dipole is an often overlooked approach when operating portable. In
addition to increased radiation at high wave angles, this eliminates the need for ground
radials and may cut the required amount of wire in half.
The biggest impediment to erecting a dipole sufficiently high in the air to be useful
is the number and spacing of suitable available supports. Trees, flagpoles (but not utility
poles), roofs of buildings, and upstairs windows can all be candidates. Depending
on the distances to be covered, a height of 30 or 35 ft for an 80-m dipole can be quite
useful.
Because of the low power level involved, the dipole can be fed with lightweight
RG-58 or RG-8X coaxial cable. Of course, open-wire line (OWL) is an excellent companion
to a dipole, but most will find it easier to keep a stored roll of coaxial cable in good
working order between emergencies or portable “expeditions” than to stow a spool of
true OWL.
In a pinch, zip cord or lamp cord makes an easily obtainable feedline; measurements
on zip cord some years ago indicated a characteristic impedance of about 72. However,
the losses can become excessive on the higher bands. In a worst-case scenario, lengths of
rotor cable, telephone cable, or Cat 5 or Cat 6 Ethernet cable can be pressed into service!
Figure 17.2 shows the common dipole and the normal equation for determining approximate
length. Typically it is cut “long” and then trimmed until the VSWR drops to
its lowest point, a nicety that might not seem altogether important in an emergency.
One or both ends of the dipole must be supported on trees, masts, or some other elevated
structure.
If only a single high support is available to use, a sloping dipole or an inverted-vee
can be substituted for the dipole. For the antenna of Fig. 17.3, assume as a starting point
for dimensions that the length of each leg is 6 percent longer than would be obtained by
using the dipole formula of Fig. 17.2.
A final approach is to combine the vertical with the horizontal and run a randomlength
wire up as high as you can and then horizontally as far as you can. A total of 140
4
4
Rope
Insulator
Center
insulator
Insulator
Rope
Coax to
transmitter
Figure 17.2 Basic dipole.