Download MULTIBAND ANTENNAS The ARRL Handbook, 1964. As suggested in the

MULTIBAND ANTENNAS
The ARRL Handbook, 1964.
As suggested in the preceding section, the same antenna may be used for several bands by
operating it on harmonics.
When this is done it is necessary to use TUNED FEEDERS, since the impedance matching
for nonresonant feeder operation can be accomplished only at one frequency unless means
are provided for changing the length of a matching section and shifting the point at which
the feeder is attached to it.
A dipole antenna that is center-fed by SOLID-DIELECTRIC line is USELESS for even
harmonic operation; on all even harmonics there is a voltage maximum occurring right at
the feed point, and the resultant impedance mismatch causes a large standing-wave ratio
(SWR) and consequently high losses arise in the solid dielectric.
It is wise not to attempt to use on its even harmonics a half-wave antenna center-fed with
coaxial cable.
On odd harmonics, as between 7 and 21 MHz, a current loop will appear in the center of
the antenna and a fair match can be obtained.
High –impedance solid-dielectric lines such as 300 Ohm Twin-Lead may be used in an
emergency, provided the power does not exceed a few hundred watts, but it is an inefficient
feed method.
When the same antenna is used for work in several bands, the directional characteristics
will vary with the band in use.
The most practical simple multiband antenna is one that is a half wavelength long at the
lowest frequency and fed either at the center or one end with an open-wire line.
Open-wire line feed is recommended for an antenna of this type, since the losses will run
too high in solid-dielectric line.
For low-power applications up to a few hundred watts, open wire-wire TV line is
convenient and satisfactory to use.
However, for high-power installations up to the kilowatt limit, an open-wire line with №14
(1.6 mm) or №12 (2 mm) conductors should be used.
73!
EW1MM (набрано вручную из “The ARRL Handbook, 1964г.”)