Practical_Antenna_Handbook_0071639586

C h a p t e r 6 : D i p o l e s a n d D o u b l e t s 177
In wires and other conductors, the length calculated here is too long. The physical
length of a conductor having a diameter or cross-sectional side much, much smaller
than its length is shortened by about 5 percent because of the velocity factor of the wire,
any insulation on it, and capacitive effects of the end insulators. A more nearly correct
approximation for such a “slender” half-wave dipole is
468
L (feet) =
F(MHz)
(6.3)
Example 6.1 Calculate the approximate physical length for a half-wavelength dipole
operating on a frequency of 7.15 MHz, the middle of the U.S. 40-m band.
Solution
468
L =
F
468
=
7.15
= 65.45 ft
or, stated in feet and inches:
L = 65 ft 5½ inches
In practice, it would be smart to spool off about 70 ft of wire, fold it in half, and cut
it at the midpoint. This leaves a foot or more of wire at each end of the two halves for
threading through end and center insulators and wrapping back on itself.
Despite the precision in the preceding calculation of L, there’s no real good reason
for worrying about the exact length of this dipole. Several local factors—nearby objects,
height above ground, and the length/diameter ratio of the conductor to name a few—
might make it necessary to either add or trim a short length of wire on each side to
achieve exact resonance. But to what end? The primary reason to aim for approximately
a half-wavelength is to make the antenna wire resonant, or nearly so, at the frequency of
operation so that we can minimize the reactive part of the feedpoint impedance that
will have to be tuned out by the transmitter, an antenna tuning unit (ATU), or other
matching network. Unfortunately, a lot of people accept Eq. (6.3) as a universal truth, a
kind of immutable Law of the Universe. Perhaps abetted by books and articles on antennas
that fail to reveal the full story, too many people spend too much time worrying
about the wrong things when constructing and installing dipoles.
Whether the choice of design frequency in the preceding equations is critical or
not depends on the percentage bandwidth of the band in question. For instance, for
a half-wave dipole cut to the center (14.175 MHz) of the U.S. 20-m band, the antenna
might be used at frequencies ranging between plus and minus 1.2 percent of the
center frequency, and there will be a generally insignificant change in the feedpoint
impedance over the entire band. An 80-m dipole, on the other hand, could conceivably
be called upon to span a range of frequencies (in the United States, at least) up
to plus and minus nearly 7 percent of the band center (3.750 MHz). For most home-