Practical_Antenna_Handbook_0071639586

256 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
Longwire Pattern and Gain
As the number of wavelengths in an end-fed wire increases—either by using a fixedlength
wire on a higher frequency or by lengthening the wire while keeping the operating
frequency fixed—both the pattern and the direction of maximum gain change, as
well. At l/2 the pattern of an end-fed wire is the broadside “doughnut” of a l/2 dipole,
perhaps very slightly skewed as a result of being fed at one end instead of in the center.
At a length of l, the doughnut-shaped figure eight pattern has broken into a four-lobed
cloverleaf with peaks at 45 degrees from the antenna’s axis, and the maximum gain of
the antenna (in the center of each lobe) has increased by almost 1 dB. As the frequency
is further increased, the pattern breaks into more and more lobes, such as that depicted
in Fig. 10.4 for a 2l longwire, but the broadest of these maintain a gradual “march” toward
the axis. At the same time, the peak amplitude of these main lobes gradually increases,
reaching an additional 5 dB compared to the l/2 wire at a length of 4l, which
corresponds to using an 80-m 135-ft l/2 dipole on 10 m. Figure 10.5 plots the increase
in gain of the main lobe relative to a l/2 dipole as a function of longwire length.
Regrettably, most longwires are fixed in one location; if the longwire is your only
HF antenna, having the direction of peak signal gradually precess from broadside to
nearly 90 compass degrees away while changing frequency may not suit your operating
needs. More typically, however, an amateur or SWL with acreage available installs
multiple longwires for a preferred band. An ideal configuration might have the fed end
of all the wires located at a common point where an ATU is located and can be switched
easily from one wire to another.
A rarely mentioned problem with very long wires: Electrostatic fields build up a
high-voltage dc charge on longwire antennas! Thunderstorms as far as 20 mi away are
known to produce substantial charge build-up on long antennas, capable of causing
serious damage to a receiver or its operator! A simple solution is to connect a transmitter-rated
RF choke (RFC) between the antenna terminal and the ground connection of
the ATU. In a pinch, a dozen 10-MΩ, 2W resistors in parallel can be substituted.
Nonresonant Longwire Antennas
The resonant longwire antenna is a standing wave antenna because it is unterminated at
the far end. A signal propagating from the feedpoint toward the open end will be reflected
back toward the transmitter from the open end; the resulting superposition of
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Figure 10.4 Radiation pattern of longwire antenna.
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