Practical_Antenna_Handbook_0071639586

258 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
2
R
I
I
R
R rope
I insulator
R 1
400–800
ATU
XMTR
4 radials
Figure 10.6 Longwire (greater than 2l) antenna is terminated in a resistance (typically 400 to 800 Ω).
Radials under the resistor improve the grounding of the antenna.
varies with frequency, even though the pattern remains unidirectional. The directivity
of the antenna is partially specified by the angles of the main lobes. It is interesting to
note that gain rises almost linearly with nl, while the directivity function changes rapidly
at shorter lengths (above three or four wavelengths the rate of change diminishes
considerably). Thus, when an antenna is cut for a certain low frequency, it will work at
higher frequencies, but the directivity characteristic will be different at each end of the
spectrum of interest.
Of course, if a terminated longwire is used for transmitting, approximately half the
transmitter power will be dissipated in the termination resistor, so one should subtract
3 dB from the overall gain relative to unterminated antennas.
Arrays of Longwires
Longwire antennas can be combined in several ways to increase gain and sharpen directivity.
Two of the most popular of these are the vee beam and the rhombic. Both forms
can be made in either resonant (unterminated) or nonresonant (terminated) versions.
Since a 1l or longer longwire is itself an array, these antennas can be considered to be
arrays of arrays.
Vee Beams
The vee beam (Fig. 10.7) consists of two equal-length longwire elements (Wire 1 and
Wire 2), fed 180 degrees out of phase and oriented to produce an acute angle between
them. The 180-degree phase difference is provided automatically by connecting the two
wires of the vee to opposing sides of a single parallel conductor feedline.