Practical_Antenna_Handbook_0071639586

216 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
Each side is
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2 2
L side = 480
F MHz
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/
Matching section
4
1:1 BALUN
transformer
52 Coaxial
cable to receiver
except that each side is now l/2 in length. If the
reactance is canceled out through slight shortening
or lengthening of the total loop, the feedpoint
impedance is a good match for either
52-Ω or 75-Ω cable directly. The difference in
feedpoint impedances between the 2l diamond
loop and the 2l quad loop is a direct result of
changing the location of the feedpoint from the
midpoint of a side to the corner between two
sides.
When operated at its design frequency and
fed as shown in Fig. 7.8, the bisquare antenna
offers maximum horizontally polarized radiation
in free space at a 45-degree elevation angle
broadside to the plane of the antenna (42 degrees
when the feedpoint is l/4 above ground).
The peaks of the vertically polarized radiation
pattern are as before—that is, in the plane of the
wire (to the left and right on the page), but the
gain is 2 dB or so less than that of a single vertical
l/2 dipole, thus proving that “more” is not
always “better”!
In general, there is little to recommend the
added mechanical complexity of a 2l loop unless
the phase relationship between opposing
sections is “corrected” with the use of phasing
lines or inductors.
Near-Vertical Incidence Skywave
(NVIS) Antenna
The preceding descriptions of large loop operation
in the presence of ground are based on
hanging each loop from one or two high supports.
However, these loop antennas can instead
be suspended so that the plane of the loop of wire is parallel to the ground beneath
it. When that is done, at practical heights above ground these loops will send most of
their radiation straight up in the air, since the innate array pattern of the 1l loops generates
maximum radiation at right angles to the plane of the loop.
Figure 7.8 Bisquare 2l square loop antenna.
Given
0111057
the
FIG
high-angle
14-10
horizontal polarization of the bisquare loop, which favors
short-haul communications links, comparable results can be obtained by laying the
same amount of wire “on its side”, supported at a more practical height. A 2l loop of
wire arranged in a diamond of uniform height l/8 or more can provide nearly the same
gain as the bisquare, albeit in a more omnidirectional pattern. Actually, the pattern of
the 2l NVIS antenna consists of two separate four-lobed cloverleafs: one with horizontally
polarized lobes at right angles to the four wires, and the other with vertically polarized
lobes midway in between.