Practical_Antenna_Handbook_0071639586

416 p a r t V I : a n t e n n a s f o r O t h e r F r e q u e n c i e s
nighttime signal on 160—especially if tall trees are available. Of course, the user has the
choice of at least four variations on a dipole: fully horizontal, sloping, inverted-vee, or
bent. Having enjoyed the use of three fully horizontal 160-m dipoles at 100+ ft for more
than a decade some years ago, the author feels qualified to observe that even though the
antennas were low (in terms of wavelength at 1.8 MHz), they were excellent for distances
out to 1500 mi or so, beyond which they generally took a backseat to a single
top-loaded vertical. And with the appropriate ATUs capable of handling high feedpoint
impedances when operating a dipole on its second harmonic, the antennas were superb
performers on 80 m, as well, where they exhibited about 2 dB more gain in the main
broadside lobe than a conventional l/2 dipole.
For limited spaces, the author prefers bent dipoles to inverted-vees, for reasons
described in detail in Chap. 6. However, the former antenna requires two high supports,
while the latter needs but one. With only one high support but no particular
space limitations, the sloping dipole is often a better choice than the inverted-vee.
Shortened Horizontal Antennas
The strategies that work for loading short vertical antennas also work for horizontal
antennas, although in the horizontal case we are simulating a half-wavelength (180-
degree) balanced antenna rather than a l/4 unbalanced antenna. Because of the large
dimensions involved, probably the only practical form of shortened horizontal antenna
is one with inductive loading in the middle of each leg, either with loading coils or with
traps that allow use of the same antenna on 80 m.
Inverted-L Antennas
One of the simplest wire antennas, but one that can produce excellent results, is the l/4
inverted-L (Fig. 18.6). Typically considered when there is no chance of having a high
enough support to hang a full l/4 wire vertically over its entire length, this antenna is
simplicity itself. Starting as close to the antenna coupler (ATU) or transmitter as is reasonable,
run as much of a 170-ft piece of wire vertically as high as you can. Then run the
remainder of the wire horizontally until the full 170 ft have been deployed. As an example,
this might result in a 65-ft vertical section and a 105-ft horizontal section, but
wide variations from these lengths are often found—even vertical sections as short as
30 ft have merit. The recommendation to make the total wire length somewhat longer
than l/4 on 160 is partly to make sure the natural high-current portion of the standing
wave is out on the vertical portion of the wire, not trapped inside the feedline, and
partly to give a little boost to the radiation resistance. It also has the effect of forcing the
input impedance to be slightly inductive so that the feedpoint reactance can be tuned
out easily with a series air variable transmitting capacitor. Fed as described, the antenna
should exhibit a reasonable match to 52- or 75-Ω coaxial cable.
Basically, the inverted-L is a cousin to a quarter-wave vertical monopole. Some prefer
to look at it as a bent vertical, while others see it as a short vertical with asymmetrical
top loading. If constructed as shown in Fig. 18.6, a small portion of the transmitter
power will be radiated from the horizontal half of the wire, providing some high-angle
signal useful for working stations within a few hundred miles. If you don’t have a
tower you can shunt-feed, but you need a good antenna for 160 that will acquit itself
well on DX yet still allow you to work stations “in close”, you will be hard-pressed to
beat the inverted-L.