Practical_Antenna_Handbook_0071639586

C h a p t e r 1 8 : a n t e n n a s f o r 1 6 0 M e t e r s 419
The second problem with using the antenna on its second harmonic is this: Much
more of the transmitter power (roughly half, in fact) is going to be radiated from the
horizontal section of wire. (The total wire is a half-wavelength on 80, and the current
maximum is at the midpoint of the wire, at the point where the vertical and horizontal
sections come together.)
A work-around that minimizes both problems is to insert an 80-m trap at the top of
the vertical section—or even a slight distance out on the horizontal section if the total
vertical run is less than 60 ft or so. The trap will also permit some shortening of the
horizontal top section, allowing the total wire length to be less than 150 ft.
Phased Inverted-L Antennas
Because it is inexpensive and normally series-fed, the inverted-L is an excellent candidate
to be the basic element or building block of an all-driven 160-m phased array. All
the considerations of Chaps. 5 and 11 for ensuring proper and stable phasing pertain to
the inverted-L, as well.
But inverted-Ls make good building blocks for parasitic arrays, too. Consider, for
instance, an inline array of three inverted-Ls as being equivalent to one side of a vertically
polarized three-element Yagi, the other side of each pseudo-Yagi element being
replaced by ground. Using interelement spacings of 0.15l results in a total baseline (or
boom equivalent) length of about 170 ft. Forward gain, F/B, etc., are then optimized by
adjusting the vertical portions of the Reflector and Director, preferably with the aid of an
antenna modeling program. Unlike the horizontal Yagi this array is derived from, a
good radial system is required under each element for maximum efficiency and stability
of parameters.
Flat-Top or T Antenna
If you are primarily interested in DXing, then you will want to minimize the amount of
signal that you put straight up in the air. One way to do that, if you have the backyard
space, is to convert the “inverted-L” into a “T” by extending another horizontal wire,
exactly the same length as the existing horizontal segment, in the opposite direction
(Fig. 18.7). Now whatever standing wave of current exists in the first horizontal wire
will be balanced by an equal current in the opposite direction in the new wire.* At distant
points nearly straight up (meaning the ionosphere directly above you), the total signal
from the antenna will approach zero. The effect of this is to re-form the radiation pattern
of the antenna such that nearly all the transmitter RF emanates from the vertical portion
of the wire, thus increasing its low-angle performance—and coincidentally eliminating
some of the azimuthal pattern skew introduced by the single horizontal wire. The flat-
*You can see this by considering an instance when the current in the vertical portion of the antenna
is flowing in the downward direction. Think of the two horizontal sections as having been created by
splitting the original horizontal section of wire in half lengthwise. Since the two horizontal sections are
part of the same quarter-wave monopole as the vertical section, the current in both of them must be
flowing into the three-way junction at the top of the vertical section. For the current in both horizontal
sections to be flowing into the junction, the current in the left-hand section must flow to the right, and
the current in the right-hand section to the left. These two segments of wire constitute a two-element
horizontally polarized array, and at any point in space directly above the antenna (or nearly so), the
fields from the two wires cancel because they are of opposite polarity.