Practical_Antenna_Handbook_0071639586

406 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
which about a half-wavelength of insulated wire is wound over an insulating form
(such as a length of PVC pipe or a wooden dowel); the turns of the coil are spread out
over the entire length of the insulated support. I 2 R losses in the winding are somewhat
higher than in a straight vertical member because the conducting element is not only
longer but often has a smaller cross-sectional area.
Linear loading (Fig. 18.1E) is an arrangement whereby a section of the antenna is
folded back on itself like a stub. Antennas of this sort have been successfully built
from the same type of aluminum tubing as regular verticals. For 160-m operation, a
length of 60 ft for the radiator represents 41 degrees, while a normal l/4 vertical corresponds
to 90 degrees. The difference between 90 degrees and 41 degrees of electrical
length is made up by the “hairpin” structure at the base. A potential disadvantage of
the linear loading approach is the requirement for an additional insulator somewhere
along the monopole’s physical support structure if the antenna is supported from the
bottom (as a tower or rigid mast might be), rather than suspended from above (as a
wire would be).
The relative cost and mechanical complexity of continuously loaded and linearly
loaded verticals for 160 m generally render them inferior to other approaches—especially
the top hat of Fig. 18.1C—and we shall not consider them further in this chapter.
Loading the Tower
Many amateur radio installations include one or more rotatable Yagi antennas on top of
an appropriate support structure, such as a ground-mounted guyed triangular lattice
tower. By far the preponderance of amateur tower heights is in the 40- to 90-ft range—
often considered too short to be an efficient vertical monopole radiator on 160 m. But
with proper attention to installation details and a modest set of radials, most such tower
and antenna combinations can become excellent transmitting and receiving antennas
for 160 m.
There are two critical components to making a “short” tower an efficient radiator on
160 (or 80) m:
• The apparent electrical length must be increased to approximately l/4 or
slightly greater. This brings the natural high-current part of the antenna out into
the open so as to increase the I × h (or I × l) product, and coincidentally raises
the radiation resistance, thus beneficially altering the resistive divider formed
by the radiation resistance and ground losses.
• A low-loss ground return path for the RF fields surrounding the vertical must
be provided. In the absence of saltwater at the base of the tower, 20 to 60 copper
radial wires—each approximately as long as the tower is tall—should be
connected (through an appropriate anticorrosion metal) to the grounded side of
the transmission line or base-mounted ATU.
As implied at the beginning of this topic, an excellent way to turn a short grounded
tower into an efficient 160-m antenna is to utilize one or more HF Yagi antennas at the
top to provide top loading, but there are at least two other ways to do the same thing:
• If the tower is guyed, allow the top set of guy wires to extend out and down
from the top of the tower by a calculated amount before inserting a set of
insulators.