Practical_Antenna_Handbook_0071639586

436 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
because it allows securing of each parasitic element with a pair of hex nuts, one on either
side of the 2- × 2-in boom. Unthreaded elements can be secured with RTV to seal a
press-fit. Alternatively, secure the rods with tie wires (see inset to Fig. 19.8): drill a hole
through the 2 × 2 to admit the rod or tubing, secure the element by wrapping a tie wire
around the rod on either side of the 2 × 2, and solder it in place. Use #14 to #10 solid bare
or tinned wire for the tie wires.
Mount the mast of the antenna to the boom with an appropriate boom-mast bracket
or clamp. One alternative is to use an end-flange clamp, such as is sometimes used to
support pole lamps and the like. The mast should be attached to the boom at the center
of gravity, also known as the balance point. If you try to balance the antenna on one hand
unsupported, there is one (and only one) point at which it is balanced (and won’t tilt
end down). Attach the mast hardware at, or near, this point in order to prevent normal
gravitational torques from tearing the mounting apart.
The antenna is fed with coaxial cable at the center of the driven element. Often, either
a matching section of coax or a gamma match will be needed because the usual
effect of parasitic elements on a Yagi’s feedpoint impedance is to make it too low to be
a good direct match to standard coaxial cable impedances.
Halo Antennas
One of the more “saintly” antennas used on the VHF boards is the halo (Fig. 19.9), formed
by bending a half-wavelength dipole into a circle. The ends of the dipole are connected
to the two plates of a capacitor. In some cases, a transmitting-type mica “button” capacitor
is used, but more commonly the VHF halo capacitor consists of two 3-in disks separated
by air or plastic dielectric. In some implementations of the halo, the spacing of the
disks is adjustable via a threaded screw. While air is a good (and perhaps better) dielectric,
the use of a plastic spacer provides
C
mechanical rigidity to help stabilize the
exact spacing and, hence, the value of the
capacitor.
/2
Quad Beam Antennas
The quad antenna was introduced in
Chap. 13. Although much of the discussion
there revolved around implementations
for HF, it has, nonetheless, also
emerged as a very good VHF/UHF antenna.
It should go without saying that
the antenna is a lot easier to construct at
VHF/UHF frequencies than it is at HF
frequencies!
Figure 19.10 depicts only one of several
methods for building a quad for VHF
or UHF. The radiator element can be any
Feedpoint
Figure 19.9 Halo antenna.