Practical_Antenna_Handbook_0071639586

C h a p t e r 1 9 : V H F a n d U H F A n t e n n a s 439
of the commonly used scanner frequencies. Although antenna performance is not optimized
for the scanner frequencies, it is also not terrible on those frequencies. If you already
have an “all channel” TV antenna installed, then it is a simple matter to connect
the antenna to the scanner receiver with a 2:1 splitter and (possibly) a 4:1 balun transformer
that accepts 300 W in and produces 75 W out. These transformers are readily
available at RadioShack and anywhere that TV and video accessories are sold.
The directional characteristic of the TV antenna can be boon or bane to the scanner
user. If the antenna has a rotator, then there is no problem. Just rotate the antenna to the
direction of interest (unless someone else in the family happens to be watching TV at the
time!). But much of the time it won’t even be necessary to rotate the TV/FM Âantenna—
partly because no antenna completely rejects signals arriving from off the sides or the
back, and partly because most of the local scanner repeaters will be quite strong.
An excellent alternative to hijacking the family’s primary TV antenna is to locate an
indoor set-top TV antenna at a garage sale or flea market (or maybe your own attic).
Some of these have a simple phasing circuit built in with a front panel knob that allows
the user to move the peak of the antenna pattern around some. With the possible addition
of a wideband TV antenna preamplifier (available at RadioShack, among other
places), such a setup may be perfectly adequate for scanner monitoring.
Scanner Skyhooks
Some popular scanners even cover much of the HF spectrum. For true SWLing on those
bands, a random-length wire antenna ought to turn in decent performance, and it may
even be able to do double duty as a VHF/UHF longwire antenna. Typically, this antenna
is simply a 30- to 150-ft length of #14 wire attached to a distant support.
Additional gain, about +3 dB, can be achieved by stacking VHF/UHF antennas together.
Figure 19.11 shows a typical arrangement in which two half-wavelength dipole
antennas are connected together through a quarter-wavelength harness of RG-59/U
coaxial cable. This harness is physically shorter than an electrical quarter-wavelength
by the velocity factor of the coaxial cable:
vF
L (inches) = 2832 (19.8)
F(MHz)
where L = length, in inches
v F = velocity factor (typically 0.66 or 0.80 for common coax)
F = frequency, in megahertz
The antennas can be oriented in the same direction to increase gain, or orthogonally
(as shown in Fig. 19.11) to obtain a more omnidirectional cloverleaf pattern.
Because the impedance of two identical dipoles, fed in parallel, is one-half that of a
single dipole, it is necessary to have an impedance-matching section made of RG-58/U
coaxial cable. This cable is then fed with RG-59/U coax from the receiver.
There is nothing magical about scanner antennas that is significantly different from
other VHF/UHF antennas except, perhaps, the need to cover multiple frequency
ranges. Although the designs might be optimized for VHF or UHF, these antennas are
basically the same as others shown in this book. As a matter of fact, almost any antennas,
from any chapter, can be used over at least part of the scanner spectrum.