Practical_Antenna_Handbook_0071639586

366 P a r t V : H i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
Large wire
loop
(Top view)
Tuner
Figure 15.2 Large loop antenna.
Parallel line
Coax to
XMTR
Keep in mind that indoor antennas are
usually inferior receiving antennas because
of their proximity to a multiplicity of noise
sources: noisy power wiring in the walls of
the structure, microprocessor waveforms,
Ethernet drivers, wall adapters for myriad
consumer electronics devices, and the like. If
you are plagued by noise that comes from a
single direction, you might consider the use
of a noise-canceling accessory, such as those
made by ANC or MFJ, for your receiver or
transceiver. These devices pick up the noise
on a sense antenna that is often nothing more
than a short piece of wire and then change
the phase of the detected noise signal so it
cancels the noise on your main antenna. The
noise canceler approach works best when
the offending noise comes from a single
source and/or a specific direction.
For those who can entertain the possibility
of hidden antennas outdoors, another
ploy is the old “flagpole trick” shown in
Figs. 15.3A and 15.3B. Some housing developments
allow homeowners to promote
their patriotism by installing flagpoles—and
flagpoles can be antennas in disguise. In the
most obvious case, you can install a brass or
aluminum flagpole and feed it directly from
an ATU. For single-band operation, especially
on the higher frequencies, you can delta-feed the “flagpole” unobtrusively and
call your flagpole a vertical antenna. But that is not always the best solution.
Figures 15.3A and 15.3B show two different methods for creating a flagpole antenna;
both depend on using white PVC plumbing pipe as the pole. The heavier grades
of PVC pipe are self-supporting to heights of 16 to 20 ft, although lighter grades are not
self-supporting at all (hence, are not useable).
Figure 15.3A shows the use of a PVC flagpole in which a #12 (or #14) wire is hidden
inside. This wire is the antenna radiator. For some frequencies the wire will be resonant,
and for others it will surely be nonresonant. Therefore, an ATU is used either at the base
of the antenna or inside at the transmitter. If the wire is too long for resonance (as might
happen in the higher bands), place a capacitor in series with the wire. Multiple settings
may be required, so use a multisection transmitting variable that has a total capacitance
selectable to more than 1000 pF. Alternatively, use a vacuum variable capacitor of the
same range.
In cases where the antenna is too short for resonance, as will occur in the lower
bands, insert an inductance in series with the line to “lengthen” it. Alternatively, use an
L-section tuner at the feedpoint.
A good compromise situation is the use of a 16-ft length of “flagpole” pipe with a 16-ft
wire embedded inside. The 16-ft wire is resonant at 20 m, so it will perform like a quarter-