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-
C h a p t e r 1 5 : H i d d e n a n d L i m i t e d - S p a c e A n t e n n a s 367 Flag End cap Wooden or plastic plug Taut #14 or #12 wire hidden inside PVC plumbing pipe (sectioned view) PVC plumbing pipe 1/2 inch to 1 1 /4 inch aluminum tubing Tuner Coax to XMTR Feedpoint Figure 15.3A Flagpole antenna made from thick-walled PVC pipe. Figure 15.3B PVC flagpole hiding an aluminum tubing vertical.
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C h a p t e r 1 5 : H i d d e n a n d L i m i t e d - S p a c e A n t e n n a s 367<br />
Flag<br />
End<br />
cap<br />
Wooden or<br />
plastic plug<br />
Taut<br />
#14 or #12 wire<br />
hidden inside<br />
PVC plumbing pipe<br />
(sectioned view)<br />
PVC<br />
plumbing<br />
pipe<br />
1/2 inch to 1 1 /4 inch<br />
aluminum tubing<br />
Tuner<br />
Coax<br />
to<br />
XMTR<br />
Feedpoint<br />
Figure 15.3A Flagpole antenna made<br />
from thick-walled PVC pipe.<br />
Figure 15.3B PVC flagpole hiding an aluminum tubing<br />
vertical.