Practical_Antenna_Handbook_0071639586

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 365
As we have mentioned many times, the length of a l/2 dipole is given approximately
by
L
ft
468
= (15.1)
F
If you do some quick calculations you will find that antennas for the 10-, 13-, 15-, and
(possibly) even 18- and 20-m bands, will fit entirely inside the typical town house attic.
This statement is also true of the 11-m citizens band antenna. But what about the lowerfrequency
bands?
Figure 15.1 hints at a possible solution for the lower frequencies (specifically, 80, 40,
and 30 m) in a town house or single-family residence where the radio enthusiast has
access to the highest interior space. In a sticky situation we can install a dipole with its
arms bent in conformity with the space available. In this example, only one of many
possible methods for accomplishing this job is shown. Here each quarter-wavelength
section is composed of two legs, AB and CD, respectively. Ideally, segments B and C are
the longest dimensions. Another method is to reverse the direction of one end leg—say,
for example, D—and run it to the other corner of the building over the peak of the roof.
In reality, you might not be able to use the exact layout shown—so ad lib a little bit.
How about performance? Will the constrained dipole of Fig. 15.1 work as well as a
regular dipole installed a wavelength or two off the ground and away from objects? Of
course not—especially for DXing, where low radiation angles make a big difference.
But that is not the problem being solved; getting on the air at all is the problem at hand.
You will find that the pattern of the constrained dipole is distorted compared with that
of the regular dipole. In addition, the feedpoint impedance is not going to be 50 or 73 Ω
(except by some fluke), so you will be required to use an antenna tuner of some kind.
Although wood (as in rafters and studs) is an insulator, the wire used in the constrained
dipole (or other forms of attic antenna) should be mounted on TV-type screwin
standoff insulators if any significant output power is employed. Almost any store
that sells TV antennas or accessories will have them. These standoff insulators are also
available in many local hardware stores and department stores that sell TV antennas, as
well as at electronics parts suppliers.
MHz
Caution Do not simply tape or staple the wire to the wooden underside of your roof. The
reason is simple: In all transmitting antennas, voltages can get high enough—not only at
the ends but at many multiple locations along the wire—to produce corona effects. The
resulting arcing can easily start a fire!
Another alternative for the attic antenna is the nonresonant loop shown in Fig. 15.2.
Although presented as a top view, the loop can be installed in any configuration that is
compatible with the available space. In fact, the best performance will be bidirectional
when the loop is installed vertically. Again, use standoff insulators and insulated wire
for the installation. Ideally, the giant loop is fed with parallel line and tuned with a balanced
ATU, but it is no crime to feed it with coaxial cable. As was true with the constrained
dipole, the performance is not to be equated with the performance of higher
antennas outside but is far superior to the prospect of not being on the air at all!