Practical_Antenna_Handbook_0071639586

220 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
A
Alligator
clip
lead
A
Coax
Insulator
B
Wood or
PVC mast
Links
B C
C
Rope
Rope
Figure 8.1C Multiband inverted-vee uses shorting links to change bands.
Some amateurs build the multiple-band dipole from four- or five-wire flat TV rotator
cable. Starting with the highest frequency, cut each wire to the required length and
strip off any unused portions.
Another possibility is the ”jumper-tuned” inverted-vee shown in Fig. 8.1C. In this
situation, a single conductor, broken into segments A, B, and C (or more, if desired), is
used for each leg of the antenna. Adjacent segments are separated by inline insulators.
(Standard end insulators are suitable.) Segment A is a quarter-wavelength on the
Âhighest-frequency band of operation, A + B is a quarter-wavelength on the next highest
band of operation, and A + B + C is a quarter-wavelength on the lowest-frequency band
of operation.
The antenna is “tuned” to a specific band by either connecting or disconnecting a
wire (see inset) jumper across the insulator between the segments. Either a switch or an
alligator clip jumper will short out the insulator to effectively lengthen the antenna for
a lower band.
A big disadvantage to this type of multiband antenna is that you must go outside
and manually switch the jumpers to change bands, which probably explains why other
antennas are a lot more popular, especially in northern latitudes. One way to get at the
higher jumpers is to make the center support a tilt-over mechanism (as described in
Chap. 28).
Log-Periodic
From a distance, the log-periodic (LP) antenna resembles a long boom Yagi (Chap. 12)
parasitic array, but it is not. Rather, it is an all-driven array that derives its name from
the fact that each element length and the spacing from that element to the next one is a
constant percentage of the previous element’s length and spacing, respectively. An
open-wire parallel transmission line that runs the length of the boom feeds the centers
of all elements, which are split and insulated at their midpoints, but the transmission
line is flipped as it passes from element to element so that each element is fed 180 de-