Practical_Antenna_Handbook_0071639586

418 p a r t V I : a n t e n n a s f o r O t h e r F r e q u e n c i e s
shelter, such as a coffee can or a child’s beach bucket. If you are using a matching
capacitor, consider investing in a plastic storage container to enclose everything.
• Don’t worry if property lines, driveways, or buildings keep you from making
all the radials the same length. Just do the best you can. Radials in close proximity
to the ground are not resonant, so their exact length is immaterial.
One popular method of constructing the inverted-L employs an existing tower for
the vertical section and a length of wire for the horizontal section. If you already have a
60-ft tower to accommodate a beam antenna used on higher frequencies, it is relatively
easy to turn it into an inverted-L for 160 m. If the base of the tower and any guy wires
are insulated, all that is necessary is to connect the shield of the coaxial feedline and the
tips of the radials together at the bottom end of the insulator and connect the center
conductor of the feedline (or one end of the matching capacitor) to one leg of the tower
above the insulator. (But never connect copper wire directly to a galvanized tower. See
Chap. 29 for additional information.) If the tower is not insulated from ground, run a
wire up the side of it, using aluminum angle-stock with insulators, rigid PVC pipe, or a
two-by-four from the hardware store to hold the vertical portion of the wire at least
a few feet away from the tower. Depending on its natural electrical length, the tower
may still affect the tuning of the inverted-L—becoming an incidental reradiator in the
process—but that’s not necessarily bad. Keep in mind, however, that the effect of the
tower may vary as feedlines to HF Yagis or other antennas at its top are switched in and
out at their far end in the radio room.
Like the little girl in the nursery rhyme, the inverted-L can be “very, very good”—
but its success is far more related to the quality of its system of radials than it is to the
exact height of the vertical section of radiator. The inverted-L can approach the performance
of a full-size l/4 monopole, but only if these two factors are optimized:
• The radial field must be extensive, so as to minimize losses in the earth beneath
it. This is more important than for a full-size l/4 or taller vertical radiator
because the radiation resistance of an inverted-L is typically lower (20 to 25 Ω)
than that of the taller (37 Ω) vertical, Hence, it takes a lower ground resistance
to dissipate no more transmitter power in ground losses, than in the case of the
full-length l/4 radiator.
• The total length of wire in the inverted-L should be at least 150 ft in order to get
the high-current portion of the standing current out of the feedline and onto the
open vertical portion of the antenna.
Dual-Band Inverted-L
Once you’ve gone to the trouble of erecting a wire antenna for 160 m, the natural question
is “Can I use this on 80 m?” The answer is “Yes, but only with a different way of
feeding it”. The problem with trying to use the 160 inverted-L on its second harmonic is
that the impedance at the feedpoint, near the ground, is very high (high voltage, low
current) because it is roughly l/2 from the far end of the wire. Instead of being close to
that of the coaxial cable, the feedpoint impedance is apt to be a few thousand ohms.
This can be matched with a good parallel-tuned ATU or a step-up transformer, but the
system is likely to tune more sharply.