Practical_Antenna_Handbook_0071639586

402 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
greatly increases absorption of these near-vertical signals in the lower layers of
the ionosphere, making daytime short-haul communication problematic. Further,
during periods of very low sunspot activity, the critical frequency, f C , can in
fact drop below 1.8 MHz, with the result that nighttime skip entirely disappears
on the topband. An efficient vertically polarized antenna can cover the same
region with a strong ground wave that is present 24/7 and is not dependent
upon conditions in the ionosphere.
What, then, is a “decent” vertical installation? Surprising to many who are wrestling
with antenna space requirements for the first time, vertical antennas require nearly
as much backyard space (or footprint) as l/2 dipoles! Although a 40-m vertical (33 ft
high) is not an unreasonable mechanical assembly and has modest guying requirements,
a full-size quarter-wave vertical for 160 stands 125 to 140 ft tall, and will likely
require a guy wire footprint having a diameter of 200 ft or greater. In addition, local
building codes may not require any special inspections or permits for the 33-ft antenna
but may impose rigid and very exacting requirements on the higher structure. Worse
yet, an antenna with overall height substantially taller than the maximum allowed for
a principal structure (often 35 or 40 ft in the United States) may not be allowed at all by
the local zoning ordinance! On suburban or urban lots, a typical 40-m vertical might
well be able to fall over without crossing the property line or landing on power lines.
Not so the 160-m vertical!
Thus, the biggest problem for most topband DXers, as you can see, is the size of
antennas for those frequencies; but there are ways to shorten an antenna—not without
penalty, mind you, because the TANSTAAFL* principle still applies—to the point
where an antenna for 160 becomes feasible with limited yard space. In return, the user
typically gives up a few decibels of distant signal strength for a given power delivered
to the antenna and suffers a narrower VSWR bandwidth before retuning of the antenna
is necessary.
As discussed in Chap. 16, to get the most (radiated field) from an antenna we must
maximize the area under its current-versus-length curve while simultaneously minimizing
the portion of the RF energy from the transmitter that is wasted in feedline losses
and ground system losses. In general, this means that our preferred antennas are those
that give maximum exposure to the natural high-current portions of the antenna
element(s), exhibit a high radiation resistance relative to other losses in the system, and
are a reasonably close match to available feedlines. Further, if the antenna is a monopole
(such as a ground-mounted or ground-plane vertical) being operated against ground, we
must not neglect the design of the RF ground system in the immediate vicinity of the
antenna.
l/4 Vertical Monopole
By far the conceptually simplest way to give maximize exposure to the high-current
portion of the antenna is to make sure that horizontal dipoles are l\2 or slightly longer
and verticals are l/4 or slightly longer. Since the title of this book includes the word
“practical”, we will concentrate on the l/4 ground-mounted vertical instead of l/2
dipoles erected at heights of 300 ft or more!
*TANSTAAFL = There ain’t no such thing as a free lunch!