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Aerials
by Kurt N. Sterba and Lil Paddle (This article ran in Worldradio, May 1998) To let all know that it is indeed the urbane and erudite who appreciate the monthly elucidation occurring here, we present this missive from John Amos, KC6TVM, of San Jose, California. "I have been a faithful reader of your column in Worldradio for
quite some time and I admire your perspicacity and perseverance greatly.
I'm an engineer, though not specifically in RF, and to put it
simply, you make technical sense. Even the old-timers who are not just
appliance operators don't always get the theory right. Your antenna
specs are the best technical articles since I lost track of The Journal
of Irreproducible Results. "Not to gush, but I want to make sure you keep writing that column.
In that line, I am enclosing the munificent sum of $10.62 (as a suffering
6) to join and receive a White Hat. I can guarantee you that it will
be worn with all the technical pride that I can support, which is a
variable since I have been doing electronic QA for thirty years, based
on a four-year degree in Asian history. I figure neither of us is cynical,
merely realistic." Mr. Amos then said, "I do a lot of operation on foot for things
like a 10K run a couple of weeks ago and RACES and such-like. I usually
carry my HT on my belt, and I am getting increasingly tired of looking
over my shoulder and getting the rubber-ducky-replacement whip in my
eye or up my nose. I have cooked up a gadget that will clamp on my web-belt
suspenders and hold a BNC connector more or less upright. I figure that
right around my shoulder blades would be a good spot for it, with something
like an MFJ 1/4-wave whip connected to it, then a foot or two of cable
to my HT wherever I want to carry it. Question is, would I gain anything
significant by adding the other half of a dipole as a wire hanging down
from this mount? I realize the HT is not looking into anything like
50 ohms with the whip, but I figure I ought to gain something from the
counterpoise. Make sense?" Answer is: Yes, indeed! Some years back Kurt suggested that very thing
in this column. It was quickly seized upon by a manufacturer and is
prominently displayed in their catalog. There has not been even a farthing
or a sou in royalties. Nor has been proffered a small word of thanks.
The addition of the correctly adjudged "missing half of the dipole"
will definitely, with significance, improve the signal. Here is a little trick for VHF. To extend the range set your walkie-talkie
on the roof of an automobile. The chassis of the WT will capacitively
couple to the mass of metal below. It is indeed gratifying to have heard from so many who promise to be
wearing their Kurt White Cap at 11:30 a.m. on Saturday (at Dayton) and
be standing in front of the "Granite Antenna Co." booth chanting
"Ohm, Ohm." Possibly, the genuine solution to a particular problem, for you or
someone you know, is an unconventional one. Previously, in this series, an antenna was described that should have
sent the soggy old dipole to its deserved place in the history books
and to its final resting place. Oddly, dipoles are still being erected
willy-nilly with nary a thought that there really is a far simpler,
and better method. I'm not surprised. With this episode, however, we'll try to send to the briny deep
another orthodox antenna. We're talking about ground mounted
1/4 WL vertical. The amateur level books and articles all speak about
the low-angle radiation of the vertical. However, they neglect to point
out a salient fact that is found elsewhere. Without a quality ground
screen the vaunted low angle does not exist. The definition of the optimum radial system that is bandied about is
120 radials with each of them being 0.4 of a wavelength long. I've
even seen it phrased that "anything less is a compromise,"
which is, if any amount of logic is applied, obviously pure poppycock.
But, in a practical matter, there would be an unmeasurable difference
in signal strength at the antipodes if the ground system were instead
64 radials, each 0.3 WL long. But, as we realize, even that is impractical
for most. Another drawback of the antenna that must be in a hundred
thousand, or more, back yards is that the immediate surroundings tend
to smother that wave as it is taking off from a position right next
to the ground. So let's solve the two vertical dilemmas with a system that will also
be (by the unwashed masses), ignored. That is the usual fate for something
that is new or different. It is much easier and reassuring to cling
to what has been, instead of trying something new. Let's start by looking at the garden (no pun intended) variety vertical.
The formula is 234/F-MHz. So for the frequency of 14.200 MHz the length
would be 16.48 feet. That's for a wire, with an antenna of aluminum
tubing (actually copper is preferable) being slightly shorter. The first key to solving the problem of all that RF (as Kurt puts it)
wallowing about on the ground and being slurped up is: We're
going to get the antenna up off the ground! But first allow me to digress.
With a 1/4 WL vertical the high current point is right there at the
feedpoint. Let's get that feedpoint up in the air so the current-created radiation
will have a fighting chance to go skyward, instead of into the automobile
in your garage, the washing machine, dryer and whatever else there may
be. Let's get the feedpoint of that vertical up in the air a quarter-wave
above the ground. Unless you have 33-foot wooden pole that you can attach
a wire to, we'll have to do something else. Using telescoping
tubing one could build a 1/4 wave vertical. It could be attached, with
a four-foot overlap, to a wooden pole. Here we've already solved one perplexing problem of the normal
vertical. The antenna's efficiency is now not so dependent on
an extensive ground system. In fact, in this new situation, four radials,
each a quarter-wave would suffice nicely, but another problem still
exists. The radius of the remaining four radials is 33 feet for a circumference
of about 104 feet. This would present quite a sight and bring the blush
of victory to the cheeks of the neighborhood antenna snoopers. Also,
the ends of each of the four wires must somehow be attached to supporting
structures. So, here's solution number two. Just drop the four insulated wires
vertically. Yes, let them drape down, enveloping the support pole and
the coax feed. I would try to keep each of the four ends at least a
foot off the ground. This antenna will handily outperform its ground-mounted counterpart
utilizing the average (and better) ground system. And in reality, only
three downward radials are needed. Probably only two. Maybe even only
one. That possibility you will have to solve for yourself using a Field
Strength Meter. Will thousands of amateurs replace their inefficient, TVI creating,
down-on-the-deck verticals with something vastly superior? Of course
not. Will even hundreds of sterling signal seekers sally forth into
uncharted (for them) waters? Doubtful. But, for the infinitesimally small number that are open to new ideas,
we'll continue. You've seen how we've taken the
ground-plane antenna (usually seen at V and UHF) and adapted it for
use on the HF frequencies. Continuing in that vein let's look
at another antenna that is usually found at U and VHF - the ubiquitous
J-pole. Here's how we'll scale it up for 20M. First take 146 MHz (which
the J-Pole is probably cut for) and divide that by the new frequency,
which will be 14.2 MHz. The answer is 10.282 times, which means that
if you multiply the dimensions of your 2M J-pole by 10.282 you will
have the dimensions for a 20M J-Pole. The advantages of the 20M J-Pole
will be whatever virtues the J-Pole seems to bring to its advocates
and a lack of ground dependency for proper operation. Outstanding performance from a simple antenna! You may ask why this
hasn't been done before. The answer is that there is a lack of
imagination out there. However, the ambitious, realizing what this antenna has to offer,
will be erecting them for 80 Meters. And can you just imagine the fine
signal that would come from the ones that will be erected for 160 Meters?
I do hope that those who put them up on Top Band will write and tell
us about the great signal reports they received from DX stations. Since we will be thrilled to hear about your results and can hardly
wait, we're breaking our vows of anonymity and giving you our
email address (keep it a secret please): Some readers have complained that our name clues for the various antenna
companies are too obtuse. To assist your quest I'll suggest that
in your almanac you look up the slogans for the various U.S. states.
Then you will see which is the company that must be using for their
dB claims, not dBd but rather dBpods. That is: dB over a Piece Of Dry
String. (How can you, in good conscience, sit there and read this knowing that you have not yet ordered a Kurt White Cap?) |
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