Tech Notes
Why Radials?
PART I
There are few subjects in amateur radio that are so clouded in mystery as radials and
ground systems for vertical antennas. That this should be so is itself something of a
mystery, for countless books and articles have examined this subject in considerable
detail over the last 50 years. The basic points are quite well known by now EXCEPT, it
seems, among the amateur community.
Why so much confusion? Some people will tell you that vertical antennas REQUIRE
them for effective operation or even for low SWR, but you'll see ads stating (a) that a
particular vertical antenna works like a bomb with no radials at all, (b) that another
doesn't need any radials because it's a “half-wave” tall on one or another band and
remotely tuned, another (c) that THEIR antenna can get by with a greatly abbreviated
radial system because its feedpoint is a few feet above ground, and our favorite (d) is
the one that claims that only a few 14 foot radials will allow it to deliver MAXIMUM
operating efficiency.
The ARRL Antenna Book tells us that more than 100 much longer radials would be
needed for that kind of efficiency on most amateur bands, even though the
advertisements say otherwise! And what is meant by "ground" anyway? Much of the
misunderstanding can be laid at the door of over-zealous dream merchants who prefer
to gloss over unpleasant truths. Let's review the basics and try to separate the facts
from the hype.
What is a ground? It can be a connection to the earth itself and often is. At power
frequencies, the earth is usually a good conductor, and most electrical codes dictate a
copper-plated steel rod driven into the earth to a depth of six feet or more.
Unfortunately, such a ground connection is next to worthless at radio frequencies,
although it's useful in preventing shocks. Too many amateurs have been electrocuted
when they contacted the "ground" side of a feedline connected to ungrounded (or
poorly grounded) station equipment while standing on damp earth'. Be especially leery
of old two-conductor house wiring, and don't count on the newer three-conductor
wiring to take the place of a good earth ground to all station equipment that plugs
into the power outlets in the shack!' An ungrounded chassis can be lethal whether the
unit is switched on or not, so drive that copper-plated steel rod into the flower bed
and connect a heavy wire between it and all station equipment while everything is still
unplugged.
But why should a ground connection that serves quite well at 60 Hz not also suffice
into the megahertz range? And why do we even worry about it? Consider a vertical
radiator installed at ground level and fed through the usual coaxial feedline, the
braided outer conductor connected to the inevitable copper-plated ground stake. What
is not so obvious is that the “business” end of a vertical antenna is also “connected”
to the earth through the capacitance of the vertical radiator to the earth itself. True,
this capacitance won't be
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PART I
very great, but it'll be great enough to cause current to flow in or along the earth all
around the antenna out to a distance greater than the length of the vertical radiator.
These "return" currents make their way back to the feedpoint to complete the circuit
and can be seriously attenuated if they must pass along or through lossy earth. Even
the most conductive earth is fairly lossy at radio frequencies, and the “return” losses
can be severe unless an extensive radial system is used to provide a number of low-loss
paths back to the feed point.
But what kind Of losses are we talking about in the average case? The ARRL Antenna
Book (any edition) suggests that 120 radials equally spaced and each a halfwave long
would make an essentially lossless ground system at R.F., and the FCC mandates such
ambitious systems for stations operating in the AM broadcast band. A lossless ground
system means that all power applied to a vertical antenna apart from conductor and
loading losses (usually only a few percent) will be radiated instead of being lost in the
earth as heat.
Amateurs must usually make do with much shorter and many fewer radials, particularly
on the lower frequencies, but one can often reduce the length of radials and their
number considerably without incurring significant loss. Still, the Antenna Book
observes that with only two 1/8-wavelength radials (about 17 feet on 40 meters)
overall efficiency is not likely to exceed 25%, in which case the difference between a
bare-minimum ground system and an “ideal” one might amount to a whopping six
decibels or more. Much depends on the natural conductivity of local soil. Sandy, arid
regions are probably the worst, but the best is none too good compared to seawater.
It's worth noting that what matters is conductivity at or near the SURFACE of the
earth. If your R.F. has to fight its way through several feet of high resistance sand or
rock to find a low-resistance path back to the antenna feedpoint you've probably lost
the battle already. Subsurface mineral deposits and high water tables don't help much
either, for these are usually too far down to do much good. Fresh water, by the way, is
not a very good conductor at R.F. , so don't look for any great benefit from nearby
lakes, ponds, rivers, creeks or swimming pools.
Some people imagine that they have a wonderful ground system because they're
connected to a well casing that goes down several hundred feet. Not so, alas!
Remember that your return currents will be flowing all around the antenna on or
slightly under the surface, so even a six-inch casing won't provide much surface area
along which current can flow. In other words, your well casing could go down 15 feet
or 1500 feet or all the way to China without doing much to reduce your earth losses in
the HF range.
PART II
Some practical considerations, however, before we take a close look at some fairly
typical installations and draw some rough conclusions: the PERFECT ground system for
a vertical antenna operating in the HF range is probably out of the question on most
residential lots, but that doesn't at all mean that nothing can be done to reduce earth
losses and turn more of your applied power into useful radiation rather than heat. The
most important thing to keep in mind as we go along is that some of your precious
R.F. will be radiated straight-away (good), a relatively small amount will be lost forever
in feed line, traps, loading coils and the like (not so good, but we can usually live with
it), and a fair amount will come raining down from the vertical radiator onto your lossy
real estate. Your main task will be to help this last portion of R.F. to work its way back
to the antenna feed point with as little wear and tear as possible so that most of it will
be available to run up the radiator again on the next cycle. How to do it? Copper-plate
your backyard? Hardly practical, but you can do quite a bit with plain old wire (bare or
insulated) in any gauge
-2-
PART II
heavy enough to stay in one piece if stepped on or if ground between rocks during a
hard freeze. Many radial wires emanating from the base of the antenna will offer a
number of low-resistance paths back to the feed point. These radial wires can be
buried an inch or two under the sod to protect them from lawnmowers and foot
traffic, or they can simply be draped on the earth. There's no point in burying them
any deeper than is necessary to get them out of the way. Space them more or less
uniformly over 360 degrees (not always possible, but that's the goal).
HOW MANY WIRES? That depends on how long they are. HOW LONG SHOULD THEY
BE? Answer: The longer the better. The hitch is that as the wires become longer more
of them are required to take full advantage of their greater length. This is because a
longer wire will intercept current on the surface out to a greater distance than will a
shorter wire (good), but for a given number of wires the separation between adjacent
wires necessarily increases as the wires become longer, in which case currents on the
surface between two highly-conductive wires must cross an ever-greater stretch of
lossy earth to encounter a low-loss path home (not so good). Of course, four 1/2-wave
radials will do a better job of reducing ground losses than will four 1/4-wave radials,
but the difference may not be very great for the reason just given and because the
intensity of currents flowing out near the end of the wires will be much less than that
of currents closer to the antenna.
It's generally reckoned that approximately half the ground loss encountered occurs
within a circle having a radius equal to the antenna's height and that most (though
not all) of the remaining loss resistance occurs in the next quarter wavelength out from
the antenna as the capacitance between the vertical radiator and the earth rapidly
decreases. In any case, it's clear that for a given amount of wire it pays to lay down a
larger number of radials when they have to be short, although some have pushed. this
sound principle to ridiculous lengths, cutting 120 ONE-FOOT radials (covering
approximately the same surface area as a garbage can lid) when a dozen 10-footers
would have done a much better job.
Perhaps you've heard or read that all radials should be some particular resonant length,
say a quarter wavelength, before they're draped on the earth or buried slightly under
the sod. Resonant radials have their uses (as we'll see shortly), but within a few feet
of the earth any practical length of wire in the HF range will have enough capacitance
to the earth to be tightly coupled to it and thus be detuned considerably, much as a
horizontal wire dipole at very low heights will be detuned from the formula lengths for
resonance by the earth. Luckily, radials at ground level need not be resonant at all, so
at ground level your only problem is to make the earth around the antenna more
conductive than it is to start with. In practice that means putting down as many
radials as possible and making each one of them as long as possible.
In essence, all we're talking about is efficiency. If you put 100 watts into an antenna,
how much of that leaves the antenna as useful radiation and how much is lost as
heat? Some of the quantities we have to deal with are elusive and usually can be
measured only indirectly, but with a little theory and seventh grade math we can begin
to evaluate things more or less logically and usually come up with useful insights into
the probable effectiveness of a proposed vertical installation.
The first basic concept we have to deal with, a RADIATION RESISTANCE. This term is
a misnomer in that it doesn't denote a real resistance, but R F. energy that is “lost” by
radiation--just what we want. In fact, we can say that radiation resistance is "good"
resistance as opposed to “bad” ground and conductor resistance which represent a
total loss.
-3-
PART II
Let's assume that the antenna is a full quarter wave tall and resonant or nearly so (the
usual case) so that we don't have to worry about any inductive or capacitive reactance
components or losses in loading coils or matching networks. Of all the several
“resistances” the radiation resistance is the easiest to estimate because that's largely a
matter of radiator height (length) and to a lesser extent, diameter. Conductor
resistance is usually negligible for radiators constructed of tubing, but loading losses
can increase rapidly as the structure is made much shorter and as the loading coils
require more inductance to bring the antenna to resonance, and the various trap
circuits required for multi-band operation add their own losses. Some of these are
lossier than others, so one should refer to the ARRL Handbook or other publications
for a more thorough understanding of such concepts as “Q” and “form factor”.
But ground losses can easily exceed combined conductor, loading, and trap losses if no
measures are taken to reduce them. Let's consider a 1/4-wave vertical at ground level
with only a 6 ft rod for a ground system (a fairly typical installation, regrettably).
Because a quarter-wave is a resonant length we can forget about loading and trap
losses, and the conductor losses will usually be low enough to ignore.
Therefore, we can assume that whatever feed point impedance we encounter will
consist of the antenna radiation resistance plus the ground loss resistance and little
else, so we attach our 50 ohm cable and measure the SWR at the antenna feedpoint.
Measurements made at the transmitter end of the line may be much less accurate.
Hmm. The lowest SWR in the center of the band is 2:1! What does that tell us? First,
we know that a SWR of 2:1 on 50-ohm line means a feedpoint impedance of either
100 or 25 ohms. Which is it? Luckily, we also know that a 1/4-wave vertical has a
radiation resistance of approximately 35 ohms, so there's no way our total feed point
impedance can drop BELOW that value, Our feed point impedance at resonance, then,
is 100 ohms, and we now have enough information to say something about the
efficiency of this antenna and its ground system. If our radiation resistance is 35 ohms
we must also have some 65 ohms of pure ground loss resistance that's doing us no
good at all.
Efficiency (the ratio of power radiated by the antenna to the total power fed to it and
expressed as a percentage) can be easily calculated by dividing the radiation resistance
by the total impedance of the antenna circuit (i.e., radiation resistance + ground loss
resistance + conductor, trap and loading losses of all kinds). In this little example
we've assumed a resonant quarter-wave antenna to simplify matters, so we can now
say that the efficiency is equal to the radiation resistance (35 ohms) divided by the
same radiation resistance (35 ohms) and ground and other losses (65 ohms) or 35/100
= 35%, meaning that a little more that one watt out of every three applied to the
antenna goes anywhere.
Suppose, however, that we put down a half dozen radials and find that our SWR
drops to 1:1? (It may or may not!) That would mean that the feed point impedance has
dropped to 50 ohms, and since our radiation resistance is still 35 ohms we can assume
that the ground loss component is down to only 15 ohms. Our efficiency, however, is
up to 35/50 or 70%--a notable improvement for a dollar or two worth of wire
additional increases in efficiency will come more slowly and require much more wire, of
course, but from zero radials to a half-dozen or so there's probably no easier or less
expensive way to make your signal louder. Just how much improvement you can
expect from adding radials to a system that previously included none is hard to predict
because we don't usually know what the local R.F. ground loss resistance is to start
with, and the technique of working back from the SWR with no radials at all permits
only a rough estimate if we have an approximate idea of the radiation resistance of the
antenna. If the antenna is much shorter than 1/4 wave and has to be loaded to
resonate on a given band (the usual case with multi-band
-4-
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