Part 2
So new questions came along, and I, once again atempt
to explain:
>OK, a dipole needs good ground reflection. I thought a friend of mine
>on Molokai was strange when he ran a half-wave ground radial under his
>dipole. Do you think it did anything--good or bad?
Did nothing useful to add to the performance of the dipole.
For dipoles, or any other balanced antenna, the characteristics of the
ground below the antenna, will impact the feed impedance of the
antenna, but not its' "far field" radiation pattern; the height of the
antenna
above the ground will also impact the antenna's feed point impedance
and also it's far field radiation pattern. See more below.
>Now for a beam, do you still want ground reflection or does a beam work
>best in free space?
We want the far field reflection, and for the earth to cut off the
lower half of the sphere, so it will operate better near the ground,
than it will in free space.
See below, again.
> Is it better for a 20M beam at 45ft to be over lava
>or on perfect ground in Nebraska?
Will always be better, for the initial launch of the signal, no matter
the height, if in Kansas/Nebraska/Texas, than when out here on
Hawaiian soil/rocks/lava!! However, we do have an advantage over
the amateur stations in the mid-West, see below.
>And what about hams who run ground radials under their beams?
Unfortunately, they misunderstand the facts, and are wasting their
time and money as far as improving the beam performance at
HF! See below.
>I just read on TowerTalk a guy who said to run lots of ground radials
>attached and out from the base of the tower to disperse lightning
>in New Mexico.
That is a total separate issue, and has nothing to do with how the beam
on the tower will operate at HF.
>May I send this and your next reply to our HWARS (Kona club) newsletter?
Of course you may use the info, anything to help dispel
misunderstandings. Of which, I will now continue in response
to your added questions, those above.
Topic #1. RF Current Flowing in the Soil Directly Below the
Antenna; Or, Why Radials are Used
The antennas we use are divided into only two types: those
which, as far as the radiated electromagnetic field, form a
complete physical circuit, and those which do not. Radials and
high soil conductivity are desirable qualities directly beneath
antennas which do not form a complete circuit for the EM field.
a. Antennas which are a complete RF Circuit "in the air":
Any antenna which is fed at, or somewhere near the center
of two conductors, each of which forms a nearly 1/4
wavelength at or near the EM field frequency, does form such
a completed circuit for the electric field energy of the EM field.
Actually any doublet antenna, that is even random lengths of wire
on each side of the feed point will also operate, but special
impedance matching techniques will be required. Only the 1/4th
wave long conductors either side of center will match well
to 50 or 72 ohm coax transmission line, and it is still a good
idea to use a balun at the feed point to insure equal currents
being fed into each side; an antenna tuner down in the shack
at the transmission line feed point can take care of the impedance
matching problem for random length doublet antennas.
For beam antennas, it is usual to actually have quarter wave
elements on each side of the feed point, or "loading" of the
element on each side can be used causing the EM
signal to believe that the total length of 1/4th wave is
present. What happens is that the loading elements give
the EM energy a place to reside as a "stored" field while
the EM current is flowing into what it believes to be a
full 1/4 wave long conductor. When this is done, the feed
point impedance is probably not going to be near 50 ohms,
so special matching techniques, of various sorts depending
upon the manufacturer, are used to couple the coax line
to the beam. Again this can introduce losses.
This is why the "Big Gun" DX'ers want full size beam antenna
elements, that is a full 1/4th wave of conductor on each
side of the beam boom! A trapped/linear loaded antenna
can be used to shorten the elements of the beam and/or to allow
one element to appear to the EM field to be at or near 1/4th
wave length long on more than one band. However, traps do
have losses, but most believe the losses are a small price to
pay for lighter weight, smaller, and less costly beam antennas.
A center fed half wave dipole antenna, the G5RV antenna, and center
fed, very long wire antennas, such as quad elements (which are
a full wave long) or V-beam and rhombic antennas which may be
many wavelengths long are all examples of antennas which form
complete circuits for the EM field.
All these and the beam antennas are considered to be balanced,
which is almost just another term for saying they form a complete
circuit for the antenna RF current. None of the current from the
transmitter output stages into the transmission line
and up to the antenna element is flowing in the ground directly
below the beam tower or in the ground below any of the balanced
wire antennas mentioned. The complete antenna circuit is "closed"
up at the wire or beam element. The elements either side of the
center feed point are coupled via displacement currents in EM field
which complete the RF circuit.
Installing radials directly beneath this type of antenna is a waste of
money and time for the work! There is no antenna current there to
assist with these added conductors.
b. Antennas which do not form a complete EM field "circuit"
I can think of at least two such antennas: the vertical monopole
and the windom, Carolina or not. These antennas, without some
very special placement and installation engineering, must use
current flowing in the soil directly beneath the antenna installation
to form a complete circuit for the current flowing from the rig into
the transmission line and to the antenna element.
This type of antenna is said to "work against the ground." That is,
current flowing in the ground (or provided conductors) back to the
antenna feed point is mandatory, or the antenna just will not
radiate any useful energy!!
The most common of these is the vertical monopole, and it is usually
in the form of a 1/4 wave long vertical element, or something shorter than
that, and again provided with traps, linear loading, or capacitive
hats, or a combination of these, to again convince the EM signal
that it is "seeing" a 1/4 wave long circuit into which current can flow
for an entire half cycle of the signal with no reflections.
Now where is the other 1/4 wave of the necessary circuit? It is in the
ground!! Sometimes this is explained as being the mirror image of
the vertical antenna; a reflection of the antenna directly below and
down into the ground. For this to occur, of course, exactly half of
the transmitters current must flow into the ground to induce the
necessary EM field in the ground to complete the circuit. To the
extent that the soil is lossy, I-squared-R power losses will occur,
and energy is lost, not radiated. Or, if the soil is "glassy" bits of
broken down volcano glasses, it will be high in dielectrics, and the
energy is lost to polarization losses of the molecules rotating about
in the soil glass under the influence of the electric field energy
of the signal in the soil.
Obviously, since half of the transmitter's output current must flow
in this path in the ground, if the ground is lossy, up to half of the
transmitters output power can be lost to heat in the ground
resistance!! That is a power loss of 3dB from what you start out
with before the signal ever leaves the area of the antenna. Or, that
much loss is suffered by a weak received signal before your
receiver ever has a chance to "hear" it!
To deal with this problem, ever since the beginnings of radio, the
ground beneath vertical transmitting antennas has been the
object of all sorts of techniques to improve the conductivity,
to approach the appearance of a complete copper metal
plane below the antenna and extending our from the base
of the vertical at least 1/4th wavelength, to form the other
half of the RF circuit. So why even use a vertical antenna with
all this complication? Because they put more energy on the
horizon. This is mandatory for medium wave broadcast; they
want to cover the local area with the best ground wave signal.
For those of us interested in DX at HF, putting energy down
toward the horizon is the path to use for greatest distance throw,
or from where the signals come from great distances to our
receivers.
All of these techniques: radial wires, setting up in salt marshes,
regular chemical treatment of the soil around the transmitting
site, have been employed for decades and all just to complete
the actual transmission circuit between the transmitter and the
antenna. The numbers of needed radial wires, their size, spacing,
etc. have all been covered extensively in the literature. Briefly, the
ARRL Antenna Book and other references show that you must
install 24 1/8th wave long radials when over lossy ground to reduce
the power loss to only 2 dB (a 1 dB improvement); it takes 90 full
1/4 wave long radials to cut the power loss down to only 0.5 dB, that is
22 1/2 full wave lengths of wire to install on/in the ground!!
This is the purpose of radial wires: to complete the circuit.
They do not radiate nor impact the radiated field other than
to allow it to be weakened if the radial field is not "perfect" and if
the soil conductivity under and immediately around the antenna
is not high.
Recent experiments and tests by DXpeditioners, backed up by the
folks at Force 12 and Bob Meyers at the Gladiator antenna firm, are
showing that elevating vertical antennas and their radials has a dramatic
impact on HF vertical antenna performance. In fact, two DXpeditions
to Jamaica in last year's DX contests used vertical antennas, and set
World class scores for that class of station, multi-multi. And they
used ONLY two elevated radials per vertical antenna!! Similarly,
elevated radial Gladiator antennas were used on the low bands
on Heard Island, VK0IR; at the Maldives, 8Q7AA; at Spratly, 9M0C;
and right now on Wake Island, /KH9! The Gladiators use four
elevated radials at each of these DX locations. Note that at
each of these DX locations the ground conductivity was very
poor, to non-existent. But they were surrounded by salt water.
The elevated radials must be 1/4th wave long and a few feet off the
ground. The base of the antenna may or may not also be raised a
few feet above the ground. It was found to be of benefit to also raise the
base of the vertical to around 10 feet high, and keep the radials below
the antenna base running out/sloping downward. In this way
the vertical antenna is decoupled from the ground, and no/very little RF
current flows in lossy ground whether it is coral or volcanic
glasses! The four elevated radials collect the vertical antenna's
E field, via displacement currents, just as do the two halves of
the elevated balanced wire or beam antennas!
Suddenly the elevated vertical antenna, with elevated radial fields
may be establishing a new paradigm for DX antennas! See
CQ-Contest magazine, both February and March 1998 issues,
for rather complete articles on these at 6Y4A in last year's
DX contests. The reason: these antennas place much more of the
radiated signal near the horizon than can a beam antenna, unless
it is mounted very, very high. The vertical also has a more uniform
distribution of radiation from the horizon on up; while the beam
forms many pattern lobes as it's height is increased.
Topic #2 Why does the beam/dipole antenna operate better in Kansas
if it does not care what the soil conductivity is directly
beneath the tower?
Because it matters greatly what the soil conductivity is a couple of
wavelengths and further beyond the antenna, and it does not matter
whether the antenna is of the first or second kind! The benefit from
being ground/near ground mounted, as opposed to being in free
space, is we want the ground reflection "gain" inherently available.
And this occurs in the so called "far field" of the antenna, beyond the
area where ground currents may be returning to the antenna if it is
not inherently a balanced antenna. Ground reflection gain cares
not what the soil conductivity is directly beneath the tower or under
our dipoles, beams, rhombic, G5RV's or even our verticals!
Precisely how this ground reflection gain operates is a bit mathematical,
and, if interested you can pursue the topic in the Antenna Book by
the ARRL. It is analogous to the math of optics and what goes on at
the interface between two EM field conducting media; as the
interface between air and water, and at what angle does all the
light reflect off the water, and at which does the light penetrate
the water, etc. Antenna theorists talk about the pseudo-Brewster
angle, below which our antennas will not radiate useful energy,
for example. The interface reflection angles between air and soil
depends upon the ground conductivity and other factors of the situation.
The reflection gain benefit is governed by the soil conductivity
a couple of wavelengths out and beyond. That's why any antenna will
operate more satisfactorily in Kansas than anywhere "in land" in Hawaii.
If you happen to live near the beach or shoreline in Hawaii, you will
benefit greatly in signal propagation in the direction of the shoreline!
But forget much reflection gain benefit in the opposite direction across
the island behind! For most of us on the islands, having this cross
island handicap is not too much of a problem anyway, since we usually
have mountains, ridges and hills toward the island centers which rise
up some degrees from the horizon. So it matters not too much that
we are not putting energy down low; in fact, best if we could
engineer things such that, back across the islands, our antennas were set
up to send the energy just over and above the ridge tops, that
would yield the greatest efficiency possible given the presence of
the ridges and mountains.
For tower mounted antennas, the ground conductivity as far out as 10 to
20 or even more wavelengths will still impact the antenna pattern; how
far out depends, of course on how high the antenna is mounted. The
higher up, the further out in-phase reflections can occur which will
add to the signal gain at radiation angles near the horizon. Thus, if
over reasonable ground, the higher the antenna is mounted, the better,
usually at least, for working DX.
In Conclusion
Now, if you have read this far, you need to know that we island
dwellers do have one great advantage over those radio users
in Kansas!! We are surrounded by salt water; thus our signals,
upon returning from the first reflection from the ionosphere,
bounce from the ocean surfaces, which are nearly perfect
conducting mirrors for our signals, no matter which direction
we want to transmit. In fact, we might have several ocean,
lossless reflections, depending upon the path and the DX we
are after.
Those hams living in the mid-West have a very difficult time scoring
well in DX contests because their signals must reflect off soils on
the way in any direction which are far more lossy than ocean water!
Even the soil in Kansas has less than 1/100th the conductivity of
the ocean, about 30 mS/m in Kansas, which is the best on the
mainland, vs. 5000 mS/m at the surface of the oceans! By the way,
this is why maritime mobile stations on sail boats, etc. "get out"
so well when at sea!
It is difficult for any amateur station in the mid-West to
set a new, or near new world class record in a DX contest;
while it has been done from more than one station here
in Hawaii, KH7R on Oahu coming close most recently.
Anyway, enjoy amateur radio from Hawaii, and don't spend too
much time worrying about our volcanic soil's lack of conductivity,
just move closer to the beach, if you can!
73, Jim, KH7M
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