Richard Harrison wrote:

Richard Clark wrote:
"Salt water is miserable as a conductor, .....

I praise the god of conductivity for the ocean`s behavior as a beniign
enabler of medium wave.....

Great info again Richard,
I recall during my early days in the Navy of
being able to listen to AM radio stations from Victoria B.C. all the way to
Hawaii. I always attributed it to the salt medium, but never really knew
enough about antennas and propagation other than to tell myself that salt
water was a better conductor.

When you think in terms of 'salt water because of its
conductivity (many times that of the best soils)' and then show the differences
in terms of Db attenuation it all makes sense.

This applies in many areas of discussion on this newsgroup. One worries
about ideal height above ground, transmission line losses, radiation pattern, etc,.
whereas the main objective is to get a signal out in the ether one way or
another, and damn the technical naysayers, full speed ahead.

Even with a limited knowledge of antennas and propagation, which thanks
to the knowledge gained on this newsgroup, I have always managed to get a signal
out of the shack and into the air without regard to the warnings of the pundits,
and thoroughly enjoyed making contacts -- many DX -- blissfully unaware of
how effective or efficient my antenna system may have been!

It frequently troubles me that when a novice asks a simple question
about antennas they are often distracted and possibly prevented from trying
something because of theoretical albeit often practical arguments against.
If you don't get the wire out there, you don't get the signals out either.
60db attenuation still beats infinity, and on a clear day you can hear
forever!

By golly, maybe I should rethink salting the back 40! :-)

Irv VE6BP


--
--------------------------------------
Diagnosed Type II Diabetes March 5 2001
Beating it with diet and exercise!
297/215/210 (to be revised lower)
58"/43"(!)/44" (already lower too!)
--------------------------------------
Visit my HomePage at http://members.shaw.ca/finkirv/
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--------------------
Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada

"Richard Clark" wrote

The electric dipole moment is clearly bridged by a conductor, by
definition. As such, at the interface, it must collapse completely
into a current which gives rise to counter emf, the two waves cancel
as a function of phase - the proof again is found in the Lambertian
distribution that vanishes completely with the removal of ground (why
horizontal antennas are held up in the air). The more remote the
ground, the greater the variation of phase and the distribution, and
yet the low angles never fully recover (the death embrace of ground is
always there).

Richard, would the dipole's performance thus be improved by bedding the
ground with sand, and hurt by adding ground radials? Same true if the dipole
was at some compromise between 1/4 wave and the desired 1/2 wave above
ground?

Regards,

Jack Painter
Virginia Beach, Va (where mostly sand exists anyway)

On Thu, 6 May 2004 12:47:58 -0400, "Jack Painter"
wrote:

"Richard Clark" wrote

The electric dipole moment is clearly bridged by a conductor, by
definition. As such, at the interface, it must collapse completely
into a current which gives rise to counter emf, the two waves cancel
as a function of phase - the proof again is found in the Lambertian
distribution that vanishes completely with the removal of ground (why
horizontal antennas are held up in the air). The more remote the
ground, the greater the variation of phase and the distribution, and
yet the low angles never fully recover (the death embrace of ground is
always there).

Richard, would the dipole's performance thus be improved by bedding the
ground with sand, and hurt by adding ground radials? Same true if the dipole
was at some compromise between 1/4 wave and the desired 1/2 wave above
ground?

Hi Jack,

A good question, and one that brings out the one of my elliptical
statements about having disproven you don't have to worry, because
there is nothing you can do.

In fact you can do something, however, it separates the discussion of
ground insofar as near field and far field issues.

IF you add a ground screen below a horizonal antenna, you CAN improve
your communications efficiency (your contact, with sufficient
resolution, could see an improved, stronger signal).

This, of course, has no strength in its argument in the far field, the
same problem exists of the complete collapse of the electric field
through its polarization giving rise to a canceling current. The near
field application (where the media does NOT exhibit a 377 Ohm
characteristic) is one of shielding the source from loss (which is
largely a dielectric loss, not a conductive, Ohmic loss).

Richard Harrison, KB5WZI, has already recalled Terman's treatment, but
having no reference handy, he hadn't really pulled it together.

The point of the matter is that for a conductive ground, the electric
fields are laid across a short. The obvious occurs and that electric
field collapses into a magnetic field (through the short circuit
current that necessarily follows) at the interface. This simple
statement is enough to evidence the reversal of fortune (magnetic
replacing electric in the face of its initiating source spells short
circuit city).

At a distance (along the magic 0° DX launch angle), BOTH the source
and its reflection (or image) in the ground below it, are at an equal
distance to the observer. Thus the distant observers (if they could)
see TWO sources that are 180° out of phase. Thus everywhere along
this meridian, those two signal completely cancel. With tongue in
cheek, let's call this 100dB down. This happens ONLY for horizontal
polarized signals. By shielding ground beneath the horizontal
antenna, you are doing nothing to change this star fixed fate; but you
are improving efficiency with a net positive gain, relatively
speaking. You simply have two stronger signals canceling.

At higher angles, lets call them 5° or higher (sometimes much higher)
the path lengths of the two sources diverge from equality (a phase
shift is introduced) as the signal strength attempts to pull toward
the free space value, some 30dB higher. If you pull your attention
successively higher, you eventual come to the point where the two path
lengths introduce enough phase difference that they combine to a net
signal that is greater than the free space value. This, by the way,
does not constitute DX opportunity and is crowed about as the great
NVIS advantage (in other words, the sufferer has no options and is
content to make lemonade). This exercise describes the Lambertian
distribution, a classic example of Optical sources.

Raising the horizontal is much the same gain story. It removes itself
from the cold embrace of earth's loss, and it introduces a new phase
combination. Thus the lobes may lower from the Zenith, but you will
never see them pulled all the way down to the horizon, such is the
fate of horizontality. ;-)

73's
Richard Clark, KB7QHC

Gene, W4SZ wrote:
"There is no need to invoke phony arguments about waves "shorting"."

Shorting waves does not annihilate them. It merely reflects them. A
short is a low-resistance conductor.

A transmission line short is a low-impedance U-turn for for the wave`s
current which forces the voltage between conductors to zero.
Cancellation of the electric field sends its energy for an instant to
the magnetic field. As these two conjoined fields continuously
regenerate each other, the electric field is immediately recreated by
the enhanced magnetic field. The electric field goes from zero at the
short to double the incidet just 1/4-wave back from the short due to
addition of the incident and reflected wave vectors (phasors).

For a complete reflection in a short, you need zero resistance.
Otherwise, resistance consumes some of the available energy.

When a radio wave strikes the ground, it is reflected. Angle of
reflection equals the incidence angle but because the earth is an
imperfect reflector the reflection is ncomplete. Reflection depends on
incidence amgle, wave polarization, frequency, and type of earth. The
reflection occurs as if the R-F wave were an optical wave.

NVIS is simple to do by using a horizontal dipole up 1/4-wave above the
earth. The wave is delayed 90-degrees in travel to the earth. It is
delayed 180-degrees by earth reflection. Then, another 90-degrees of
delay is experienced in the reflected wave`s return to the vicinity of
the dipole. The 360-degree total round-trip delay puts the reflected
wave back in-phase with newly emerging radiation from the dipole in its
travel toward the zenith. If the ionosphere can reflect this high-angle
energy, it can cause reception fairly close to the transmitter.

Best regards, Richard Harrison, KB5WZI

Every time I see a post that begins "-----BEGIN PGP SIGNED MESSAGE-----", I
can't help but think of a kid saying "hey, you want to see my secret decoder
ring?"

"Jack Twilley" wrote in message
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I have seen what appear to be AM broadcast towers in or on the edges
of salt marshes, and it seems pretty obvious to me why that's a good
place to go. However, the environmentalists being a little more
noticeable than they were once upon a time, this particular method of
siting is probably a little more challenging than it used to be.

I recognize that salt water is far more conductive than fresh water,
but fresh water's still superior to sand and the like. That being
said, I am wondering about using a pond as a ground screen and
mounting the antenna itself on an island (or a raft) in the middle of
the pond.

What I don't know is just how large a pond do I need in order for
something like this to work? Obviously it depends on type of antenna
and band and a bunch of other things, but even a wild-ass guess (with
some math or physics behind it) will help make the difference between
whether I bother trying or not.

For those who absolutely require less variables in their equations,
imagine a standard dipole tuned for 20m strung roughly 45 feet above
ground level between two trees, one on either side of a fresh water
pond. How wide does the pond have to be at that point (and others)
for it to work right? Even answers like "the pond will have to be
wider than the dipole is long" or "there will be no noticeable impact
on performance" are fine if they're based in reality, and ideally in
math and physics I can understand.

Oh, and another question: what difference, if any, would frozen
versus liquid water make in this situation?

Jack.
(exploring new antenna options.)
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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