Chuck wrote:
"Or that only vertically polarized signals can be intercepted by ships
at sea?"

I served on a navy ship in WW-2. Our antenna was a low-L. It could
intercept either polarity but responded only to line of sight and
high-angle signals. This was a deliberate design. The Navy did not want
our emissions QRM-ing the world. Our range was limited to about 500
miles. We could contact our shore destinations at about 2 days travel
from them (our ship was slow).

Best regards, Richard Harrison, KB5WZI

On Thu, 18 Nov 2004 08:49:44 -0600, (Richard
Harrison) wrote:

Gary, K4FMX wrote:
"Are you saying that my low (less than 1/2 wavelength high) horizontal
antenna will be next to useless if I live on the sea shore?"

No. Your antenna will do whatever it does. I said that sea water
reflects so well that the reflected ray from the sea is almost as strong
as the incident ray. At low angles they cancel when horizontally
polarized, being equal and of opposite polarity, and this eliminates
low-angle radiation. This is demonstrated in Figs. 13 & 14 on page 3-12
of the 19th edition of the "ARRL Antenna Book".

The description for the radiation patterns say they are over flat
ground. Would the patterns be similar over water?

bob
k5qwg


Low horizontal wires tend to send most energy straight up. This can
provide near vertical incidence contacts.

For distance, when the reflecting surface is good (sea water) and the
antenna is low, the antenna had better be placed vertically. Results are
shown in Fig 16 on page 3-13 of the same book.

Best regards, Richard Harrison, KB5WZI

A local here used a 20M Hamstick in a canoe -- worked Japan from Arizona

Hamsticks at URL:
http://www.hamstick.com/

--
The Anon Keyboard
I doubt, therefore I might be

Hello Richard,

I think Gary and I were taking issue with your statement:

"Recall that the ground wave is vertically polarized. There is no
horizontally polarized wave propagation over the sea."

While the first sentence is correct, the second would be a bit of
surprise if it were true. Indeed, your experience on the naval vessel
utilized horizontal propagation over the sea. But it is not correct to
equate horizontal polarization with low-angle polarization.

I think I understand what you meant to say.

73,

Chuck

Richard Harrison wrote:
Gary, K4FMX wrote:
"Are you saying that my low (less than 1/2 wavelength high) horizontal
antenna will be next to useless if I live on the sea shore?"

No. Your antenna will do whatever it does. I said that sea water
reflects so well that the reflected ray from the sea is almost as strong
as the incident ray. At low angles they cancel when horizontally
polarized, being equal and of opposite polarity, and this eliminates
low-angle radiation. This is demonstrated in Figs. 13 & 14 on page 3-12
of the 19th edition of the "ARRL Antenna Book".

Low horizontal wires tend to send most energy straight up. This can
provide near vertical incidence contacts.

For distance, when the reflecting surface is good (sea water) and the
antenna is low, the antenna had better be placed vertically. Results are
shown in Fig 16 on page 3-13 of the same book.

Best regards, Richard Harrison, KB5WZI

Bob, K5QWG wrote:
"The description for the radiation patterns say they are over the
ground. Would the patterns be similar over water?"

The legend says:
"The solid-line curves are the flat, perfect-earth (read sea water)
patterns, and the shaded curves represent the effects of average flat
earth---."

Best regards, Richard Harrison, KB5WZI

Chuck wrote:
"While the first sentence is correct, the second would be a bit of a
surprise if true."

Terman writes on page 803 of his 1955 edition:
"The ground wave is vertically polarized, because any horizontal
component of electric field in contact with the earth is short-circuited
by the earth."

On page 808 of the same book:
"Examination of the vector diagrams shows that with a perfect reflector
(read sea water) the horizontal components of electric field will
exactly cancel each other at the surface of the perfect reflector. In
contrast, the vertical components of the electric field of the incident
and reflected waves do not cancel, but rather add at the reflector
surface with (small values of elevation angle from the surface) Psi.

It`s true. Terman has been tested for 7 decades.

Best regards, Richard Harrison, KB5WZI

Gary, K4FMX wrote:
"Are you saying that my low (less than 1/2 wavelength high) horizontal
antenna will be next to useless if I live on the sea shore?"

No, but at some distance from the water, its benefits fade away. I`ve
read, and I don`t remember where, that the benefit of high conductivity
only extends 1 or 2 blocks back from the water`s edge and then it is
gone.

More money is spent on Viagra and breast implants than on Alzheimer`s
research. With aging baby boomers, we may soon have crowds of salient
people who don`t know why.

Best regards, Richard Harrison, KB5WZI

This could be a little misleading.

Horizontal antennas perform just about the same over salt water as they
do over regular ground, with one exception: If the horizontal antenna is
low, very high angle waves will be attenuated when over real ground, but
won't be attenuated nearly as much if over sea water. The radiation at
zero elevation angle is zero for any antenna height and ground
conductivity. So there's nothing about sea water that would make a
horizontal antenna work worse than over ground.

However, a vertically polarized antenna has much stronger low angle
radiation when over sea water (*) than when over plain ground. And this
low angle radiation can be much stronger than anything but a very high
horizontal antenna. So a vertically polarized antenna is usually the
best choice for a boat.

I also don't think that an L antenna is the best idea, especially if the
horizontal part is low. If it is, the horizontal portion radiates mostly
straight up, wasting part of your power. It's a better idea to make a T
shaped antenna. Then the top will radiate very little, leaving most of
the radiation to the vertical section.

(*) The important factor for good low angle radiation is the ground
conductivity at the point where reflection occurs, rather than the
conductivity just under the antenna. For lower and lower angles, the
reflection occurs at farther and farther distances from the antenna.
Also, the reflection occurs farther away as the antenna gets higher. For
typical HF antennas and low angle propagation, the important region is
on the order of up to a few hundred feet from the antenna. Efficiency is
a separate issue, and for that, the important thing is the conductivity
just under and close to the antenna. Having the antenna directly over
salt water makes getting a good low loss ground connection easy, as
another poster pointed out.

Roy Lewallen, W7EL

Richard Harrison wrote:

Gary, K4FMX wrote:
"Are you saying that my low (less than 1/2 wavelength high) horizontal
antenna will be next to useless if I live on the sea shore?"

No. Your antenna will do whatever it does. I said that sea water
reflects so well that the reflected ray from the sea is almost as strong
as the incident ray. At low angles they cancel when horizontally
polarized, being equal and of opposite polarity, and this eliminates
low-angle radiation. This is demonstrated in Figs. 13 & 14 on page 3-12
of the 19th edition of the "ARRL Antenna Book".

Low horizontal wires tend to send most energy straight up. This can
provide near vertical incidence contacts.

For distance, when the reflecting surface is good (sea water) and the
antenna is low, the antenna had better be placed vertically. Results are
shown in Fig 16 on page 3-13 of the same book.

Best regards, Richard Harrison, KB5WZI

Roy, W7EL wrote:
"The radiation at zero elevation angle is zero for any antenna height
and ground conductivity."

It is the equal and opposite reflection which cancels the incident wave
at an equal distance from the reflection point, i.e. toward the horizon.

Extremely low conductivity would produce radiation as in free space; no
cancellation in the horizontal plane.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Roy, W7EL wrote: "The radiation at zero elevation angle is zero for
any antenna height and ground conductivity."

It is the equal and opposite reflection which cancels the incident
wave at an equal distance from the reflection point, i.e. toward the
horizon.

Extremely low conductivity would produce radiation as in free space;
no cancellation in the horizontal plane.

No, that's not true. The radiation at zero elevation angle is zero for
any antenna height and ground conductivity, even "extremely low".

Sometimes it's necessary to look beyond simplified quotes read in books,
and read and understand the underlying math. In this case, the math can
be found as equation 1(*) on p. 717 of Kraus' _Antennas_, 2nd Ed. (I
thought this was also in the 1st Ed., but can't find it there.) This is
the equation for reflection coefficient from ground for horizontal
polarization. It's easily seen that it equals -1 at an elevation angle
(alpha) = 0 for any value of ground conductivity or permittivity. (Note
that epsilon-sub-r is the complex permittivity, as shown in eq. 4(**),
which contains the conductivity.) The total field is given in equation
3(***), which shows that its value is zero when the elevation angle is
zero and the reflection coefficient is -1.

This is, however, entirely theoretical. As the elevation angle gets
lower and lower, the reflection point becomes farther and farther from
the antenna. So a reflection at zero elevation angle takes place at an
infinite distance from the antenna. Among other problems, this requires
an observation point an infinite distance away and a perfectly flat
ground plane that's infinite in extent. While this is the standard for a
lot of theoretical analysis and in programs like NEC, MININEC, and
EZNEC, it of course can't be constructed in reality. In real life, the
Earth curves away, so if the terrain is perfectly flat, extremely low
angle radiation never strikes the ground. (Without working through the
numbers, I'd guess this to be below a small fraction of a degree for a
moderate antenna height. But it would be easy to calculate.) Again
theorectically, still using a simple reflection model but now with an
idealized model of a spherical ground, you'd get a free space pattern at
zero elevation angle and up to a fraction of a degree *regardless of the
ground conductivity*. But when slicing things this thin, you probably
need to use a better reflection model, which takes into account
dispersion and refraction. I'm not sure how that would modify the result.


(*) rho(horiz) = (er*sin(a) - sqrt(er - cos^2(a))) / (er*sin(a) +
sqrt(er - cos^2(a))) where

er = epsilon-sub-r = (complex) relative permittivity of the ground
a = alpha = elevation angle

(**) er = er' - j * sigma/(omega * e0) where

er' = epsilon-sub-r prime = (DC) dielectric constant of the ground
sigma = ground conductivity
omega = radian frequency = 2 * pi * f
e0 = permittivity of free space

(***) E(horiz) = 1 + rho(horiz) * [cos(2*Bh*sin(a) + j*sin(2*Bh*sin(a)]
where

B = beta = 2 * pi / wavelength
h = height of horizontal antenna above ground
a = alpha = elevation angle

Roy Lewallen, W7EL