Hello All,

I have these questions that I hope you can help me with:

1) We are told that vertical antennas over salt water are highly effective. Why? What would an ideal antenna to take advantage of this look like?

2) We are told that a yagi should be mounted as high up as possible. Is this really true? Why? What are the physical/electrical facts behind this phenomenon?

3) When a yagi is mounted high above ground, it still performs better if the ground is pretty conductive. Why?

Thanks for helping me out.

73

Wassim
WN6WJN

Brian Morrison wrote:
On Fri, 25 Apr 2014 09:10:34 -0700 (PDT)
Wassim wrote:

Hello All,

I have these questions that I hope you can help me with:

1) We are told that vertical antennas over salt water are highly
effective. Why?

This is particularly true of a monopole antenna, this type of antenna
constructs an image of itself that appears to be reflected in the
plane of the ground beneath it. Since salt water is more conductive
than fresh water, this ground plane will allow a better, more complete
image to be created and hence the antenna will radiate at a low angle.
This tends to enhance signals over long paths as the RF signal has to
refract from the ionosphere (for HF paths) fewer times and hence loses
less energy.

In addition since water is flat and by definition it's nearly always at
sea level, there is a clear horizon and plenty of uncluttered space for
the far field to be formed and launched or for the received signal to
be captured.

What would an ideal antenna to take advantage of
this look like?

Any vertical antenna will work well, naturally the longer this is the
better (allowing for achieving the correct feed impedance) as this
increases the antenna aperture (and hence gain) for a given frequency.


2) We are told that a yagi should be mounted as high up as
possible. Is this really true? Why? What are the
physical/electrical facts behind this phenomenon?

The earth is curved, hence the radio horizon is increased by a ratio
proportional to the square root of height above ground. Antennas work
better when they are in unobstructed space, an antenna is simply a
transducer that converts from the (typically) 50 ohm impedance of
coaxial cable (or indeed any other impedance such as open wire or twin
feeder) to the impedance of free space which is 377 ohms (it's
proportional to the ratio of permittivity to permeability of free
space).


3) When a yagi is mounted high above ground, it still performs better
if the ground is pretty conductive. Why?

This is not always true, it depends on various factors. In the case of
plane earth propagation the direct and ground-reflected ray will go in
and out of phase as the distance changes, what seems to be improved at
a particular range can easily enter a deep fade and at the transition
region the path loss increases from an R-squared loss to an R^4 loss.


Thanks for helping me out.

73

Wassim
WN6WJN


HTH

A yagi, or a dipole for that matter, mounted over ground has the pattern
skewed upward by an amount that depends on the antenna height in wavelengths
because of ground reflections.

At heights less than around 1/2 wavelength, most of the energy goes to
warming clouds, which is fine if you are trying to communicate with satellites
and not ground stations.

As the height increases the less the effect becomes.

This can easily be seen by using EZNEC and looking at the vertical pattern
of a dipole at various distances (in wavelengths) over ground.


--
Jim Pennino

On Fri, 25 Apr 2014 09:10:34 -0700 (PDT), Wassim
wrote:

1) We are told that vertical antennas over salt water are
highly effective.

Very effective. We have a local AM broadcast station with two
antennas (KSCO/KOMY) located in the middle of a brackish lagoon. With
10KW/5KW day/night power, they're stronger than most other stations at
the same range.

Why?

With a vertical, the ground acts as a counterpoise which makes up the
other half of a vertical dipole. You want this counterpoise to act as
an RF conductor. Dirt is an absorber. Salt water is a conductor.

Incidentally, modeling such an antenna is covered in the current issue
of QST, May, 2014 Pg 50 "Modeling a Ground Mounted Monopole Can be
Tricky".

What would an ideal antenna to take
advantage of this look like?

A better question would be how to avoid the grounding problem. A half
wave vertical is a good way. No counterpoise (ground) is required. I
don't quite agree with this page:
http://www.qsl.net/v73ns/vert.html
but it does give you the general effect of using something other than
a 1/4 wave monopole.

I'm not sure I can answer you question on an ideal antenna without
knowing the frequency of operation. The trick will be getting the
best ground, without going overboard and creating an un-necessarily
large grounding system.

2) We are told that a yagi should be mounted as high up as possible.
Is this really true?

True. Again, the answer depends on the frequency of operation. At
VHF frequencies, the effects of the ground is minimal. On the lower
HF frequencies, antenna height has a big effect on the antenna
impedance, pattern, and takeoff angle.

Why?

At VHF, because the earth is not flat and the RF horizon is further
away as you get higher. There are calculators available to give you
the approximate range at various altitudes (at both ends).
http://www.qsl.net/w4sat/horizon.htm

For HF, because the ground is a dandy RF absorber. Getting away from
the absorbent ground sends more RF in the desired direction.

What are the physical/electrical facts behind this phenomenon?

Too much to cover here. Plug your mythical antenna into a modeling
program and see what happens. That should be sufficient to illustrate
the effects of altitude and different grounding. Incidentally, I did
a rough model of a 1/2 wave 300 MHz vertical dipole at various heights
above ground. Note the changes in takeoff angle:
http://802.11junk.com/jeffl/antennas/vertical-dipole/slides/animated-v-dipole.html

3) When a yagi is mounted high above ground, it still performs
better if the ground is pretty conductive. Why?

Again, the effect is different at HF and VHF/UHF. If you're too close
to the ground or buildings, you're going to make a mess of the antenna
pattern, takeoff angle, and antenna impedance. You're also going to
have some of the RF produced absorbed by the ground. At VHF/UHF, it's
fairly easy to get a few wavelengths away from the ground to reduce
the effects. At HF, not so easy.

Another part of the puzzle is that a conductive ground reflects the
signal, while a not so conductive ground absorbs it. The reflected
signal may not be going exactly in the direction you want, making the
ground system part of the antenna, but at least it's not going into
heating the ground or building. With luck, it may be what the
receiving station is hearing.

Thanks for helping me out.

Sure, but next time, please include some numbers.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

El 25-04-14 18:10, Wassim escribió:
Hello All,

I have these questions that I hope you can help me with:

1) We are told that vertical antennas over salt water are highly
effective. Why? What would an ideal antenna to take advantage of
this look like?
This is true if you want low elevation angle (for example for HF DX).


2) We are told that a yagi should be mounted as high up as
possible. Is this really true? Why? What are the
physical/electrical facts behind this phenomenon?

This is not generally true, it depends on the elevation angle where
you want maximum radiation. For line of sight where elevation angles
are near zero, more height gives more signal, so for that case, the
statement is true.




3) When a yagi is mounted high above ground, it still performs
better if the ground is pretty conductive. Why?

For horizontally polarized antennas that are well enough above ground
level to avoid direct influence of the soil/water and low elevation
requirement (HF DX), soil conductivity hardly affects overall gain
(that is the gain including the ground reflection).



Thanks for helping me out.

73

Wassim WN6WJN

All your questions have to do with reflection on earth (or water).
Mother earth more or less reflects waves, depending on soil
conductivity, polarization and elevation angle.

The reflected wave interferes with the direct wave and this can result
in an increase or decrease of the signal.

You can treat the reflection as an image transmitter with antenna
that replace earth (or water). The phase of the image transmitter and
power depend on soil conductivity, polarization and elevation angle.

For horizontal polarization and very small elevation angle, the
reflection has almost same strength as the incident waves, but has
opposite phase. (path length difference)/lambda ratio between the
direct ray and the ray from the image transmitter below ground,
determines whether you get more, or less signal.

When the path length difference is almost zero (valid for extremely
small elevation angle in case of line of sight), the direct and
reflected wave almost cancel eachother (for horizontal polarization).
Therefore the path loss is well above that for real line of sight.

For vertical polariztion it is more complex because of the (pseudo)
brewster angle. If you search on reflection, Fresnel equations and
brewster angle, please note that many references use incident angle
(that is with respect to the normal) instead of elevation angle.

For sea water, even at around 5 degrees elevation, the reflection is
almost 100%, and in phase for HF. Therefore the direct wave and
reflected wave reinforce each other. Over land, the reflection is not
100%, but almost out of phase resulting in partial destruction under
low elevation angle (required for HF DX).

The best is try to understand reflection on land and seawater for
vertical and horizontal polarization. Make sure you know how to
calculate phase delay due to path lengh difference and addition of
phasors (vector presenation of sinusoidal waveforms).


--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM