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