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
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