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On 8 mayo, 10:35, Szczepan Białek wrote:
I start reading about acoustic analogy.
I found that: "Over long distances, the atmosphere can cause the
polarization of a radio wave to fluctuate, so the distinction between
horizontal and vertical becomes less significant."
From:http://whatis.techtarget.com/definit...843762,00.html
The my question a
1. What means "long distances" in km (or miles),
2. What is the best orientation of the antenna for long distances.
S*
Hello,
Under normal circumstances, polarization change in line-off-site
conditions (think of max 40 mile) is not that much, so antenna
polarization does matter (unless you use at least circular
polarization on one side).
In a propagation path that is dominated by multi-path effects
(reflection at buildings, hills, foliage, etc), you get almost random
polarization and then the polarization is not that important. Your
cell phone and indoor WIFI are examples.
Extreme weather conditions can also lead to polarization changes or a
random polarization component (ducting superrefraction).
For sea water up to VHF, reflection depends on polarization. For
ground-ground links (for example ship shore) mostly vertical
polarization is used (as the sea water helps in this case). So if you
want to receive these communication, you use a vertical polarized
antenna.
The largest change in polarization you will get when the waves have to
travel through the ionosphere. At HF (ground-ground link via
ionosphere), the polarization vector rotates many times. This is due
to Faraday rotation. Also ground-satellite links suffer from this
effect. The higher the frequency, the less the change in polarization.
For example at 100 MHz you should think about 30 full rotations (that
is more then 10k degrees), while at 10 GHz the change in polarization
will be about 1 degree. Circular polarization may help to mitigate the
influence of Faraday rotation.
At HF sky wave (100....1000 mile via ionosphere) polarization of the
antenna matters. This is not because of the polarization change of the
waves due to Faraday rotation, but because of the reflection
characteristics of mother earth. In HF antennas, reflection on mother
earth is used (in combination with antenna height) to get the required
elevation radiation pattern of the antenna. Reflection on earth
depends on polarization.
Hopefully this helps you a bit.
You do not use the words "transversal" and "EM". The only evidence of
polarization is antenna directional sensitivity.
In the acoustic analogy a radio waves are normal spherical electric waves
emitted from the two sources (ends of the dipole).
So the sources are polarised, not the waves. Waves interfere. Do you agree?
See my topic "frequency doubling" . I am only a science hobyist.
The second question was: " What is the best orientation of the antenna for
long distances?
For old radio antennas. Very long horizontal wire.
Best regards,
S*