"christofire" wrote in message
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"Szczepan Białek" wrote in message
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"christofire" wrote
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Now I understand what you meant by 'total field' - sum of powers of
components in all polarisations.
Does one wave has many polarizations, or one antenna has many
polarizations? Which one: transmitter or receiver? Could you teach me?
A*
You appear to have changed your identity from S* to A* !
The answers according to the physics that real-life radio communication
depends upon, and was designed by, a
A single EM wave is plane polarised. It is composed of a magnetic field H
that acts in a direction perpendicular to the direction of propagation,
the magnitude and sign of this field varying as a travelling wave in the
direction of propagation, and an attendant electric field E that also acts
in a direction perpendicular to the direction of propagation. The
magnitude and sign of the electric field varies as a travelling wave,
coherent and in phase with the magnetic field and the magnetic field is a
direct consequence of current flowing in the transmitting antenna. The
directions in which the H and E fields act, in the plane transverse to the
direction of propagation, are mutually perpendicular and the direction in
which the E field acts, by convention, defines the polarisation.
Thus a single EM wave has a single, plane, polarisation. Different
combinations of waves are possible such as circular polarisation and, more
generally, elliptical polarisation, but these can always be resolved into
orthogonal plane components.
Simple antennas like straight-wire dipoles and loops transmit and respond
to plane polarised EM waves. More complicated antennas can be made to
transmit and receive circular polarisation of one sense or the other, and
generally an antenna will tend to transmit or be sensitive to some
combination of different plane polarisations. In addition to radiated EM
waves, there are also induction fields in a region close to the antenna.
In a system that contains no anisotropic material (e.g. magnetised
ferrite), when the distance between transmitting and receiving antennas is
at least tens of wavelengths, the principle of reciprocity applies. By
this principle the properties of an antenna when transmitting are the same
as when it is receiving - the properties including the polarisation,
radiation pattern and terminal impedance.
If you find any of this interesting, please don't believe what I've
written here but go to a technical library (e.g. at a University) and look
up the authoritative sources - books on antennas and propagation by Kraus,
Jasik, Jordan and Balmain, Terman, etc.
Please _do not_ respond here telling me or the group that EM waves are
longitudinal and are not polarised.
Chris
.... but the libraries are probably closed today so, for an instant, online
source you could do worse than visit
http://www.globalsecurity.org/milita...icy/navy/nrtc/, download
the NEETS module 'ELECTRONICS TECHNICIAN, VOLUME 07--ANTENNAS AND WAVE
PROPAGATION ' and read it. It's based on the same, real world physics.
Chris