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HB9ABX > TECH 05.05.07 19:59l 26 Lines 998 Bytes #999 (0) @ WW
BID : JHL0JHHB9EAS
Read: GUEST OE7FMI
Subj: Generation of EM waves
Path: DB0FHN<DB0FOR<DB0SIF<HB9EAS
Sent: 070505/1856z @:HB9EAS.CHE.EU [Basel JN37TM] DP6.00 $:JHL0JHHB9EAS
From: HB9ABX @ HB9EAS.CHE.EU (Felix)
To: TECH @ WW
Many articles about antennas mention this:
A good antenna needs much wire in the air.
Is this statement generally valable ?
It is correct for wire antennas, as there the waves
are generated by cyclic acceleration of charges.
EM waves are generated by two different principles:
Either by cyclic acceleration of charges or by
cyclic changing of electric fields.
The third Maxwell equation (Faradays law of induction)
says (simplified), that the time variable change of the
magnetic flux equals the circulation of the electric field.
See Maxwell equations:
http://en.wikipedia.org/wiki/Maxwell%27s_equations
and electromagnetic radiation:
http://en.wikipedia.org/wiki/Electromagnetic_radiation
The second principle, cyclic time-variable electric fields
can be realized by feeding correctly a capacitor
which is open into space by a high voltage RF source.
In this type of antenna, the length of the antenna
becomes very short.
Felix HB9ABX
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