Neglecting some practical disdvantages that may occurr, a generator can be
placed wherever one likes most along an antenna. In any case, to maximize
the energy transfer, the generator impedance must match the feed point
impedance.
The impedance in a certain point and the current distribution along the
antenna only depends by it's electrical lenght, and the current amplitude is
maximal when match is obtained and losses are minimized.
In other words, the electrical lenght is the only parameter able to alter
the current distribution.
Looking at the antenna as a receiving device or as a parasitic element it's
easy to understand that varying the feed point doesn't move anything in the
current distribution while can have an impact on the efficiency
(amplitudes).
In theory and practice, reactive devices like inductors and capacitors vary
the electrical lenght and actualy move the current distribution of a stated
antenna without changing its lenght but often introduce so unacceptable
losses and unbalances that all the benefit is lost and the overall
efficiency is poorer.
73,
Mauri I4JMY
--
FAQ on WWW: http://www.contesting.com/FAQ/topband
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|