R: R: R: [antennaware] Feeding cable lenght

Maurizio Panicara Maurizio Panicara" <i4jmy@iol.it
Wed, 4 Aug 1999 10:05:27 +0200

An half wave line of different impedance than source and load is a matching
device (a special case when the line matches itself) and like any device in
the real word has inherent losses and bandwidth.

The half wave line, infact, can be explained as 2 quarter wave transformers
connected back to back, having specific Q and consequent losses
and where the mid impedance, likewise the impedance in any point of the
line vary, depending by line and load impedances.

When instead line, load and source impedances are the same, the impedance
is also the same along any point of the line and no other device than a line
transmission exist in the electrical circuit.

The fact that at the end of an half wave line of any impedance the line
voltage and the current have the same values of the input, doesn't eliminate
the fact  that a line, when used like as an impedance transformer, has a
finite bandwith and extra losses, things that do not exist when the line has
the same impedance of source and loads.

If these extra losses and bandwidth characteristics are or not acceptable,
depends by individual considerations and practical situations. Definitely,
coaxial lines with solid dyelectric have more losses than open wire lines,
expecially when used as impedance transformers.

Practically speaking, with complex loads as the antennas, an half wave
line having the same impedance of source and load is quite useful to
approximate real antenna feedpoint at RTX edge (for measurements) and
avoids unwanted impedance transformations due to odd line lenghts when
load is not pure resistive (out of resonance) or equal to the line and
source impedance.

Mauri I4JMY

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