GENTLEMEN, behave yourselves.

The ONLY way of modelling the effect of a loading coil in an antenna wire is
to consider it to be a section of a continuous-inductance-loaded
transmission line which also has a radiation resistance.


Which, of course, is what it actually is. It can then be incorporated in
the remainder of the system which consists of other radiating transmission
line sections.


As with all other lines it has length and diameter (that of the coil
former).


It has a uniformly distributed capacitance (to the rest of the world) per
unit length.


It has normal distributed inductance per unit length PLUS the MUTCH extra
inductance due to being wound as a coil.


It has the normal RF wire loss resistance.


It has a uniformly distributed radiation resistance according to the length
of the coil former. (NOT of the length of the wire on the coil.)


R, L and C are all calculable, or at least can be estimated, from
dimensions.


So in an antenna system, in general, we have 3 consectutive transmission
lines sections with the loading coil forming the center section.


Because of the high inductance of the loading coil, Zo = Sqrt(L/C) will have
a much higher value than that of a wire of the same straight length.


If a generator (transmitter) is applied at one end then currents, voltages
and phase relationships at any point along the overall length can be
calculated. Mismatches between Zo's of the various sections are
automatically taken into account.


But we are ultimately interested only in input impedance, efficiency and
power radiated. All the intermediate stuff which may be available in the
process is just so much waffle for the old wives to haggle about.


As is well known, the coil alone, a simple helix, if of appreciable former
length relative to a 1/4-wavelength, will radiate.


Download in a few seconds program HELICAL and run immediately. Enter the
following values -


Height = 2.5m
Coil dia = 50mm
Coil turns = 750
Wire dia = 2.5mm
Rod length = 0mm
Rod dia = 0mm
Ground loss = 8 ohms

Computed results are the performance of a Helical 160 meter band antenna
which was popular a few years back amongst UK mobile amateurs.


Its a case of the height being so low and the loading inductance being so
big that it occupies the whole length of the antenna. Why waste the space?
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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