THE ONLY WAY TO ACCURATELY MODEL A LOADED VERTICAL


and similar coil and capacitance loaded structures, is as follows.


Let's take the most simple structure consisting of 3 sections - top, centre
and bottom.


The top is usually referred to as a whip or a rod.


The centre section is a loading coil.


The bottom section can be referred to as the mast.


All three sections have length and external diameter.


All three sections have uniformly-distributed inductance, capacitance and
resistance per unit length. All calculable from dimensions.


It is inescapable, therefore, the structure consists of -


THREE CASCADED TRANSMISSION LINES SECTIONS.


As with ALL transmissions lines, each section has a characteristic impedance
Zo, attenuation and phase-shift per unit length. All caculable from
dimensions.


The centre section, a solenoidal loading coil, clearly has inductance extra
to that due to the length of the coil former. The extra inductance is
calculable from the number of coil turns per unit length. If necessary
self-capacitance can be included.


ALL sections have a UNIFORMLY DISTRIBUTED radiation resistance which is
extra to conductor loss resistance. The two resistances are combined by
simple addition. Radiation resistance is itself obtained from the length of
a section.


CALCULATING PROCEDURE / PRINCIPLES.


This is done by using classical transmission formulae or by the well known
sufficiently accurate approximations.


Looking upwards from the base of the antenna -


(1) Calculate the input impedance at the bottom of the whip (or rod). The
whip is a transmission line open circuit at the top end.


(2) Calculate the input impedance at the bottom end of the loading coil,
the centre section. The centre section is a transmission line terminated by
the input impedance of the whip above it.


(3) Calculate the impedance looking up from the base of the antenna. The
feedpoint impedance. The bottom antenna section being a transmission line
terminated by the input impedance of the centre antenna section, the loading
coil.


(4) Connect a generator, the transmitter, between the base of the antenna
and ground. The ground will have its own 'input' resistance depending on the
ground electrodes or perhaps the vehicle characteristics.


(5) Using classical transmission line formulae, or their close
approximations, the amps, volts, relative phase-angles, can be calculated,
IF NECESSARY, at any point along the antenna, at any point along the coil,
from its base to the extreme tip of the whip


(6) But a full analysis is unnecessary. Nevertheless there's a big bunch of
calculations to be done to calculate radiating efficiency, a single number,
the ultimate solitary objective. Of what use to anybody is the difference
between input and output currents of the loading coil?


(7) The radiation pattern is eventually available. But even CB-ers are
familiar with the radiation pattern of a simple vertical antenna, loaded or
not, which is less than 5/8ths wavelengths in height.


(8) By an obvious extension of the above principles, a top hat on the top
of the whip, such as a set of radial wires within a ring, can be
automatically accounted for.


Now, you old-wives, what can be more simple than THAT? KISS!


I'm very sorry, due to recommendations by the medical profession, I have to
decline invitations to tea-parties in Boston, Mass.


And stop haggling amongst youselves. You have quite enough problems to sort
out on your side of the Atlantic.


As examples of practical use of the foregoing principles, download in a few
seconds and run immediately short, self-contained programs LOADCOIL and
TOPHAT2 from website below. Free to USA citizens.
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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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