MATCHING END-FED 1/2-WAVE ANTENNAS TO THE TRANSMITTER.

Instead of depending on hearsay and old-wives tales, there is a
logical way of selecting tuned-circuit L and C values.

To begin, in the case of impedance matching it is always required to
maximise power efficiency.

It is the power lost in the tuned circuit which matters.

To minimise power lost in the tuned circuit it is necessary to
minimise the current circulating in it. (Circulating current is the
antenna current times working Q but there's no need to know what Q
actually is.)

For a given radiated power the voltage across the tuned circuit and
antenna when at resonance is fixed. So to minimise circulating
current the reactances of L and C should be as high as practicably
possible. That's obvious!

So for a high reactance of C (which is approximately the reactance of
L), C should be mimised.

The limit on the smallness of C is the minimum setting value of the
tuning capacitor, plus coil self-capacitance, plus stray capacitance
associated with wiring and connections to the end of the antenna, plus
an allowance for the antenna not being exactly at resonance, plus a
small 'safety' margin to allow the tuning capacitor to be adjusted to
either above or below resonance.

For half-wave antennas at frequencies below 11 MHz a tuning capacitor
setting of 20 or 25 pF is suitable. This choice automatically sets
the value of the inductance (which is usually higher than the
inductance value of a roller inductor and can be home-brewed quite
easily.)

The maximum setting of the tuning capacitor allows the circuit with
the antenna to be tuned to the lower frequency bands. But efficiency
will be reduced on lower frequency bands due to smaller reactances of
L and C and the greater circulating current. Note that the reactances
of L and C are always approximately equal to each other.

Theoretically, if the antenna length is slightly shorter than
1/2-wavelength, then it is impossible to obtain an impedance match
with this particular circuit. You can't have a tuning capacitor less
than zero pF. It is prudent to select the overall antenna length to be
a few feet longer than 1/2-wavelength at the desired operating
frequency.

On the other hand, it is just as well to remember, a non-resonant
end-fed antenna wire is no less efficient than an exactly resonant
wire provided it can be matched to whatever load resistance is
required by the transmitter. (And the load resistance presented to
the tansmitter as indicated by a TLI has nothing to do with the
so-called, and imaginary, SWR).

As Walter Maxwell has peviously pointed out, SWR = Short Wave Radio.

But, despite its diagramatic simplicity, a parallel tuned circuit is a
not-so-simple, quite complicated arrangement when used as an impedance
matching transformer between an end-fed antenna and the transmitter.
No maths needed - just arithmetic.

Slight change of topic -

I am presently imbibing White, Argentinian, Chardonnay Torrentes. I
do hope my use of plain English language does not lead to
misunderstandings.

May I add that I much prefer a decent, intoxicated, political
opposition-party leader to a sober, war-mongering Tony Blair. At
least a few of you USA citizens must be acquainted with the relatively
trivial events taking place in the 'mother' country. In sympathy with
what's happening in the US.

When will you impeach your so-called president and rid yourselves of
your secret Nazi/OGPU CIA police. I'm sure the Constitution does not
allow such people to exist.

We would be happy to confine both Blair and Bush, here, in the
historic Tower of London, chained to the cold stone walls, for an
indefinite period, on bread and water, without access to legal
representation, as so many other kidnapped prisoners (disappeared
people), for years, are being presently detained without sound reason
or charge.

Have you no conscience? Or have you no idea what a map of the World
looks like?

To obtain an elementary understanding of a simple tuned circuit as an
impedance-matching circuit, download program TUNEHALF from website
below. There are other programs available related to the same
subject.
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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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