I received an e-mail asking for a little more about 1/2-wave dipoles and
tuned circuits. The guts of the reply is reproduced here. It's a popular
subject. It may be of interest to other learners.

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You can't really get away from the arithmetic. Read the following,
draw a circuit, think about it, then read it again. ;o)

If you have a tuned circuit, L, C and R in series, and apply a voltage
across the ends then a current will flow.

Current = volts divided by impedance Z.

Z = Squareroot ( Square(R) + Square( XL - XC ) )

Where XL and XC are the reactances of L and C. Reactances cannot be
directly added to resistances. All are measured in ohms. XC has a negative
sign.

Magnitudes of both reactances XL and XC can be much larger than R.

As frequency increases the reactance of L increases and the reactance of C
decreases.

At some frequency, the resonant frequency, XC becomes equal in magnitude to
XL and they cancel each other out in the formula.

We are then left with Z = R. There is only the resistance R to limit the
current. So the current at resonance can be much larger than at frequencies
higher and lower than the resonant frequency.

So the large current develops large voltages across L and C. These
voltages, at resonance, can be much larger than the source voltage which is
driving the circuit.

At resonance the voltage across the coil is Q times the input driving
voltage.

A dipole is very similar to a tuned circuit. The wire has inductance and
reactance. The circuit resistance is the antenna's radiation resistance. The
wire has capacitance to the rest of the universe. Like inductance it can be
calculated from the wire's dimensions, length and diameter. Radiation
resistance can also be calculated from the antenna's dimensions. After all,
there's only two of them available.

And that's how a large voltage appears at the ends of the entenna,
considerably larger than the driving voltage. It's a sort of voltage
transformer.

It is also an impedance transformer. It is possible to calculate the
impedance seen looking into one end. There are such things as end-fed
dipoles such as 1/2-wave verticals.

The volts at the ends of the dipole are in antiphase with each other and and
zero in the center. The current is a maximum in the center and zero at the
ends. Graphs of amps and volts versus distance along the wire are not
straight lines - they are sinewave curves.

Tuned circuits, as you know, are used all over the place. The only
resistance in circuit may be the RF resistance of wire in the coil itself
and very high Q values are obtainable. Q's of several hundreds are common in
transmitter coils, in antenna loading coils and in antenna tuners to keep
power losses very small.

In antennas, of course, the power "developed" in the fictional radiation
resistance is made good use of - it is radiated!

I trust the foregoing will be helpul. Have fun.
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Yours, Reg, G4FGQ
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Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
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