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Updating Circuit Theory: Dipole Transients


By Ian Darney

A transient model of a dipole antenna is developed from an earlier model which used an LCR circuit to simulate the frequency response of single conductor. A transformer at the centre provided the source voltage. The resistive component of the model was that defined by electromagnetic theory as the ‘radiation resistance’.

The new model uses a waveform generator to deliver current to the central section and shift registers to simulate the propagation of charges along the conductor. When the waveform is sinusoidal, the response is shown to be exactly the same as that of the LCR circuit.

It is reasoned that the radiation resistance provides a measure of the power used in converting the radial flow of charges in the conductor to that of photons emanating from the surface. 

Frequency Analysis

Figure 1a shows a general circuit model of a length of conductor acting as a dipole transmitter. During the first half cycle of a sinusoidal signal, the voltage source at the centre draws current from the left hand monopole and delivers it to the right hand monopole. During the second half cycle the process is reversed. When the frequency of the source corresponds to the half-wavelength of the conductor, the current reaches a maximum value, as does the radiated energy.

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