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Getting an HF antenna to work (Part 5)

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Getting an HF antenna to work (Part 5)


In the last 2 previous articles Inductive and Capacitive reactance were described. These values have something to do with a resonant circuit. What is it?

Besides a reactance, other things go on in the 2 components inductor and capacitor). When an Alternating electrical source is applied to an inductor, the voltage and current have a little argument. They argue, who is first. In this case the voltage wins and it leads the current by 90 degrees. In other words the voltage and current which normally have waves that are in phase, separate. So when the voltage reaches its peak and starts on its way down, the current starts increasing from 0 to its peak behind the voltage.

With a capacitor, the current wins the argument and leads the voltage 90 degrees.

Why be concerned about this argument?

Connect an inductor and capacitor in series. One end of each component is connected together. The other side of the inductor is connected to your electrical source and the same for the capacitor.

When the frequency hits that amazing point where the 2 reactance's are equal, we have resonance. Because of the described phasing and being connected in series, the circuit develops a minimum resistance at resonance. At resonance, this resistance looks just like a regular resister. No phasing argument. As you move up or down in frequency away from resonance, then the reactance starts to kick in and will increase.

Actually the resistance this circuit produces is really termed IMPEDANCE - Z. The definition of Z is, the total opposition to an alternating current. RESISTANCE is related to a direct current or a device that creates no Reactance - X, such as a carbon resister - R.

From this point on IMPEDANCE or Z will be the terminology for alternating sources.

Resistance that is caused by the reactance of an inductor or capacitor, is termed REACTANCE or X.

Resistance that is caused by a non-reactive device such as a carbon resister, is simply termed RESISTANCE or R.

This Series Resonant configuration is the same configuration of dipoles, verticals and (huh?), Isotrons.

Since this applies to most of the antennas used, we will continue on this configuration next.


Ralph WD0EJA

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