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Getting an HF antenna to work (Part 4)

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Getting an HF antenna to work (Part 4)

Part 4 of Ralph Bilal's email stream, (the final one) is concluded below.

Last time there was information about inductors, one component of a resonant circuit. Now capacitance, the other component will be described.

A capacitor is made of 2 metal surfaces parallel to each other in close proximity, but not making electrical contact. The larger the surface area of the metal, the higher the value of the capacitor.

When we energize a capacitor by putting an alternating current across each plate, a static charge develops. + on one side and - on the other. This starts current flowing much like a battery. When the current reverses, the current flow does also. There is no current flowing through the capacitor because it is highly insulated. It is like a miracle except we can make this happen any time we want.

Since the capacitor has a very high resistance between the plates, it offers a resistance to the Alternating current we apply to it. This resistance is called "CAPACITIVE REACTANCE" or XC. Like inductive reactance it also is measured in ohms like a resister.

As the frequency applied increases, the capacitive reactance or resistance decreases and vice versa. It reacts opposite to an inductor.

A capacitor is rated in Farads. Given this value it can be looked up on a reactance chart and the resistance can be determined for any frequency.

Capacitors not only come with a Farad value, but also with a maximum voltage rating. If exceeded the capacitor can be ruined.

Therefore, as frequency increases to the capacitor, the reactance decreases. With an inductor, as frequency increases, reactance increases. One is decreasing while the other is increasing.

When the 2 are in the same circuit, at some special point in frequency the 2 reactances will be the same or equal. Then a wonderful thing happens - RESONANCE!

Next articles we will see more how this works and how it applies to your antenna.


Ralph WD0EJA


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