If there is reflection from the end of, a wire, the number of standing waves on the wire will be equal to length of the wire divided by a half wavelength. Thus,if the wire is two half wave long there will be two standing waves, and so on. There longer wires,each multiples of a half wave in length,will also be resonant,therefore,at the same frequency as the single half-- wire. When an antenna is two or more half waves in length at the operation frequency it is said to be harmonically resonant,or to operate at a harmonic. The number of the harmonic is the number of standing waves on the wire. For example, a wire two half waves long is said to be operating on its second harmonic ; one three half waves long on its third harmonic, and so on.

Harmonic operation is often utilized in antenna work because it permits operating the same on several harmonically related amateur bands. It is also an important principle in the operation of certain types of directive antenna.

Fig.2. Harmonic operation of along enough to contain several half waves. The current and voltage curves cross.

The heavy line representing the wire to indicate that there is reversal in the direction of the current,and a reversal, in the polarity of the voltage,at intervals of a half wavelength. The reversals or current and voltage do not coincide,but occur at point a quarter wavelength a part.

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