Read this: The standing wave forms with this desired pattern at the design frequency, f0, and antennas are normally designed to be this size. However, feeding that element with 3f0 (whose wavelength is 13 that of f0) will also lead to a standing wave pattern. Thus, an antenna element is also resonant when its length is 34 of a wavelength. This is true for all odd multiples of 14 wavelength. This allows some flexibility of design in terms of antenna lengths and feed points. Antennas used in such a fashion are known to be harmonically operated.

What multiple is essential for wavelengths?
What is the answer? (If it cannot be answered, return "unanswerable")
14