Odd multiples of a half wavelength offer a low impedance match point at the center of an antenna. Even multiples of a half wave have a high impedance at their center, but this can be overcome by a 1/4 wave matching section or by moving the feed point off center. All resonant antennas are essentially some multiple of a half wave, the design considerations are usually chosen for mechanical practicality and/or matching to the feed line. Multiple-wave antennas do perform better, but on the lower bands they often become impractical.
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and check out the antenna primers and evaluation files. The first antenna you build will likely be your worst... They (and your understanding) will get better with practice!