A guitar string produces harmonics because it vibrates in a non-linear fashion. An electronic oscillator can be made to generate a much purer form of vibration (near sinusoidal) than a mechanical device such as the guitar string. Hence its harmonic level, while not zero, is much lower. For example, the harmonic distortion of a guitar string is probably on the order of 10% or so. A good electronic oscillator might be 0.01% or even lower. It has harmonics but they are hard to see on a spectrum analyzer because they are so low. For 0.01% the harmonic levels are -80 dB or less below the fundamental and may not be visible unless the spectrum analyzer has a dynamic range of at least 80 dB. That being said, the existence of harmonics in the radio spectrum is a function of the quality of the oscillator producing the communication signal as well as any power amplifiers between the oscillator and the antenna. There is nothing inherent about having harmonics in the radio spectrum. Most communications devices use high quality oscillators and power amplifiers, and often have a bandpass filter just before the antenna, to minimize harmonics. FCC requirements specify the permissible harmonic levels depending on the frequency band, power level, type of modulation, and other characteristics. Typically harmonics are kept at least 40 dB below the fundamental which is equivalent to a power level ratio of 1 to 10,000.