What determines the frequency of electromagnetic waves emitted by antennas?
In terms of antenna as a transmitter the Source of current on the antenna and its geometry can generate energy transportation but depending on the MAxwell equations that allow to compute around the antenna the energy flow by means of Poynting vector. If the source is sinusoidal and field is measured at a distance greather than a value called far field distance the frequency of the field received is near to be the some of that of the source but some spectral armonics can happen.
The frequency of waves emitted by an antenna is determined by the frequency of the source of radiation that is being used, such as a signal generator. Some things. like rust on an antenna, result in harmonics of the source frequency also being present.
Anything with an alternating current on it will radiate, but it is often necessary to increase the current using resonant structures - such as a resonant dipole, for instance, for the radiation power to be significant compared to the power available from the generator. When the resonant frequency of the dipole or other resonant structure is close to the frequency of the signal source then the radiation will be more efficient.
Radiators such as dish antennas or waveguides do not need to be resonant to radiate efficiently, but still only work over a band of frequencies for which they are designed.
The frequency of electromagnetic waves emitted by antennas is determined by the physical characteristics of the antenna and the properties of the transmitting and receiving devices. Specifically, the frequency is determined by the length of the antenna, the shape and size of the antenna, and the electrical properties of the transmitting and receiving devices, such as the capacitance and inductance of the antennas and the impedance of the connecting wires. In general, the frequency of the electromagnetic waves emitted by an antenna is directly proportional to the length of the antenna and inversely proportional to the wavelength of the signal being transmitted. For example, a long-wire antenna that is several wavelengths long will emit lower frequency waves, while a short, stubby antenna will emit higher frequency waves. The frequency of the waves also depends on the input signal to the antenna, which can vary depending on the type of transmission and the intended use of the antenna.
What is fed to the antenna is emitted (or received). But with very different efficiency depending on the frequency. An antenna for a certain frequency must be noticeably effective at this frequency what depends on its design.
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