Final answer:
The far field distance for an antenna with a maximum dimension of 0.5 m at a frequency of 900 MHz is approximately 1.5 m, and for 1800 MHz, it is approximately 3 m, calculated using the Fraunhofer distance formula.
Step-by-step explanation:
To find the far field distance for an antenna, we typically consider a distance that is at least several wavelengths away from the antenna’s maximum dimension. The far field distance, also known as the Fraunhofer distance, ensures that the antenna's radiated field pattern is well-formed and can be considered planar for practical purposes.
The general formula to calculate the far field distance (D) for an antenna is D = 2Dmax2 / λ, where Dmax is the maximum dimension of the antenna, and λ is the wavelength of the operating frequency.
For an antenna with a maximum dimension of 0.5 m at 900 MHz, the wavelength (λ) can be calculated using the formula λ = c / f, where c is the speed of light (approximately 3 x 108 m/s) and f is the frequency. At 900 MHz, the wavelength is approximately 0.333 m. Plugging these values into the far field formula, we get the far field distance to be around 1.5 m. Similarly, for 1800 MHz with a wavelength of approximately 0.167 m, the far field distance is around 3 m.