The attenuation of a microwave signal due to rainfall is highly dependent on the signal's frequency. Raindrops absorb and scatter radio wave energy, an effect that becomes more pronounced as the signal's wavelength approaches the size of the raindrops. Higher frequency signals have shorter wavelengths. Consequently, a 15 GHz signal (wavelength ≈ 2 cm) is more severely attenuated by a given amount of rainfall than a 7 GHz signal (wavelength ≈ 4.3 cm). Therefore, the attenuation on the 7 GHz link is smaller than the attenuation on the 15 GHz link under the same rain conditions.

A. This is incorrect. Rain attenuation is a strong function of frequency. The effect is significantly different between 7 GHz and 15 GHz, with the higher frequency experiencing greater loss.

C. This is incorrect. It states the opposite of the established physical principle. Lower frequency signals are less affected by rain than higher frequency signals in the microwave spectrum.

1. International Telecommunication Union (ITU-R). Recommendation ITU-R P.838-3: Specific attenuation model for rain for use in prediction methods. (2005). Section 2 and Table 1 show that the coefficients used to calculate specific rain attenuation (in dB/km) increase significantly with frequency. For example

the coefficient k for horizontal polarization is approximately 0.006 at 7 GHz and 0.047 at 15 GHz

directly indicating higher attenuation at 15 GHz for any given rain rate.

2. Huawei Technologies Co.

Ltd. OptiX RTN 900 Radio Transmission System V100R005C00 Product Description. Section 1.3.1

"Network Application." This document explains that lower-frequency bands (e.g.

6-8 GHz) are suitable for long-haul transmission due to their low rain attenuation

while higher-frequency bands (e.g.

15 GHz and above) are used for shorter distances precisely because they are more susceptible to rain fade. This implicitly confirms that 7 GHz experiences less attenuation than 15 GHz.

3. Freeman

Roger L. Radio System Design for Telecommunications (3rd Edition). (2007). John Wiley & Sons. Chapter 5

"Radio Propagation

" Section 5.4

"ATTENUATION OF RADIO WAVES IN THE ATMOSPHERE." This academic textbook states

"Attenuation due to rainfall increases with frequency... Rain attenuation is a major limiting factor for radio systems operating at 10 GHz and above." This supports the principle that the 15 GHz link will suffer greater attenuation than the 7 GHz link.