The main disadvantage of using a coaxial cable at microwave frequencies is the occurrence of signal loss and attenuation. As the frequency increases, coaxial cables become less efficient at carrying signals due to the following factors:
Attenuation: Coaxial cables exhibit higher attenuation (signal loss) at higher frequencies. The higher the frequency, the greater the attenuation. This is primarily caused by the resistance and dielectric losses within the cable, which result in the gradual reduction of the signal strength as it travels through the cable. At microwave frequencies, the attenuation can be significant, leading to a weaker and distorted signal at the receiving end.
Higher order modes: Coaxial cables can support multiple modes of signal propagation, including higher-order modes. At higher frequencies, these higher-order modes become more prevalent, leading to signal dispersion and distortion. This can cause a loss of signal integrity and affect the overall performance of the system.
Impedance matching: Coaxial cables require careful impedance matching between the cable and the connected devices or components. At microwave frequencies, achieving precise impedance matching becomes more challenging due to the smaller wavelengths involved. Any mismatch in impedance can lead to reflections and signal loss.
Size and weight: Coaxial cables designed for microwave frequencies tend to be larger and heavier compared to cables used at lower frequencies. This can make installation and maintenance more cumbersome, especially in applications where space and weight restrictions are a concern.
Due to these limitations, alternative transmission mediums such as waveguides or fiber optics are often preferred for high-frequency applications where lower signal loss and higher bandwidth capabilities are required.