The conductive wall of circular, elliptical or rectangular closed waveguides can facilitate the propagation of guided electromagnetic waves. Each waveguide device has several different guided modes, and the frequency characteristic equation of each waveguide device is different. For rectangular waveguides, the TE10 guided mode has the lowest attenuation, so it is the most commonly used.
In addition, rectangular waveguides are often the most commonly used microwave waveguide components. However, when the wiring distance is long, circular or elliptical waveguides are often used for connection. Although waveguide devices can operate in multimode, the signal integrity will be worse than that of each single mode due to the interaction between various guided modes (such as sudden wave coupling between higher-order modes). Therefore, this method is not ideal.
The lower frequency limit of the waveguide is a sharp cutoff point, and the attenuation level increases exponentially as the frequency decreases, causing the transmission to be interrupted. The design aspect ratio of most rectangular waveguides is 2:1, which can achieve a maximum bandwidth ratio of 2:1, that is, the ratio of the highest frequency to the lowest cutoff frequency is 2:1. In this way, waveguide devices can withstand the maximum power before microwave breakdown, dielectric breakdown, or secondary electron multiplication breakdown occurs.
In contrast, the maximum bandwidth ratio that can be transmitted by circular waveguides is 1.3601:1, that is, the ratio of the highest single-mode frequency to the lowest cutoff frequency is 1.3601:1. The recommended operating frequency of rectangular waveguides is 30% higher than the cutoff frequency and 5% lower than the cutoff frequency of the second highest guided mode. These recommended values prevent frequency dispersion at lower frequencies and multimode operation at higher frequencies.
Even if the waveguide device works within the recommended frequency range mentioned above, the group delay and phase delay over the entire bandwidth will account for a large percentage of the speed of light. This is different from the flat group delay of TEM mode transmission lines such as coaxial transmission lines at the operating frequency. Nonlinear phase in the group delay or large variation can cause fidelity errors in wideband radar systems and even inter-symbol interference in wideband digital communication systems. Phase shifters and other devices can be used to calibrate the delay, component delay, and unequal length wiring in the waveguide response.
When the length of the waveguide interconnection is short, the insertion loss, phase delay or VSWR will be difficult to measure. The values of insertion loss and other performance parameters on short distances are very low, so except for the highest performance vector network analyzer (VNA), others will not be able to measure beyond their measurement capabilities. This is because the above insertion loss is only a small part of 1dB, and the dynamic range of other VNAs may not be sufficient to distinguish it from noise.
Although waveguide devices generally have very high performance, coaxial cable wiring is sometimes more preferred. For example, when multiple insertions and extractions are required, or the wiring is denser or more complex, or cost reduction is desired, coaxial cables are more suitable.
In a system using a low-noise amplifier (LNA), a waveguide section can be used to introduce RF energy from the antenna into the input of the LNA, while the output of the LNA can be connected to a coaxial port. This is because the loss upstream of the LNA input has the greatest impact on signal-to-noise ratio (SNR) and dynamic range (DNR).
In addition, high-power systems can use waveguide devices to transmit high-power signals at the input and output ends. In short, when the requirements for insertion loss and VSWR performance are not high, the system can use coaxial components instead of waveguide devices to achieve lower costs.
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