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Waveguides and coaxial cables are both used in microwave systems. State two ways in which their transmission properties differ.
- TLs propagate frequencies down to DC, while waveguides have a low cut-off f, thus high-pass characteristic.
- TL - TEM waves. only components at right angle to propagation
- waveguides - TE or TM (transverse), always a field component in direction of propagation
At what distance from a conductor, carrying an alternating current having a frequency of 30 MHz, would the “near” and “far” fields have the same amplitudes?
- use f=c/lambda to calculate wavelength, then insert:
Why are half-wave dipoles preferred over “short” dipoles in antenna arrays?
How are the maximum gain and directivity of an antenna related?
Estimate the diameter of a 30db gain, circular parabolic dish antenna, operating at a frequency of 10 GHz.
Why are S-parameters widely used for describing the properties of networks at radio and microwave frequencies?
At high frequencies it is increasingly difficult to measure voltage and current accurately. It is much easier to measure the incident and reflected waves from a network under test and therefore define the electrical propoerties without prior knowledge of the form of network.
A microstrip line is operated at a frequency of 10 GHz and the effective dielectric constant of the substrate is 1.8. Calculate the physical length of line required to form a half-wave stub.
A microwave circuit has an input impedance of (25 + j45) Ω. Write down the internal impedance of a signal source designed to maximise the power delivered to this circuit. Justify your answer briefly.
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