Antennas and Microwave Engineering MCQs | Page - 13

 in a backward wave oscillator, the rf wave travels along the helix from the collector towards the electron gun. thus the signal for oscillation is provided by the bunched electron beam itself and oscillation occurs.

 magnetrons are capable of very high power outputs, on the order of several kilowatts, and with efficiencies of 80% or more. but disadvantage of magnetron is that they are very noisy and cannot maintain frequency or phase coherence when operated in pulse mode.

 crossed filed amplifiers have very good efficiencies – up to 80%, but the gain is limited to 10-15 db) in addition, the cfa has a noisier output than either a klystron amplifier or twt. its bandwidth can be up to 40%.

 gyratron is a microwave device in which the frequency of operation is determined by the biasing field strength and the electron velocity, as opposed to the dimensions of the tube itself. this makes the gyrator especially useful for microwave frequencies.

 with a slotted line, swr and the distance of the first voltage minimum from the load can be measured, from this data, load impedance can be found.

 although slotted lines used to be the principal way of measuring unknown impedance at microwave frequencies, they have largely been superseded by the modern network analyzer in terms of accuracy, versatility and convenience.

 ┌= (swr-1)/ (swr+1). substituting for swr in the above equation for reflection co-efficient, magnitude of the reflection co-efficient is 0.2. to find θ, θ=π+2βlmin, substituting lmin as 1.48cm, θ=86.4⁰. hence converting the polar form of the reflection co-efficient into rectangular co- ordinates, reflection co-efficient is 0.0126+j0.1996.

 at higher frequencies, if higher bandwidth is desired, a compromise on maximum achievable gain is made. but at these higher frequencies, the ports of the amplifier are not matched to 50 Ω.

 negative feedback can be used to increase the gain response of the transistor, improve the input and output match, and increase the stability of the device.

 in conventional amplifiers, a flat gain response is achieved at the cost of reduced gain. but this drawback can be overcome by using balanced amplifiers. this is overcome by using two 900 couplers to cancel input and output reflections from two identical amplifiers.