9.a. Transmitters Flashcards
What does the transmitter do?
Generates RF energy
How does a Magnetron work?
Power is applied to the cathode. A magentic field is applied at a right angle, causing electrons to travel in a curved path from cathode (center) to the anode (outside), into the cavities and resonate, emitting microwave radiation. The waves generated are channeled by waveguide to the antenna.
The cavities act like capactive plates with the outer wall as the inductor, so there is an LC oscillation in charge, resulting in an EM field (microwaves).
How does a Linear Beam Device (Klystron) work?
Electrons flow from cathode to collector in an electron beam. The beam flows through cavity resonators. As weak RF is applied to the cavity resonator, an EM field is generated within. As the electron beam flows through the cavity resonator, the electrons are sped up and slowed as the EM field alternates, creating bunches of electrons. As the beam passes through the second oscillator, it induces an alternating current, inducing an EM field, generating another electric current in the coax cable to the resonator.
The reason you get an amplification out of the second resonator is that the bunches are slowed down at the second cavity resonator, transferring a portion of their kinetic energy to the EM field.
In this way we inputing a weak RF wave to the Buncher Cavity, applying a voltage drop accross the cathode/anode, and getting an amplified RF wave at the Catcher Cavity.
How does a Helix Travelling Wave Tube work?
Electron gun emits a beam of electrons through the center of the Helix. Magnets keep the beam focused.
The RF travels through the Helix. The electron velocity varies as it interacts with the RF waveform. This in turn amplifies the RF waveform along the length of the helix.
Why would you use a TWT?
Less efficient than Klystron but has a wide bandwidth. This is useful for jamming or anti-jamming.
Less efficient than Klystron.
Draw a Helix TWT
Draw a Klystron amplifier.
Draw a Magnetron type amplifier.
What is an advantage of a solid state amplifier?
No warm-up time (no hot cathode required for electron generation)
Lower voltages.
Very high duty cycle. Can be compensated by pulse comnpression.
Describe Spectrum Management
Coordination and control of usage of the EM spectrum
What is the RSEC approach?
Radar Spectrum Engineering Criteria.
Ensure a radar’s emissions fit within a spectral mask.
Transmitter Errors
Power supply ripple causes time varying errors in the pulses.
MTBF
Mean TIme Between Failures
Reliability: Tube Based vs. Solid State
Tube Based: Up to 40 000h
Solid State: 100 000s of hours, Graceful degradation
What are operational considerations of radar transmitters?
Reliability
Cooling
Safety
