summery: CH2 P1,2,3,4 Flashcards
Define transmitter:
The source of the electromagnetic radiation emitted by a radar. It generate the high frequency signals which leaves the radar’s antenna and goes out into the atmosphere
Determine the wavelength of the antenna. (antenna must match the transmitter’s wavelength)
Kinds of transmitters:
- Magnetron
- Klystron
- Solid state transmitter
Transmitters are designed to:
Optimize particular characteristics
Compare the three transmitters:
Solid state:
- By combining multiple transmitters into an array and controlling the timing of each element it is possible to achieve useful power outputs
- Used in phased array antenas to improve time taken to collect radar data
Define modulator:
- an electromagnetic device that control the transmitter
Purpose of modulator:
- switch transmitter on and off
- correct the wave form of transmitted pulse
- tell the transmitter when to transmit and for what duration
- stores energy between transmitter pulses so later it will have stored energy available to use
How to operate weather radar
Using an interface between the operator and the radar to translate our wishes into radar commands
What is the control panel of radar:
Provide a number of choices and a way to select them to communicate with the radar.
Define circuits:
Used to convert selections to control signals
What do operators usually select?
- Range of display
- Elevation angle of antenna
- Azimuth to scan
- Number of pulses in each second
- How long are the pulses
Master clock:
(old radars) generate the appropriate signals and send them to the components
Ubiquitous computers:
- (modern radars) control operation of radars
- (signal processors): control the processing of received data
Signal processors can:
Main function of the mater clock/computer:
Control how often and how long the transmitter transmits
PRF:
The rate at which the radar transmits. Short for pulse repetition rate or pulse repetition frequency. (200-3000Hz)
PRF is usually measured in:
Pulses of cycles per second or HZ (1 HZ=1 cycle/second)
PRF values for different radars:
Non Doppler ground based weather radars
150 – 300 HZ
Doppler weather radar
Capable of detecting the speed of targets moving toward or away from the radar
700 – 3000 HZ
Weather radars used onboard aircraft
500 – 1500 HZ
Pulse duration (t):
Measured in units of time (0.1-10цs)
Pulse length (h):
Units of distance
How to convert (t) to (h) or the opposite:
Distance = rate x time or (h=c x t) where c is the speed of light
Wave guide:
The conductor connecting the transmitter and the antenna
Regular wires pros and cons:
- Pros: conducting electricity and low frequency signals
- Cons: very lossy “energy is lost”
Coaxial cables:
- Special conductors that carry radio frequency signals (works well at many radio frequencies)
- Contain a center conductor surrounded by insulation and then by a layer of shielding conductor and another layer of insulation
- Very loosy
Waveguide shape:
- Usually hollow
- Rectangular (sometimes round) metal conductor
- Interior dimensions depend on the wavelength of the signals being carried
- Cross section is usually rectangular
Waveguide connections:
- Long pieces connected together by joints
- to connect the transmitter/receiver and antenna
- Has to conduct signals around corners
Reason for waveguide connection:
Allow transmitter and receiver to be located at one place while the antenna is mounted else where (up on a tower is the conventional location of a ground based weather radar)
Signals conducted inside waveguide consist of both:
- Electric component
- (E-plane bend): Direction of electric field: Shorter dimension
- magnetic components
- (H-plane bend): direction of magnetic field: longer direction
Which plane band is better? Why?
E-plane bends, because it has less looses
Flexible waveguide
- Special form of waveguide The metal part of the wave guide can bend to accommodate:
- Slight misalignment in wave guide or
- Allow for slight movement between adjacent components
Flexible waveguide shape:
Outside is usually rubber coated to make it air and water tight
Rotary joint:
- Special form of waveguide used to allow antennas to rotate to be able to:
- Scan horizontally (azimuthally)
- In elevation
- Used between waveguide fixed to the radar tower and waveguide fixed to the antenna
- Second rotary joint is used so the antenna can scan up and down
Shape of wave guide in rotary joint:
Circular in cross section
