1. 1 Waveguides This is the conduit that links the transmitter to the duplexer and antenna It is a hollow conducting tube along which the microwave energy moves A 30 m run of waveguide is typical for the radars The two-way signal loss is about 1 dB

2. 2 Transmitter and receiver at base Waveguide runs up centre of stairs to antenna Waveguide Diagram

3. 3 Transmitter Magnetron and Modulator

4. 4 Transmitter The transmitter produces the microwave pulses emitted by the radar The transmitter’s main components are the source and the modulator There are three basic types of microwave sources used in transmitters: –magnetron (co-axial) –klystron –solid-state

5. 5 Basic Principle of a Magnetron Electrons emitted from negatively charged cathode in centre of magnetron Background magnetic field is directed along axis Electrons spiral outward, passing cavities in the anode Resulting microwave signal is radiated into the waveguide by the “probe”

6. 6 Magnetron Emitted pulse of random phase Makes Doppler signal processing more challenging, but allows certain techniques to be used to advantage (see discussion on Doppler signal processing)

7. 7 Klystron True amplifiers - can control waveform of emitted pulse Thus, phase can be the same for many cycles with little drift Often found on higher-powered weather radars (e.g. WSR88D-NEXRAD)

8. 8 Microwave Sources Magnetron –used in all NRP radars and typical in Europe –relatively low transmission power: 250 kW –pulses transmitted with different phases: “random phase” Klystron –used by NEXRAD radars –higher power possible (NEXRAD is 750 kW) –successive pulses are “in phase”

9. 9 Coherent Pulses vs Random Phase Pulses Coherent Phase - KlystronRandom Phase - Magnetron

10. 10 Magnetron Chosen for NRP systems on the basis of cost and good experience –lower initial cost: about 20% of a comparable klystron –good track record - some in service for 25 years

11. Summary NRP radars use magnetron transmitters Low cost – high reliability Magnetrons produce random phase pulses (Implications for processing will be discussed in another session). Differences from coherent phase.