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Density is one of the main parameters to characterize a plasma. Microwave reflectometry has been used for the electron density profile and fluctuation measurements in many plasma devices. The advantages of the reflectometry are: 1. obtaining information about local plasma electron density (not line averaged value like in the case of interferometry); 2. requiring only a small port for access to the plasma ; In this thesis a very young type of microwave reflectometry is treated: pulsed radar reflectometer. We have developed the pulsed radar reflectometer for Compact Helical System (CHS). Major emphasis will be placed on technical aspects. It must be also stressed that this work is the first trial for helical systems. In the conventional reflectometry a wave of the specific frequency is launched towards the plasma to be reflected from the layer in the immediate vicinity of the critical density. The phase difference of the launched wave and reflected wave contains the information on the position of the reflecting layer. In the case of pulsed radar reflectometry, short (～1ns) microwave pulses are launched into the plasma. The pulse is reflected from a corresponding critical density layer and received by the time of flight measurement system. The measured quantities for those types of reflectometers are phase or time delay. The advantages of the pulsed radar are as follows in comparison with the conventional reflectometry: 1. temporary losses of the reflected signal are not so serious, each pulse carries all information about the position of the critical density layer; 2. false reflection from window, horn lens or waveguides could be easily filtered, because they all occur outside of interested time window; 3. due to the very short time for the microwave propagation, plasma fluctuations seem to be "frozen" and they will not affect the measurements significantly ; 4. effect of mode coupling becomes less critical because reflected ordinary and extraordinary mode pulses arrive at the different time; simultaneous measurements of O- and X-mode reflection are in principle possible yielding the total magnetic field; Numerical simulation has been done to study the propagation and detection processes of the microwave pulses. Due to the dispersion in plasmas, the pulse share of the microwaves is deformed. This deformation causes timing error in the Constant Fraction Discriminator (CFD), which yields a logic pulse. This discriminator is inevitable for our system to do amplitude insensitive measurements. The pulse deformation is more serious for shorter pulse width, and leads to a larger error in the timing. We have found that the errors decrease with the increase of the pulse width and errors are negligible (less than 1%) for pulse width longer than about 1 ns. A 1-channel (with changeable frequency 51, 54, 57 GHz) pulsed radar system has been designed for CHS. It is similar to that developed by RTP tokamak group, where pulsed radars were introduced for the first time. The difference is that our system uses bandpass filters at the IF stage, and can be upgraded to a multichannel system more easily. The system consists of: 1. microwave pulse production part; 2. microwave transmission, vacuum window and antenna, bandstop filter; 3. mixer and IF pulse electronics; 4. time of flight measurement electronics To produce a suitable microwave pulse (short, large amplitude and well shaped) we tested special nonstandard fast switches (Millitech fast PIN switch and IRE varactor-diode fast modulator). The minimum pulse widths are 1.9 ns and 0.28 ns, respectively with heterodyne type detection, and the output power is around 50 mW at the peak of amplitude. Since a serious pulse broadening occurs in the long fundamental waveguide (WG), its length has been minimized. Although two independent launching and receiving antennas are used for most reflectometers, we use only one antenna for launching and receiving. This causes the problem that a small reflection (～2%) at the vacuum window makes a false pulse. In principle, the problem does not affect the measurements, because the false reflection occurs at a time window different from that for the true reflection (from plasma). This is the advantage of pulsed radar reflectometry. By using another PIN switch in front of the mixer we have succeeded for the first time in reducing the effect of false reflection to a practically acceptable level. Since the frequencies of ECH and reflectometer are rather close each other, we use a bandstop filter to reject strong ECH microwaves. Instead of an RF detectors, we use a mixer and IF electronics, so that the system has a good signal to noise ratio (SNR), and the potential for the multichannel system. A CFD yields the peak timing of the reflected pulse, and the timing is independent of the amplitude. This is very useful for the measurements, because large amplitude change occurs often. A Time-to-Amplitude Converter (TAC) is used to measure the time delay of the reflected pulse. The output of the TAC is sampled by an Analog-to-Digital Converter (ADC). Free space time measurements have been done to calibrate the timing electronics. Instead of the plasma cut-off layer, a metallic mirror is used and moved by 50-90 cm in front of the antenna. A spatial resolution of 0.3 cm has been reached, that comes from the accuracy of TAC output reading. A 51GHz 1-channel pulsed radar reflectometer has been installed in CHS. As a wave polarization, ordinary and extraordinary modes are launched. We probed the plasma with microwave pulses of 1.9 ns. The plasmas were initiated by IBW (Ion Bernstein Wave) or ECH and heated by NBI and ECH. These measurements were done for discharges with the magnetic field of 0.85 T and the maximum electron density of about 6×10 19 m-3. At the first stage, we used an oscilloscope instead of CFD-TAC time of flight measurement electronics. Due to the function of oscilloscopes, we could get only one waveform for each discharge. The result showed that the reflected waveform was distorted but not significantly, and that in most cases the amplitude of the reflected pulses is rather small. When we read the peak timing by eyes, the relation between the time delay and the line averaged density agreed with the calculated curve both for 0-/X-modes launching experiments. As the next step, we used the CFD-TAC electronics by which we could get the time behavior for each discharge. The repetition rate of the pulse launching and measurement is 200 kHz. The launching polarity is adjusted to X-mode to measure relatively low density plasma. When the discharge starts, the density gradually increases. Below the critical density, the pulse passes through the plasma and is reflected at the back wall of the vacuum vessel. The reflected pulse passes through the plasma again and detected by the receiving system. This situation is called as delayometry. When the density increases, the cut off appears and moves towards the antenna. The time behavior of the time delay agrees with the calculation of this scenario by assuming the parabolic density profile. Instead of assuming the whole profile, we assume a linear profile between the plasma edge and the cut-off layer. Then the estimated positiorn of the cut off layer coincides well with Thomson scattering measurements. The spatial accuracy of the determination of the position of the reflected layer during plasma measurements was about 5-8 mm. While the reflectometer can follow the fast and large changes in density, the HCN interferometer fails, which is partly shown in the ice pellet injection experiment. The fast movement of the cutt-off density layer during the density collapse was successfully measured with the pulsed radar reflectometry. This fact demonstrates the reflectometer has a high temporal resolution. The first pulsed radar measurements in a helical system have been done. We have succeeded in measuring the density with one antenna system, that means reflectometry requires only a small port. The density fluctuations could also be measured because of its high spatial and temporal resolution of pulsed radars.