• Home
  • Download PDF
  • Order CD-ROM
  • Order in Print
RING TIME MEASUREMENTS
RECEIVER PERFORMANCE CHECKS - 14190_143

Neets Module 18-Radar Principles
Page Navigation
  122    123    124    125    126  127  128    129    130    131    132  
4-8 permit the comparison of a particular radar with a standard of performance; however, you can use the readings to tell whether or not its performance is deteriorating. Because ring time measurements are the most valuable single feature of the echo box, they must be measured properly. Ring time measurements are made on the A-scope or on the ppi. In measuring the ring time, you should make sure the echo-box ringing (not some fixed-target echo or block of echoes) is being monitored. You can determine this condition by adjusting the radar gain control and noting if the ring time varies on the scope. The echo box ringing will change in duration; fixed target echoes, however, will not change duration. To obtain the best results, you should repeat every ring time measurement at least four times; then average the readings. You should take special care to ensure that all readings are accurate. If two or more technicians use the same echo box, they should practice together until their ring time measurements agree. TRANSMITTER POWER MEASUREMENT Because high peak power and radio frequencies are produced by radar transmitters, special procedures are used to measure output power. High peak power is needed in some radar transmitters to produce strong echos at long ranges. Low average power is also desirable because it enables transmitter components to be compact, more reliable, and to remain cooler during operation. Because of these considerations, the lowest possible duty cycle (pw x prf) must be used for best operation. The relationships of peak power, average power, and duty cycle were described in chapter 1. Peak power in a radar is primarily a design consideration. It depends on the interrelationships between average power, pulse width, and pulse-repetition time. You take power measurements from a radar transmitter by sampling the output power. In one sampling method, you use a pickup horn in front of the antenna. Air losses and weather conditions make the horn placement extremely critical and also affect the accuracy of the sample. A more accurate and convenient method can be used. In this method, you sample the output power through a directional sampling coupler located at the point in the transmitter where a power reading is desired. Power-amplifier transmitters usually have sampling couplers after each stage of amplification. Some radar sets have built-in power-measuring equipment; others require the use of general purpose test equipment or a special test set. In any case, the measuring instruments are most often referenced to 1 milliwatt; readings are taken in dBm (a discussion of the decibel measurement system was presented in NEETS, Module 11, Microwave Principles). When taking power measurements, you must allow for power losses. You must add the directional coupler attenuation factor and the loss in the connecting cable to the power meter reading. The sum is the total power reading. For example, the directional coupler has an attenuation factor of 20 dB, the connecting cable has a loss rating of 8 dB, and the reading obtained on the power meter is 21 dBm. Therefore, the transmitter has an output power that is 49 dBm (21 + 20 + 8). Power readings in dBm obtained by the above procedure are normally converted to watts to provide useful information. Although the conversion can be accomplished mathematically, the procedure is relatively complex and is seldom necessary. Most radar systems have a conversion chart, such as the one shown in figure 4-7, attached to the transmitter or the test equipment. As you can see on the chart, 49 dBm is easily converted to 80 watts average power.






Western Governors University

Privacy Statement
Press Release
Contact

© Copyright Integrated Publishing, Inc.. All Rights Reserved. Design by Strategico.