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.
