6.9.7 Why Are Radar Transmitters Described by Peak Power Instead of Average Power?
- Why Are Radar Transmitters Described by Peak Power Instead of Average Power?
- What Is Peak Power?
- What Is Average Power?
- Why Do Radar Specifications Quote Peak Power?
- What Does This Mean for RF Safety?
Radar systems are often reported to have transmitter powers of hundreds of kilowatts or even several megawatts. These impressive figures can be misleading because they usually refer to the transmitter's peak power, not the average power transmitted over time. This FAQ explains why radar engineers use peak power, how radar pulses differ from continuous transmissions, and why peak power alone is not a measure of RF exposure.
Why Are Radar Transmitters Described by Peak Power Instead of Average Power?
Few pieces of RF equipment appear more powerful than radar.
It is not uncommon to read that an airport surveillance radar has a peak power of one megawatt or that a military radar transmits several million watts. Such figures naturally raise questions about RF safety.
If radar really transmits at these enormous power levels, why are people able to work safely near radar installations?
The answer lies in understanding the difference between peak power and average power.
Radar Does Not Transmit Continuously
Unlike broadcasting stations, which normally transmit continuously, most radar systems operate by transmitting a succession of very short RF pulses.
Each pulse may last only a few microseconds before the transmitter switches off and the radar listens for echoes returning from aircraft, ships, weather systems, or other objects.
After receiving the echoes—or waiting long enough for distant echoes to return—the transmitter emits another pulse.
This sequence is repeated many hundreds or even thousands of times each second.
Consequently, the transmitter is actually operating for only a small fraction of the time.
What Is Peak Power?
Peak power is the instantaneous RF power produced during one of these transmitted pulses.
Because the pulse is extremely short, the transmitter can generate very high instantaneous power without operating continuously at that level.
This high peak power is essential because radar echoes are usually extraordinarily weak after traveling to the target and back again.
The stronger the transmitted pulse, the more likely it is that a detectable echo will return from a distant object.
What Is Average Power?
Average power takes into account both the transmitted pulse and the periods between pulses when the transmitter is switched off.
Imagine a radar transmitting one-microsecond pulses every one thousand microseconds.
Although the transmitter reaches its peak power during each pulse, it is actually transmitting for only one-thousandth of the total time.
Its average transmitted power is therefore only a small fraction of its peak power.
For this reason, a radar with a peak power measured in megawatts may have an average transmitted power that is hundreds or even thousands of times lower.
Duty Cycle Explains the Difference
The relationship between peak power and average power is determined by the duty cycle.
The duty cycle is the proportion of time during which the transmitter is actively radiating RF energy.
Most radar systems have relatively low duty cycles because the transmitted pulses occupy only a very small proportion of each operating cycle.
Consequently, quoting only the peak power gives an incomplete picture of the radar's actual RF emissions over time.
Rotating Antennas Reduce Exposure Further
Many surveillance radars introduce another important factor.
Rather than transmitting continuously in one direction, the antenna rotates, sweeping a narrow beam around the horizon.
A person standing near the radar is therefore illuminated only briefly each time the antenna points in their direction. For most of the antenna's rotation, the main beam is directed elsewhere.
This intermittent illumination further reduces the time during which a particular location experiences the strongest part of the transmitted field.
Modern electronically scanned radars may not rotate mechanically, but they also direct their beams only where required rather than illuminating every direction continuously.
Peak Power Does Not Equal Human Exposure
Because of these characteristics, peak power alone tells us very little about human exposure.
Engineers assessing radar safety must also consider:
- the pulse width;
- the pulse repetition frequency;
- the duty cycle;
- the average transmitted power;
- antenna gain;
- beam width;
- antenna rotation or beam scanning;
- the distance from the antenna; and
- the time a person remains within the beam.
Only after considering all of these factors can the actual exposure be evaluated.
Why Do Radar Specifications Quote Peak Power?
Peak power remains an important engineering specification because it largely determines radar performance.
A stronger transmitted pulse allows weaker echoes to be detected, increasing the radar's maximum detection range or improving its ability to detect small targets.
For this reason, radar engineers naturally describe the transmitter by its peak output capability.
RF safety engineers, however, are interested in a much broader range of operating characteristics because human exposure depends on the complete transmission pattern rather than on peak power alone.
What Does This Mean for RF Safety?
Radar systems can certainly produce strong RF fields, particularly close to the antenna and within the main beam. This is why access to radar antennas is carefully controlled, maintenance procedures are well defined, and exclusion zones are established where necessary.
However, it would be incorrect to assume that a radar with a peak power of one megawatt exposes nearby people to a continuous one-megawatt RF field.
The transmitter's pulsed operation, low duty cycle, antenna characteristics, and beam movement all play important roles in determining the actual exposure.
This is another example of a fundamental principle introduced in this chapter: human exposure depends on the complete exposure scenario rather than on a single transmitter specification.
Summary
Radar transmitters are usually specified by their peak power because this determines their ability to detect distant targets. Most radar systems, however, transmit very short pulses separated by much longer listening intervals, so their average power is considerably lower than their peak power. In addition, rotating or electronically steered antennas illuminate any particular location only intermittently. Consequently, peak power alone provides little indication of human RF exposure, which depends on pulse duration, duty cycle, beam characteristics, distance, and exposure time as well as the transmitter's maximum output power.
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