The impacts of frequency and antenna selection for free-space radar transmitters

Accuracy of free-space radar transmitters depends on frequency, beam angle, antenna configuration, and installation.

06/09/2017


Figure 1: Time-of-flight radar calculates level based on the time it takes for a microwave pulse to travel to and return from the surface of the material being measured. Courtesy: Endress+HauserFree-space radar transmitters for level measurement typically use 6, 26, and 80 GHz frequencies. Recently, there has been a lot of hype concerning high-frequency radar transmitters, where some manufacturers claim: the higher the frequency the better the performance. This is not necessarily true. Instead, accuracy depends on frequency, beam angle, antenna configuration, and installation, but most importantly the dielectric constant of the product itself.

Free-space radar transmitters

There are two major operating principles for free-space radar transmitters: time-of-flight (TOF) and frequency modulated continuous wave (FMCW). Each has "time" as the base for distance measurement but calculates it in a different way.

TOF radar uses a microwave pulse launched from a transmitter. When the microwave energy reaches the material being measured, there is a change of impedance due to the dielectric constant (gas phase to liquid or solid surface) that causes the energy to be reflected. The amount of energy reflected is dependent on the dielectric of the material being measured. High dielectric materials, such as water, reflect all or most of the energy emitted. Low dielectric materials, such as hydrocarbons, reflect smaller amounts of energy.

The radar times how long it takes for the microwave pulse, moving at the speed of light, to reach the surface and return. This time divided by two provides the distance to the surface being measured. The transmitter then subtracts the distance from the span of measurement, resulting in the level of product inside a tank or silo. (See Figure 1).

FMCW radar also uses microwave energy directed at the surface of the material being measured. Like TOF radar, the amount of energy reflected is based on the dielectric of the material. FMCW radar transmits a continuous stream of energy instead of a pulse, and the frequency is continuously modulated or varied. So, for an 80 GHz FMCW radar, the transmitter frequency may start at 79 GHz and ramp up to 81 GHz.

Figure 2: Frequency-modulated continuous wave radar measures level based on the difference in frequency sent versus frequency being received. Courtesy: Endress+HauserThe transmitter compares the frequency it's receiving back from the product surface to the frequency it's sending out. The difference between the frequencies is equal to the time it took for waves to hit a surface and come back. Just like for the TOF radar, distance subtracted from the span of measurement results in a level application (see Figure 2).

Though manufacturers will cite all sorts of reasons why one technology is better than the other, both should be considered, depending on application details. Both technologies use microwave energy traveling at the speed of light and energy reflected based on the dielectric of the material being measured, and both measure "time" to determine distance or level.

Frequency effects

There are a number of factors that impact a measuring signal's accuracy and availability, including frequency, antenna type, installation conditions, and the dielectric constant of the material being measured.

Figure 3: This is a comparison of 6 GHz and 26 GHz radar transmitters in the same application showing the sharp peak using 26 GHz to the rounded peak of a 6 GHZ radar reflection. Courtesy: Endress+HauserTransmitter frequency affects accuracy, beam angle, and antenna size. Low frequency transmitters are generally less accurate compared to higher frequency transmitters due to the poorer resolution of the signal generated by the lower frequency transmitters. Figure 3 shows an envelope curve comparison between 6 GHz and 26 GHz radar transmitters. The 26 GHz radar (red line) generates pulses that are approximately one half the length of the 6 GHz pulse (blue line). This provides a sharper return and higher accuracy.

The 6 GHz pulse is much wider than the 26 GHz pulse. The transmitter interprets this pulse and determines where the level is located. The blue arrows indicate that the interpretation can be several points. The transmitter can interpret the leading edge, the center, or the following edge as level, which affects accuracy. Since the 26 GHz transmitter pulse is much sharper, this limits interpretation to a single point, shown by the red arrow. The result is that accuracy for the 6 GHz transmitter is typically 6 to 10 mm while the 26 GHz transmitter provides 2 to 3 mm accuracy in process applications. Advanced algorithms can be applied to come to less than 1 mm accuracy for tank gauging applications.

Reflection peaks from 80 GHz radars also are quite sharp, which makes evaluation of the exact level quite easy. Process radars using 80 GHz can have an accuracy of 1 mm in process applications, while tank gauging and custody transfer radars using 80 GHz can have accuracies of less than 0.5 mm.

Frequency also affects the beam angle of the signal propagated by the transmitter. Lower frequencies generate wider beam angles than higher frequency transmitters. A wide beam angle may be more suitable than a narrow beam angle in some applications. 

Antenna considerations

Antenna size and type also affect the beam angle. The lower the frequency and smaller the antenna, the wider the beam angle (Figure 4). Increasing frequency and/or increasing antenna size reduces the beam angle.

Figure 4: This illustration includes beam angle comparisons of frequency and antenna size. Courtesy: Endress+Hauser

Many believe the smallest beam angle is the best choice for level measurement, and this is true as a general rule. Reducing the beam angle allows the microwave energy to more easily avoid the vessel wall, agitators, and other internal tank obstructions. Providing a radar transmitter with a beam angle that does not intersect the vessel wall or obstructions is ideal.

However, it's important to understand that even if the beam intersects obstructions or the wall, this does not necessarily mean the installation will be unsuccessful. Intersecting the wall will cause some energy loss but is often inconsequential. Obstructions in the beam path can be mapped out—eliminating them from signal evaluation.

It is often thought that if the beam angle intersects the vessel wall it disqualifies the application for free-space radar. While radar installation where the beam angle does not intersect the vessel wall is ideal, it's rarely attainable due to mounting nozzles and other interferences. Through proper selection of frequency, antenna size, and tank mapping, a successful application is achieved most often. 

Measuring ranges

Frequency and antenna size have an influence on measuring range. But the dielectric constant of the material being measured and the installation conditions of the measurement also greatly influence the measurable range.

Low frequency signals have a longer wave length and will travel farther than higher frequencies. Radar transmitters with high frequency emitters are capable of measuring ranges of approximately 100 feet, which is okay for most applications.

Figure 5: This illustrates the radar transmission range by size of a horn style antenna. Courtesy: Endress+Hauser

Antenna size and type also have an influence on transmission range. Larger antennas provide greater range and energy focus than smaller antennas. Figure 5 shows a range comparison for standard horn style antennas. As shown, larger antennas provide greater ranges. The style of antenna also affects the range of measurement. Figure 5 shows typical horn style antennas, but there are also "tear drop," parabolic, rod, and planar antennas. All of these antenna styles are used to provide solutions to different application requirements and measuring ranges. 


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