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.
By Gene Henry, Bill Sholette June 9, 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. 

Installation conditions for radar transmitters

Even if the best radar is chosen for the application, it will give poor results if it’s not installed properly. Conditions that are present in the installation must be considered. The dielectric constant of the material being measured, mounting type, location, and the surface of the target will all make an impact.

Figure 6: These are antennas that are designed to shed condensation. Courtesy: Endress+Hauser

Condensation, usually from water forming on the antenna of a radar transmitter, can cause problems. Water has a high dielectric of about 80 dc. In a typical horn style antenna, water droplets condensed on the inside of the horn can cause interference and affect the launch of the electromagnetic signal. This interference causes "noise" in the signal and reduces energy. Enough condensation can cause the radar to lock in the near zone, resulting in failure.

To combat condensation problems, several special antenna designs have been developed to help "shed" condensation, reducing the loss of energy and signal noise (Figure 6).

If the traditional horn style antenna must be used for application conditions, it’s possible to add a purge connection to keep the radar free of condensation or dust build up. Since air is very low in dielectric strength, it does not interfere with the microwave signal. Purging with 60 to 70 psi air, such as a blast activated by a solenoid, will keep the horn clear and is the most commonly-used method.

Some manufacturers provide diagnostic capabilities to monitor the area of incoupling, which indicates when condensation or buildup reaches a level that may start to interfere with the signal. These advanced diagnostic capabilities can be used to perform maintenance, such as purging the antenna prior to a failure condition. The ability to provide preventive maintenance reduces downtime and emergency maintenance requirements.

Turbulent and irregular surfaces can cause transmitter energy to scatter or reflect, with lost energy not returning to the transmitter. In the past, the recognized solution would be to use a low-frequency radar with a large beam angle. Consider a ball being bounced off an irregular surface such as a piece of corrugated roofing material. A small ball (tight beam angle) most likely will bounce off in any direction other than straight up. A large ball (wider beam angle) will hit the surface over a greater area and will tend to bounce straight back up again.

While this is all true, special algorithms have been developed to provide level measurement in turbulent surfaces using high frequency transmitters. This provides more or less the best of both worlds: tighter beam angles and reliable measurements. But is not a fix for all applications.

Figure 7: This is an envelope curve showing an agitator signal mapped out. Courtesy: Endress+HauserInternal obstructions in the vessel must also be considered. Agitators, heating coils, thermowells, and other components inside the vessel can interfere with the radar transmission. The narrow beam angle of the 80 GHz radar does a good job of avoiding most obstructions; the remaining ones need to be eliminated from the signal evaluation to prevent the transmitter from seeing these as false level signals.

Typically, the interfering signals are "mapped out" so the radar transmitter ignores them. Figure 7 shows the level measurement signal evaluation with an agitator mapped out. The red line in Figure 7 is the signal provided by the return of the microwave energy. The black line is the superimposed map that eliminates the agitator reflection from evaluation and allows the transmitter to follow only the actual level signal. For a signal to be valid and evaluated as a level signal it must be higher than the map.

Mounting transmitters

The radar transmitter’s mounting location on the vessel also must be considered. It’s often assumed mounting the radar in the center of the vessel is the best placement as it puts the antenna farthest from the vessel wall. But mounting the transmitter in the center of the vessel, especially a doomed roof vessel, is a bad idea because the roof can act like a parabolic dish and concentrate stray secondary microwave reflections and direct them back to the antenna (Figure 8). These secondary reflections can be stronger than the primary level reflection.

Figure 8: Mounting in the center of a tank can result in generating strong secondary reflections. Courtesy: Endress+HauserA good rule of thumb is to mount the radar transmitter at approximately one-third the diameter of the vessel if possible. Mapping, as previously discussed, can solve many mounting location problems. This, along with filters and properly setting up the transmitter, will often provide a reliable measurement in mounting locations that are not ideal.

Mounting a free-space radar transmitter in a stilling well or bypass installation can be challenging. It’s important to understand how microwave energy is transmitted in a stilling well to select the radar that provides the best accuracy. A standard horn antenna can be used for stilling wells but the accuracy will be less than normally specified. This is due to the way energy is transmitted from a horn style antenna in combination with how it propagates in a pipe instead of through free space.

Figure 9: Radar signal from a horn antenna (left) in a stilling well does not follow a straight line. A planar antenna (right) is specially designed for stilling well applications. Courtesy: Endress+HauserFigure 9 shows a horn style radar installed in a stilling well. As indicated, the microwave energy bounces from side to side as it travels down the stilling well and then again on the way back. This causes the energy to travel a longer distance which reduces the accuracy. A horn style radar that provides 2 mm accuracy in an open vessel installation will not have accuracy anywhere close to that when installed in a stilling well. For best accuracy in a stilling well, a planar style antenna should be used. The planar antenna provides an elliptical energy transmission which results in the energy following a straight line path in the stilling well.

If a horn style antenna is used, it’s important the horn diameter matches the inside diameter of the stilling well. If there is a space between the stilling well and the horn, energy will wrap around behind the horn causing excessive noise and reduction in energy.

It is not just the frequency of a radar that is important, but antenna design, smart algorithms and location play a big role in successfully measuring the level in a tank or silo. The more difficult the application and higher the accuracy requirement, the more critical it is to have the optimal frequency and antenna design. Reviewing the application details and working with a supplier or consultant who has experience with radar transmitters will provide the best results.

Gene Henry is a senior U.S. level product marketing manager and Bill Sholette is a senior regional level product business manager in the Northeast region of the U.S. for Endress+Hauser. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media, eguenther@cfemedia.com.

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Key Concepts

  • Selecting frequency for free-space radar transmitters.
  • Best practices for mounting and installing transmitters.
  • The importance of antenna design and location.

Consider this

How can radar level measurement improve application measurement accuracy?