In today's modern industry, factories compete to produce more products with less cost consumption, and the slim profit margin requires enterprises to continuously increase output and quality. Digital transformation makes it more and more worthwhile for enterprises to consider investing in order to obtain more factory data from process automation systems and instrumentation. However, to do this, new quality processes and products are needed to access data from every aspect of the factory and obtain more value in every production operation. The adoption of the new Ethernet-APL of advanced physical layer and the automation protocol that defines the structure and meaning of the information of the field devices, will become a key driving factor for IIoT in process automation in the future. It will provide an important prerequisite for expanding the digital world to process automation and instrumentation.

Ethernet-APL is an enhanced physical layer based on 10BASE-T1L Single Pair Ethernet (SPE). It communicates at a full duplex speed of 10MBit/s over cables up to 1000 m, which is more than 300 times faster than current technologies such as HART or field buses. It is a logical extension of Ethernet, providing the attributes needed for reliable operation in the processing plant field. Ethernet-APL is a physical layer that can support EtherNet/IP, HART-IP, OPC-UA, PROFINET, or any other higher-level protocol.

Ethernet with an advanced physical layer (Ethernet-APL) will communicate and power through two wires, enabling long cable lengths and explosion-proof protection. Based on IEEE and IEC standards, Ethernet-APL supports any Ethernet-based automation protocol and will evolve into a single, long-term, and stable technology for the entire process automation community.

CHINASIMBA has recently launched the ANL-9080-APL and AiW-4120MG-APL radar level meters, which is a two-wire system product with an APL communication interface that can be quickly connected to the control system and other business systems. The communication distance is 200 meters, and the rate is 10Mbps; through the APL Coupler AEP6101-1E-S, it realizes flexible applications based on the TCP/IP application layer, and realizes the interoperability of the device. It supports various types of field instruments (APL meters, general-purpose meters, wireless meters, etc.).


Process radar level transmitters operate at microwave frequencies between 24GHz and about 120GHz. Manufacturers have chosen frequencies for different reasons ranging from licensing considerations, availability of microwave components and perceived technical advantages.

There are arguments extolling the virtues of high frequency radar, low frequency radar or every frequency radar in between. In reality, no single frequency is ideally suited for all of the radar level measurement applications.

If we compare 26GHz radar with 80GHz/120GHz radar, we can see the relevant benefits of high frequency and low frequency radar:

Antenna size - beam angle

The higher the frequency of a radar level transmitter, the more focused the beam angle for the equivalent size antenna.
With lens antennas, this allows smaller nozzles to be used with a more focused beam angle.

For example, a G1-½" (40mm) lens antenna radar at 80 GHz has approximately the same beam angle as a4" (100 mm) horn antenna at 26 GHz.

However, this is not the complete picture. Antenna gain is dependent on the square of the diameter of the antenna as well as being inversely proportional to the square of the wavelength. Antenna gain is proportional to:

Diemater 2 /wavelength 2

Antenna gain also depends on the aperture efficiency of the antenna. Therefore, the beam angle of a small lens antenna at a high frequency is not necessarily as efficient as the equivalent beam angle of a larger, lower frequency radar. A 4" horn antenna radar at 26 GHz gives excellent beam focusing.
For a full description of antenna gain and beam angle at different frequencies, please read 'cSIMBA's Application Note on Radar Antennas'.

Antenna focusing and false echoes

An 80GHz beam angle is more focused but, in some ways, it has to be. The wavelength of an 80GHz radar is only 3.75mm compared with a wavelength of 11.5mm for a 26GHz radar. The short wavelength of the 80 GHz radar means that it will reflect off many small objects that may be effectively ignored by the 26GHz radar. Without the focusing of the beam, the high frequency radar would have to cope with more false echoes than an equivalent lower frequency radar.

Agitated liquids and solid measuring

High frequency radars are susceptible to signal scatter from agitated surfaces. This is due to the signal wavelength in comparison to the size of the surface disturbance. The high frequency radar will receive considerably less signal than an equivalent 26GHz radar when the liquid surface is agitated. The lower frequency radars are less affected by agitated surfaces. It is important that, whatever the frequency, the radar echo processing software can cope with very small amplitude echo signals. Note: Normally pulse radar has an advantage in this area no matter what the frequency.

Condensation and build up

High frequency radar level transmitters are more susceptible to condensation and product build up on the antenna. There is more signal attenuation at the higher frequencies, such as 80GHz. Also, the same level of coating or condensation on a smaller antenna lens naturally has a greater effect on the performance. ANL-8260AG2 lens antenna with 26GHz frequency is virtually unaffected by condensation, it is more forgiving of product build up.

Steam, dust and foam

Lower frequencies such as 26GHz are not adversely affected by high levels of dust or steam. These frequencies have been very successful in applications ranging from cement, fly ash and blast furnace levels to steam boiler level measurement.

ANL-8260AG2 26GHz pulse radar

In steamy and dusty environments, higher frequency radar will suffer from increased signal attenuation.

ANL-9127 with 80mm lens / 120GHz

Note: Normally, for a radar of higher emitting frequency, using a larger lens antenna has an advantage in this area no matter what the steam.

■ Foam

The effect of foam on radar signals is a grey area. It depends a great deal on the type of foam including the foam density, dielectric constant and conductivity. However, low frequencies such as 26GHz cope with low density foam better than higher frequencies such as 80GHz. For example, an 80GHz radar signal will be totally attenuated by a very thin detergent foam on a water surface. A 26GHz radar signal will see through this type of foam and continue to see the liquid surface as the foam thickness increases to 150 mm or even 250 mm.

ANL-9080 with 50mm lens

Note: For such thick foam measurement applications, an 80 GHz radar with a small lens (50 mm) is not an optimal product choice, which often leads to instability and level jumps. It is recommended to use an 80 GHz radar with a lens antenna at least 80 mm in diameter, which has advantages in this regard.

The thickness of foam will cause a small measurement error because the microwaves slow down slightly as they pass through the foam. When foam is present, it is important to ask us with as much information as possible on the application.

■ Minimum distance

Higher frequency radars have a reduced minimum distance (near blind) when compared with the lower frequencies. When measuring in small containers and still tubes, 80GHz/120 GHz may be a preferred choice.

■ Summary of the effects of radar frequency

1. Better focusing at higher emitting frequency means higher antenna gain (directivity), less false echoes and reduced antenna size.

2. Reduced signal strength caused by signal damping (Signal fluttering) at higher emitting frequency caused by condensation, build-up and steam and dust.

3. Higher damping caused by agitated medium surface (wave movement, material cones with solids, signal scattered).


‘To suggest that any one type of level transmitter technology could be regarded as 'universal' would be unrealistic and potentially irresponsible due to the variation and complexity of available applications when liquids, powders and solids are all considered. However, the rate at which radar based level transmitters have established themselves over the last couple of years would tend to suggest that this technology is closer to that definition that any principle has ever been.’ --- By Mel Henry


With over 40+ years of continuous development, radar level transmitters have become the preferred technology for level measurement in many of today's industrial applications. For non-contacting radars level transmitters, the microwave frequency transmitted by the radar is one area in which there have been recent developments. Normally, four different frequency bands have been used for level measurement: The C-band (~6 GHz), the X-band (~10 GHz), the K-band (~26 GHz) and the W-band(~80GHz). These frequency bands combine many attractive properties for accurate and reliable millimeter-precision measurement. Recently, radars using 120 GHz frequencies (the lower part of the D-band) have been introduced as a further option. The use of 120GHz radars is mainly driven by the development of 3D radar applications, high-precision measurement and penetration measurement of non-polar materials.

Frequency is a fundamental property of any radar as it has direct effects on measurement performance. It is important to remember that different frequencies are not equally suitable for all applications. Indeed, radars using different frequencies are required to solve different problems.

This paper will first describe the fundamental physical properties of different frequencies and thereafter explain what practical effects these properties have in some common, real-life level measurement applications. To this end, this paper differentiates between non-contacting radar transmitters using low microwave frequency (6/10GHz), mid frequency (26GHz), high frequency (80GHz) and terahertz frequency(120GHz).

Frequency and wavelength impact

At the fundamental level, radar level transmitter instruments emit microwaves to measure distance. Microwaves are commonly defined as electromagnetic radiation with wavelengths (λ) between 300mm and 3mm. The wavelength is inversely proportional to the microwave frequency (ƒ), i.e. shorter wavelength = high frequency. In the case of microwaves λ=300 mm corresponds to ƒ=1GHz and λ=3mm corresponds to ƒ=100 GHz. The properties of low, mid and high frequency level radars are summarized in below. These different physical properties have direct impact on the suitability of each frequency for different level measuring applications and conditions.

First and foremost, high frequency microwave signals suffer more attenuation (i.e. they are absorbed to a higher degree) when propagating through a medium, resulting in weaker signal return. For a simplified analogy, think of when you hear loud music played by your neighbor: low frequency sound (i.e. bass) will travel long distances and be heard clearly even through walls. High frequency sounds (i.e. treble) however are quickly absorbed and do not carry over long distances or through objects. When it comes to level measurement this means that high frequency radars are more likely to have problems with condensation, vapor, foam, build-up on the antenna, and dust. Low and mid frequency signals with wavelengths in the range of 50 mm to 10 mm are less affected by these kinds of challenges and more likely to pass through them unaffected.

Another important effect of the frequency is that it impacts the antenna beam width and beam angle, i.e. how focused the microwave propagation is. The beam angle and beam width are determined by the antenna design in combination with the microwave frequency. High frequency signals can achieve small beam angles with small antennas. Equally, small beam angles can be achieved with low frequency radars, but this requires larger antennas. The benefit of a small beam angle in level measurement is that it can make it easier to avoid hitting installations in the tank.

However, a narrow beam width can also be a disadvantage. For example, if there is an obstruction directly below the radar a narrow beam will be completely blocked, but a radar with a wider beam will be only partially blocked and still able to measure the product level.

A practical limitation established from experience is that the beam angle should if possible, not be smaller than about 3°, unless it is used for the measurement of straight pipes and the liquid level is statically flat, i.e. in oil tanks. A narrower beam makes installation sensitive to misalignment of the antenna. Consider the extreme case: if a radar has a beam like a laser it is almost impossible to align it with the plumb line on a real-life tank. Consequently, the reflected beam will miss the antenna and the signal will be lost.

Finally, waves and ripples on a liquid’s surface are common in industrial applications and may cause problems for radar level measurement. Instead of reflecting back upwards towards the antenna, microwaves hitting a turbulent surface may scatter and disperse. Thus, a lot of the signal strength (as much as 90 percent) can be lost and give the radar problems with obtaining an accurate and reliable level measurement. Microwaves remain unaffected by surface irregularities such as turbulence if the wavelength is larger than the ripple size. For example, the signal return of high frequency radars will be affected and scattered by ripples down to 3.8 mm in size, whereas mid-range frequencies will remain unaffected by turbulence up to 2.5 times as large and be reflected as if from a flat surface.

Low, mid, high and terahertz frequencies: strengths and weaknesses

It is clear then that the frequency has a major impact on the type of application a radar is best suited to. The following is a guide to the challenges that exist within some common level measurement applications and the implications of frequency choice.

Application suitability

For dirty and contaminating applications

During operating of a radar level transmitter, dirt and contaminants can build up on the antenna lens over time, which can affect the strength and direction of the radar signal. With radar level gauges operating at low and medium frequencies transceiver, the echo signal is less sensitive to this contamination and can pass through the build-up more or less unaffected. For radar signals operating at high frequencies, more energy is absorbed by the dirt covering the antenna lens, and the direction of the beam may also be shifted. A deposit of uneven thickness covering part of the antenna can redirect the beam by about 1.5°. For radars with narrow beam angles, this can cause serious problems because the return echo is not directed at the antenna, resulting in a loss of signal strength. As a result, low- and medium-frequency technologies are better suited for dirty and polluting applications.

For tanks with condensation and/or vapor applications

Condensation and vapor are sometimes a challenge for radar level measurement. Water reflects microwaves much more strongly than most industrial liquids. Condensation and vapor can therefore cause the reflection from the product surface to be obscured by 'noise' from water droplets. This is more problematic for high frequency signals because their shorter wavelengths also reflect strongly from very small particles like steam and aerosols. Low and mid frequency technology is therefore a better choice for applications with steam and condensation. It should be noted, however, that for condensation the design of the antenna is also of critical importance. Antennas with flat, horizontal surfaces should always be avoided.

Applications with turbulence, waves and ripples

In general, low and mid frequencies perform best in applications with turbulence, waves and ripples. Small ripples on the liquid surface is especially detrimental to high frequency measurements. The short wavelength means that the signal reflection will be scattered also by small surface movements causing loss of returning signal strength. Longer wavelengths are reflected as if from a flat surface and are therefore better suited to this type of application.

Applications with foam

Just like dirt and condensation, a layer of foam on top of the liquid will absorb the radar signal and make accurate measurement more difficult. Foam can have very different properties depending on which product it comes from, but once more, lower frequency generally provides better measurement reliability and accuracy. For dense and thick foam (e.g. from beer, molasses and latex) 6 or 10 GHz works best. For lighter foam, 26 GHz performs very well. High frequency technology should be avoided in applications with foam.

For Bulk liquid storage tanks application

In very large tanks, for example those used for bulk liquid storage at tank terminals, the size and placement of nozzles are typically not a restraint when it comes to choose of radar device. Obstructions and disturbing objects in the tank are usually not an issue either. Due to the vessel size, waves and ripples are often present on the liquid surface. Condensation is also common. As previously explained, this causes problems for high frequency technology.

Many bulk storage tanks are floating roof tanks where measurement is performed through still-pipes. Low frequency radars are preferred as they are less sensitive to build-up on the pipe wall, slots, and pipes that are not completely straight. High frequency radars have difficulties in such situations.

Furthermore, bulk storage tanks often have significant roof movements due to sunlight and shade, wind, and tank bulging. This causes problems for high frequency radars because their narrow beam width makes them very sensitive to tilting, if the axis moves from the vertical plumb line. Tilting can result in the reflected signal 'missing' the antenna opening. It also makes installation of high frequency radars challenging as they must be installed absolutely level in order to perform correctly. Low frequency technology is therefore the most appropriate choice for this kind of application.

For Small to medium size vessels application

This kind of vessel, typically 1~20 m tall, is among the most common in process industry. They are used for a broad range of tasks: from intermediate storage to blending, separation, and as reactor vessels. Process connections are usually 2–4-in. and conditions inside the tank are often difficult with one or several challenges such as condensation, contamination, turbulence, foam, etc. Mid frequency technology is a good choice in this kind of tank due to its versatility – it combines small antennas with good reliability in difficult conditions. Low frequency radars may be less suitable due to the small nozzles and high frequency technology is less able to cope with the tough process conditions.

For Small tanks/buckets

In very small tanks, about 0.5~1.5 m the size and placement of nozzles can be a limitation. The short measuring range and need for small antennas mean that high and mid frequency technology are attractive options for these applications. But of course, previously mentioned challenges such as condensation, contamination, turbulence and foam must be considered where applicable. For Solids For measuring the level of solids, the best frequency to use is very much application dependent. Each technology has its strengths and weaknesses. Low and mid frequencies are able to cope with dust, condensation, and with coarse solids. High frequency works well with very fine powders. Condensation is generally challenging for high frequency radars, but with solids yet another problem arises: condensation in combination with certain solids causes rapid material build up. This will quickly clog small nozzle openings and cover the small antennas of high frequency radars.


Radar level measurement has come a long way since its introduction 40+ years ago and the technology will continue to develop and improve. The recently introduced radars using the high frequency range may have benefits in level measurement applications for very small tanks with small process connections and very short measuring ranges.

However, the fundamental suitability of 6~11 and 24~29 GHz frequencies for reliable mm-accuracy level measurement cannot be overlooked. High frequency technology will usually perform well in less challenging process conditions, but fundamental microwave physics shows us it is less suitable when the going gets tough.

In contrast, low and mid frequency technology was developed specifically to meet the toughest level measurement challenges in the most demanding industrial applications. And the reason these instruments have been such an incredible success is because they provide reliable and accurate measurements in almost all applications.

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