80 GHz radar level measurement

The 80 GHz technology used in the OPTIWAVE series is the most recent and versatile radar technology for level measurement of liquids and solids. Over an identical distance, it presents a highly focused beam with a smaller diameter compared to lower frequency radars, ideal for dusty atmospheres or low reflective media. The small dead zone and narrow beam angle allow for use in both small and tall vessels.

How can 80 GHz technology be that universal?

To answer this question, we need to understand that the 80 GHz technology is based on the FMCW technology, which is currently the favoured technology that all major industrial process instrumentation manufacturers rely on. FMCW is short for Frequency Modulated Continuous Wave. FMCW radar continuously emits radar waves whose frequency is modulated over a bandwidth and receives their reflections. It measures the difference in frequency between the transmitted and received wave, which is proportional to the distance to the surface at which it was reflected.

Thus, the level measurement by radar is primarily a non-contact distance measurement from the measuring device (mounted on top of the vessel) to the surface of a medium to be measured. By entering the vessel geometry and medium properties such as density, the device can calculate level, volume or mass. In contrast to ultrasonic level measurement, radar is independent of pressure and temperature; additionally, viscosity and density do not affect the measurement.

Despite this insensitivity, there are some factors that have an influence on FMCW measurement. The 80 GHz technology is currently the most advanced technology to overcome these influences.

Signal dynamics and bandwidth

As each emitted frequency is reflected and received by the radar, a large spectrum results from this. However, not only do the waves reflect from the medium, they also reflect from all surfaces which can be found in a vessel, as for example, tank internals. The exact differentiation of all reflected signals detected by the radar is only possible via a high signal dynamic, also known as high measuring sensitivity: the more signals reflected by a target and received by the device, the clearer or higher this point rises in the spectrum over other noises and can therefore be identified.

As the bandwidth of the radar widens, the resolution of the spectrum increases and the individual targets are indicated by narrower, more accurate peaks: the bandwidth over which the frequency is modulated determines the number of different signals reflected from a target. A 24 GHz radar typically modulates between 24 and 26 GHz and thus has a bandwidth of 2 GHz, while an 80 GHz typically modulates in the range between 78 and 82 GHz and thus has a bandwidth of 4 GHz. With 4 GHz, for example, it is possible to differentiate between targets that are only 10 cm/4" apart. With 2 GHz, these cannot be distinguished under the same conditions.

Focusing and antenna size

For a long time, the bandwidth was limited by the performance of the microchips. Today it is limited by the antennas and their designs that have to transmit the frequency spectrum. Radar waves do not propagate point-focused like a laser signal, but rather in the form of a lobe or angular beam.

In order to influence the opening angle or the focusing of the angular beam, there are two possibilities. First is the frequency used: the higher the frequency, the smaller the aperture angle due to the shorter wave length. The angular beam width of an 80 GHz with 4 GHz bandwidth is at 10 m/ 33 ft distance only 30% as wide as that of the 24 GHz with 2 GHz (0.5 m to 1.75 m/ 1.6 to 5.7 ft). The second possibility is the antenna diameter: the larger the diameter, the more focused is the angular beam.

For the process industry, this can be easily transferred to the possible areas of application: in high narrow silos, the radar beam should not come into contact with the silo wall or tank internals, since both should not be measured. Therefore, the radar angular beam must both be focused and kept as narrow as possible, as provided by an 80 GHz radar with a large antenna.

Reflectivity and frequency

In addition to the angle, the properties of the product surface also determine how many radar signals are reflected and how they are received: the higher the reflectivity or dielectric constant, the higher the amplitude of the reflected signals.

In contrast to liquids that reflect the signals very well, bulk solids generally reflect the signals very poorly: an Er value of approx. 1.4 is stated as the lowest value that can still be measured reliably and safely. While the reflection coefficient of a flat liquid surface does not change over the frequency, the backscattering on fine-grained bulk goods such as granules or powders increases significantly with a higher frequency.

The 80 GHz radar is therefore the first choice here, because of the high dynamics it is able to clearly display the level line, even in the case of a heavy dust development (e.g. during the filling process of a silo or stock pile). The better resolution of its 4 GHz bandwidth also helps to distinguish the signals from interference and medium, even if they are close together.

Summary

80 GHz is the frequency with the highest focus and thus suited for all containers sizes in order to avoid interference reflections. In addition, the short wavelength is reflected very well, this is especially advantageous for bulk solids, even with granules and powders with very small particle sizes and / or high dust levels. Another advantage is the high signal focussing by design that needs no additional focus from a large antenna: the flush-mounted plastic (PEEK) lens antenna type available with the OPTIWAVE series is sufficient and very popular in these applications. Due to the small size, it can be used with threaded connections and sometimes makes the flange obsolete, which may save a lot of money. Also, 80 GHz has an enormous measuring range with a small dead zone, thus the vessel can be filled almost up to the antenna.