Select the right frequency for radar level transmitter

Radar level transmitter technology has continued to improve over the past 40 years, and it's now one of the most widely used technologies for industrial level measurement. Radar level transmitters generally fall into two types: guided wave radar (GWR), which uses a contact probe or cable, and non-contacting radar, which sends microwaves through an antenna without touching the medium.

In recent years, non-contacting radar level transmitters have also seen new developments in how they choose their microwave frequency. These devices typically come in three ranges: low frequency (6 to 11 GHz), mid frequency (24 to 29 GHz), and high frequency (80 GHz), and radar level gauges across these ranges are already being used to deliver reliable, accurate measurements in the field.

The wider the range of devices available, the better for end users. That said, the most important thing when choosing one is matching the frequency band to the actual application. Every application has a frequency band that suits it best, and not every frequency band works for every application. Because of this, users should carefully weigh the strengths and weaknesses of each frequency, along with the site conditions that can affect performance, before making a choice.

How Frequency Bands Affect the Accuracy of Measurement

A radar instrument measures the distance between itself and a surface using the microwaves it emits. It does this by calculating the time it takes the pulse to reach the measured surface and reflect back to the instrument.

Frequency determines wavelength: the higher the frequency, the shorter the wavelength.

Signal Absorption and Attenuation

Frequency is the most basic property of any radar level sensor, and it affects measurement performance in a number of ways. For example, high frequency microwaves passing through a medium get absorbed more, which weakens the signal that returns.

It's a bit like hearing music through a wall. The low frequency bass gets through because it can penetrate the wall, while the high frequency treble mostly can't.

In the same way, foam, dust, vapor, and condensation tend to work against level measurement because they interfere with high frequency radar signals in particular, weakening the return signal and affecting overall accuracy.

Signal strength always drops as it travels through a medium. So high frequency signals typically experience more attenuation than mid or low frequency ones.

Beam Angle and Antenna Size

Frequency also has a major effect on a radar antenna's beam angle and width. A high frequency signal can achieve a narrow beam angle with a small antenna, and narrow beam angles are useful because they help avoid obstructions inside tanks and vessels.

That said, low frequency radar can also achieve narrow beam angles, just with a larger antenna. So users need to factor in how much space is actually available in the vessel for mounting.

Narrow beams have their own downside too: any obstruction in the tank can block a narrow beam completely, whereas a wider beam might only be partially blocked and still give a reliable reading.

Turbulence and Signal Scatter

Turbulence in the process liquid can also affect radar measurement. Waves and ripples on the surface cause the microwaves to scatter instead of reflecting cleanly back to the antenna, and this can cause signal strength losses of up to 90%, which hurts both reliability and accuracy. Short wavelengths in high frequency transmissions are especially sensitive to this.

Newer high frequency radar units, often built around 80 GHz, use a much shorter wavelength and a very narrow beam angle. That narrow beam works well on small vessels and fine powders, where a wide beam has nowhere to avoid internal obstructions. But it carries the same drawbacks as other high frequency radar: foam, vapor, and turbulence still weaken the signal, so 80 GHz isn't a fix for the dirty or foamy tanks described earlier.

Turbulence and Signal Scatter

Radar's level measurement can be affected by turbulence within the processed liquid. This is because on the surface of the liquid, microwaves hit waves and ripples. Unfortunately, this microwave would disperse and scatter rather than reflect back onto the antenna because of the turbulence. This can make loss to signal strength of about 90% to occur and thus affect the reliability and accuracy of measurements. Therefore, this will also affect short wavelengths in high frequency transmissions.

Newer high frequency radar units, often built around 80 GHz, use a much shorter wavelength and a very narrow beam angle. That narrow beam works well on small vessels and fine powders, where a wide beam has nowhere to avoid internal obstructions. It carries the same drawbacks as other high frequency radar: foam, vapor, and turbulence still weaken the signal, so 80 GHz is not a fix for the dirty or foamy tanks described earlier.

Choosing the Right Frequency for Different Applications

There are many different radar level measurement applications across the process industries, and each one comes with its own set of challenges. That's why it's important for users to figure out which frequency band actually suits their situation. Here are a few common scenarios worth considering.

Antennas with dirt and contamination

Dirt and contamination that build up on an antenna over time can affect the direction and strength of the radar signal. Mid and low frequency signals are largely unaffected by this, since they have low sensitivity to contamination. High frequency signals are a different story: any dirt covering the antenna absorbs a good chunk of the signal's energy and can also shift the beam's direction. Even a small, unevenly distributed buildup can deflect the beam by around 1.5 degrees. This is especially problematic for narrow beam radar, since the antenna won't receive the return echo properly and signal strength drops significantly. Cement silos and ore processing plants, where dust levels are especially high, are good examples of where mid and low frequency radar tends to hold up better.

Tanks with vapor or condensation

Noise from water droplets can make it harder to get a clear reflection off the product's surface when vapor or condensation is present. This mainly affects high frequency signals, so mid and low frequency technologies are the better choice in this situation. Antenna design still matters here too. Antennas with flat, horizontal surfaces aren't a good fit for condensation-prone environments. This kind of setup shows up often in steam boiler feedwater tanks or chemical reactors with heated steam jackets.

Applications with turbulence, ripples, and waves

Liquid in a large tank often has ripples and waves on its surface, and this kind of turbulence is a real obstacle for high frequency measurement. High frequency devices have short wavelengths, so even small surface movement can scatter the signal instead of reflecting it back cleanly, weakening the return signal. Mid and low frequency instruments handle this better, since they emit longer wavelengths. Large crude oil storage tanks and aeration basins at wastewater treatment plants, where the surface is constantly agitated, are typical examples where mid and low frequency radar is the more reliable choice.

Foamy applications

Accurate measurement gets harder when the surface is covered in foam, for similar reasons to condensation and dirt: foam absorbs the radar signal. Low frequency instruments tend to work well here and give more reliable readings. Since foam properties vary a lot, low frequency is better suited to thick, dense foam like latex, molasses, or beer, while mid frequency can handle lighter foam. High frequency instruments, on the other hand, aren't a good fit for foamy applications at all.

Bulk liquid storage tanks

Level measurement in bulk storage, like floating-roof tanks, is sometimes done through still pipes. Low frequency radar is usually the preferred choice here, since it's less sensitive to buildup on the pipe wall and to pipes that aren't perfectly straight. High frequency radar tends to struggle in this setup. Crude oil and refined product storage tanks at petrochemical plants are a classic example of low frequency radar paired with still pipes.

On top of that, wind, tank bulging, shade, and sunlight can all cause some roof movement in bulk storage tanks. High frequency radar is more sensitive to this kind of tilting because of its narrow beam angle. If the antenna's axis shifts even slightly out of vertical alignment, it can miss the reflected signal entirely.

Solids level measurement

The right frequency for solids measurement really depends on the application. Mid and low frequency radar can handle coarse solids, dust, and condensation, while high frequency radar works well with fine powders. Grain silos and coal bunkers, with their coarse particles and heavy dust, are a good fit for mid and low frequency, while fine powders like flour or milk powder tend to measure more accurately with high frequency radar. Condensation is generally an issue for high frequency radar, and when it combines with certain solids, it can speed up buildup that clogs small nozzle openings and blocks high frequency antennas, which tend to be small to begin with.