Home News Radar Level Transmitter: High vs Low Frequency – Which Has Better Penetration?

Radar Level Transmitter: High vs Low Frequency – Which Has Better Penetration?

Wrong frequency choice means signal loss. Understanding the principles is the first step to selecting the right model.

1. How Radar Level Transmitters Measure Level

Radar level transmitters operate on the Time of Flight (TOF) principle: microwaves are emitted from the antenna, reflect off the liquid surface, and return to the transmitter. By measuring the round-trip time and multiplying by the speed of light divided by two, the distance to the liquid surface is calculated.

Distance D = (c × t) / 2

c = Speed of light (~3×10⁸ m/s)
t = Round-trip time (picosecond level)

Frequency directly determines wavelength, beam angle, and energy concentration – which in turn affects measurement accuracy, beam width, and application suitability. This is the first critical decision in transmitter selection. Leading suppliers like Endress+Hauser (E+H), a pioneer in radar level measurement technology, offer comprehensive product lines spanning 6 GHz, 26 GHz, and 80 GHz frequencies to address diverse industrial applications.

🔊 Low vs High Frequency: Wavelength Comparison

6 GHz wavelength ≈ 50mm (fist-sized)
26 GHz wavelength ≈ 11mm (fingernail-sized)
80 GHz wavelength ≈ 3.7mm (needle tip)

↑ Higher frequency = shorter wavelength = narrower beam = higher resolution

2. Low vs High Frequency: Complete Comparison

🟠 Low Frequency Radar

Representative: 6 GHz / 10 GHz

Wavelength: 30-50 mm
Large antenna size
Wide beam angle (15°-30°)
Accuracy: ±5-10 mm
Range: up to 20-30 m
✅ Strong foam penetration
✅ Good corrosive gas resistance
❌ Poor for small tanks
Lower price

🔵 High Frequency Radar

Representative: 26 GHz / 80 GHz

Wavelength: 3.7-11 mm
Small antenna (planar possible)
Narrow beam angle (3°-8°)
Accuracy: ±1-2 mm
Range: 30-100 m (80G)
❌ Weak foam penetration
⚠️ Sealing considerations
✅ Excellent for small tanks
Higher price (80G especially)

⚠️ Important Note: The term “penetration power” requires careful clarification in radar level measurement – low frequency penetrates foam and steam better due to longer wavelengths, while high frequency offers concentrated energy and higher resolution but is more sensitive to foam and vapor attenuation. Neither penetrates the liquid surface itself – radar measures surface reflection, not penetration!

3. Why Low Frequency Penetrates Foam Better

Foam consists of numerous tiny bubbles forming an inhomogeneous medium. When microwaves pass through foam, scattering and absorption cause signal attenuation.

Scattering and Wavelength Relationship

According to Rayleigh scattering theory: when obstacle size is much smaller than wavelength, scattering is minimal; when obstacle size approaches or exceeds wavelength, scattering increases dramatically.

Practical Understanding:

Foam bubble diameter typically ranges 0.1-5 mm

Low frequency (6 GHz) wavelength ≈ 50 mm → bubbles much smaller than wavelength → weak scattering, good penetration
High frequency (80 GHz) wavelength ≈ 3.7 mm → bubbles comparable to wavelength → strong scattering, significant signal loss

Water Vapor and Condensation Effects

Water molecules strongly absorb microwaves (microwave ovens use this principle). Higher frequencies experience more absorption by water vapor molecules. Therefore, in high-temperature tanks and reactors with concentrated steam, low frequency radar experiences less signal attenuation and better steam penetration.

Interference Medium	Low Freq (6 GHz)	High Freq (26 GHz)	High Freq (80 GHz)
Foam layer (< 100mm)	✅ Penetrable	⚠️ Slight attenuation	❌ Severe attenuation
Foam layer (> 200mm)	✅ Still measurable	❌ High signal loss	❌ Basically fails
Water vapor (light)	✅ Minimal effect	✅ Minimal effect	⚠️ Some attenuation
Water vapor (high temp/conc.)	✅ Good penetration	⚠️ Affected	❌ Significant effect
Corrosive gases	✅ Minimal effect	⚠️ High sealing req.	⚠️ High sealing req.
Agitation/turbulent surface	⚠️ Moderate	✅ Faster response	✅ Fastest response

4. Beam Angle – Will It Fit Your Tank?

Beam angle refers to the divergence angle of microwaves emitted by the antenna. Higher frequencies enable larger antenna apertures relative to wavelength, producing more concentrated (narrower) beams. E+H’s Micropilot FMR6x series, for example, achieves beam angles as narrow as 3° with its 80 GHz planar antenna design, making it ideal for installations in small tanks with internal obstructions.

📡 Typical Beam Angle Data:

6 GHz + DN100 antenna → beam angle ~24°
26 GHz + DN80 antenna → beam angle ~8°
80 GHz planar antenna → beam angle ~3°-4°

Impact of Beam Angle on Installation

If the tank diameter is small but the beam angle is wide, microwaves will hit the tank wall or internal components (agitator blades, heating coils), generating false reflection signals that cause erratic readings or fixed interference peaks.

Tank Diameter	Recommended Frequency	Reasoning
DN < 500mm (small tanks, pipes)	80 GHz	Extremely narrow beam, won’t hit walls
DN 500mm-2000mm	26 GHz or 80 GHz	Depends on internal components
DN > 2000mm large storage tanks	6 GHz or 26 GHz	Large space, low frequency has longer range
Tanks with agitators	80 GHz	Narrow beam avoids agitator interference

💡 Installation Blind Zone Note: Radar level transmitters have a near-end blind zone (typically 50-200 mm from antenna), with high frequency types having smaller blind zones. When liquid level enters this zone, the transmitter will report an error. Always leave adequate margin based on actual maximum liquid level.

5. Why 80 GHz Became Mainstream After 2015

Leading instrumentation manufacturers like Endress+Hauser (E+H), Emerson, and Siemens have driven the industry shift toward 80 GHz technology. E+H’s Micropilot series, in particular, has demonstrated the practical advantages of high-frequency radar in diverse industrial applications.

Around 2015, 80 GHz FMCW (Frequency Modulated Continuous Wave) radar level transmitters entered the market in large numbers and quickly became the industry standard. Several key factors drove this shift:

Measurement accuracy leap: 80 GHz + wideband FMCW technology achieves ±1 mm accuracy, an order of magnitude better than older 6 GHz models at ±5 mm
Miniaturized antennas: 80 GHz wavelength of only 3.7 mm enables planar array antennas in compact packages, ideal for small vessels and pipes
Ultra-narrow beam angle: ~3°-4° means minimal interference from tank internals, greatly simplifying false echo suppression settings
Extended range: Some models reach 100 m, covering virtually all industrial storage applications
Rapid price decline: Mass production has brought 80 GHz prices close to 26 GHz levels, creating compelling value

80 GHz Limitations:

Heavy foam conditions (fermentation tanks, black liquor) – foam absorbs high frequency signals; low frequency or guided wave radar recommended
High temperature/high pressure steam requires more stringent sealing design
Strong corrosive gas applications need careful evaluation of antenna seal lifetime

6. Frequency Selection Recommendations by Application

✅ Prioritize 80 GHz For:

Small vessels (DN < 1000mm)
Reactors with agitators or heating coils
Side-mounted pipe applications
Solid silo level measurement
High-precision custody transfer tanks
Very small installation ports (below DN50)

🟠 Prioritize 6 GHz Low Frequency For:

Continuous heavy foam on medium surface
High temperature tanks (concentrated steam)
Large vertical storage tanks (range > 30 m)
Strong corrosive vapors (chlorine, ammonia)
Cost-sensitive projects with lower accuracy requirements

🔵 26 GHz Positioning:

Transitional choice between low and high frequency. Suitable for medium tanks (DN 1000-3000mm), light foam conditions. Mature technology with established spare parts availability.

❌ Radar Level Transmitters NOT Suitable For:

Media with extremely low dielectric constant (liquefied gas ε < 1.5) – minimal reflection; use guided wave radar
Metal mesh tank structures (signal leakage)
Severe turbulence/spray with no clear liquid surface

✅ Vacuum Tank Clarification: Microwaves (electromagnetic waves) propagate without issue in vacuum (speed equals light speed). Radar level transmitters work perfectly in vacuum tanks, often more accurately than in gas environments (eliminating gas refractive index correction errors).

7. Three Common Misconceptions

❓ Misconception 1: “Higher frequency is always better – choose the most expensive 80 GHz for high accuracy”

❌ Wrong. Fermentation tanks, paper mill black liquor tanks, detergent production tanks with heavy foam will cause 80 GHz signals to be absorbed by foam, frequently reporting “signal lost.” In these cases, low frequency or guided wave radar is the correct choice, not the most expensive option.

❓ Misconception 2: “Low frequency has strong penetration, so it works for every application”

❌ Wrong. Low frequency wide beam angles cause false echoes when installed in small diameter tanks; low frequency accuracy (±5-10mm) creates measurement errors in high-precision applications; low frequency large antennas may not fit some installation ports.

❓ Misconception 3: “Radar signals penetrate the liquid surface to measure tank bottom distance”

❌ Fundamentally wrong. Radar level transmitters measure surface reflection, not penetration. Microwaves hit the liquid surface and reflect back – that’s the measurement. If you need to measure below the liquid surface, that’s the job of ultrasonic or displacer level transmitters.

Key Takeaways

Radar level transmitters measure liquid surface reflection, not penetration
Low frequency (6 GHz): Long wavelength, strong foam/steam penetration, suitable for large tanks and heavy foam, lower accuracy
High frequency (80 GHz): Extremely narrow beam, high accuracy (±1mm), ideal for small vessels and complex internals, foam-sensitive
26 GHz: Transitional type, medium accuracy, widely applied
80 GHz has become mainstream but isn’t universal – for heavy foam, choose low frequency or guided wave radar
Narrower beam angle = less interference from tank walls and internals, prioritize high frequency for small tanks

For technical consultation and radar level transmitter selection assistance

📧 sales@yunrui-controls.com | 📱 WhatsApp: +86 187 1078 4030