Polarization Switching: Working Principle & Commissioning Tips for Horizontal/Vertical and Circular Polarization LNBs

Introduction

Satellite communications adopt polarization multiplexing to improve spectral efficiency, enabling two independent signals to be transmitted over the same frequency. Linear polarization covers Horizontal (H) and Vertical (V), while circular polarization includes Left‑Hand Circular Polarization (LHCP) and Right‑Hand Circular Polarization (RHCP). Numerous field‑site reception anomalies stem from incorrect polarization identification, failed polarization switching and misaligned LNB polarization angle. This article analyzes the underlying operating mechanisms of linear and circular‑polarization LNBs, and summarizes polarization‑switching logic as well as practical guidelines for on‑site commissioning and troubleshooting.

Horizontal Polarization (H): The electric‑field vector of the electromagnetic wave runs parallel to the ground plane. Vertical Polarization (V): The electric‑field vector runs perpendicular to the ground plane. The satellite receiver outputs 13 V / 18 V DC to control the internal polarization‑switching unit of the LNB for H/V signal selection. This scheme is widely deployed in C‑band and Ku‑band broadcast and communication satellite systems.

For circular‑polarized electromagnetic waves, the electric‑field vector rotates continuously over time, categorized as LHCP and RHCP. A circular‑polarization LNB integrates a 90° phase shifter to phase‑process two orthogonal linear‑polarized signals and synthesize circular‑polarized RF signals. Circular polarization shows low sensitivity to antenna mechanical tilt and ground electromagnetic disturbance, and is commonly used for meteorological satellites and selected Asia‑Pacific communication satellites. Circular‑polarization LNBs do not support polarization switching via 13 V / 18 V DC control.

In linear‑polarization receiving systems, the receiver supplies 13 V / 18 V DC control voltage to actuate the internal polarization probe or relay inside the LNB, selecting either the horizontal or vertical polarization path for subsequent processing. This DC level serves as a control signal rather than an RF signal.

Note: Certain multi‑output LNBs assign fixed polarization per port, independent of 13 V / 18 V control.

1. The dish feedhorn collects satellite RF signals. Two orthogonal probes inside the feedhorn capture horizontal‑ and vertical‑polarized electromagnetic waves respectively.

2. The satellite receiver delivers 13 V / 18 V DC control voltage to the LNB over the coaxial cable.

3. The internal LNB switch activates the corresponding polarization path according to the input DC level, feeding the selected signal to the low‑noise amplifier.

4. The RF signal is mixed and down‑converted into an intermediate‑frequency (IF) signal, which is transmitted back to the receiver via coaxial cable.

Typical symptom: No signal from partial transponders during channel switching. This failure is normally caused by a defective internal polarization relay or physical misalignment of the feedhorn probe.

1. The feedhorn simultaneously captures two orthogonal linear‑polarized RF signals.

2. The built‑in LNB phase shifter introduces a 90° phase offset to one signal path.

3. Two orthogonal signals with a 90° phase difference are combined to generate either LHCP or RHCP circular‑polarized waves.

4. Circular‑polarization LNBs are factory‑locked to either LHCP or RHCP. Rotation direction cannot be modified by receiver voltage; full LNB hardware replacement is mandatory to swap rotation sense.

Important Reminder: Circular‑polarization LNBs cannot receive linear‑polarized satellite signals, and linear‑polarization LNBs cannot demodulate circular‑polarized signals. These hardware types are not interchangeable.

• Polarization‑angle calibration: Rotate the LNB around the feedhorn axis; do not simply mount it in purely horizontal or vertical orientation. The polarization angle is calculated from satellite orbital longitude and site latitude / longitude. Minor angular misalignment brings significant signal attenuation.
• Voltage verification: Measure LNB supply voltage on the coaxial cable with a multimeter. When toggling between H/V channels, verify normal switching between 13 V and 18 V DC. Static voltage indicates a fault in the receiver or coaxial link.
• Port‑wiring check: For Twin / Quad multi‑output LNBs, some ports are hard‑assigned to fixed polarization. Strictly follow datasheet port definitions during installation.
• Fault diagnosis: Complete signal loss on one polarization while the other works normally usually points to a failed internal LNB polarization relay.

• Confirm satellite rotation specification: Verify whether the target satellite adopts LHCP or RHCP. An LNB with matching rotation sense must be selected; hardware for the two rotation directions is non‑interchangeable.
• No polarization‑angle rotation required: Circular‑polarization LNBs require no rotational polarization‑angle offset. Only antenna azimuth and elevation alignment are needed.
• Prohibit mixed‑hardware assembly: Do not fit a linear‑polarization feedhorn with a circular‑polarization LNB. Severe signal loss and demodulation failure will occur.
• Typical troubleshooting: High signal‑level reading but near‑zero SNR. First cross‑check whether LNB rotation sense matches satellite specification.

• Signal present but partial transponders missing: Prioritize polarization‑angle inspection and receiver polarization‑switching voltage check.
• Weak signal across all frequencies: Verify antenna alignment accuracy, feedhorn mounting condition and coaxial‑cable loss.
• Receiver output voltage unchanged during polarization toggle: Inspect receiver configuration, coaxial‑cable short‑circuit and F‑connector contact integrity.

Linear‑polarization systems accomplish H/V switching via 13 V / 18 V DC output from the receiver, and require precise LNB polarization‑angle calibration. Circular‑polarization relies on internal phase‑shifter signal synthesis; rotation sense is factory‑fixed and cannot be altered by voltage control. Most field‑site reception failures arise from mismatched polarization parameters, angular misalignment or defective polarization‑switch components. Before commissioning, always confirm the target satellite’s polarization specification, distinguish between linear‑ and circular‑polarization hardware, and complete system alignment with voltage measurement and angular calibration.

Notes

1. To change rotation sense of circular‑polarization LNB, physical hardware replacement is required; it cannot be achieved via receiver configuration.

2. Some multi‑output linear‑polarization LNBs feature fixed‑polarization ports independent of 13V/18V control. Always refer to product datasheet.