How Feed Offset Affects LNB Receiving SNR & Commissioning Tips

Feed Displacement: Why SNR Remains Low Even With Antenna Aimed Correctly

Introduction

Many satellite ground station technicians encounter a typical problem: the antenna azimuth and elevation are aligned toward the satellite, the receiver can lock the carrier, but the signal-to-noise ratio (SNR) never reaches the theoretical maximum with low fade margin and easy lock loss during rain. After checking antenna, LNB, cable and LO drift with no defects, the root cause is often feed (LNB) installation displacement. Dish antenna concentrates satellite electromagnetic waves to the focal point. If the LNB feed deviates from the focal point, the converged RF energy cannot fully enter the LNB, directly resulting in gain reduction and degraded SNR. This article introduces classification, principle, fault characteristics and field correction methods for feed displacement.

1. Three Types of Feed Displacement

Feed displacement includes axial offset, transverse offset and tilt offset. Multiple deviations often coexist in field installation:

1.1 Axial Offset (Focal Length Error)

The LNB moves forward or backward along the antenna central axis, either too far or too close to the focal point. The reflected waves converge optimally at the focal plane. If the LNB feed horn is placed before or behind the focal plane, the beam diverges and energy cannot be fully coupled into LNB.

1.2 Transverse Offset (Lateral Decenter)

The LNB feed horn deviates left/right or up/down away from the central focal point on the focal plane. The energy center of reflected waves falls outside the feed horn, so most RF energy fails to enter LNB. This is the most common offset type on site.

1.3 Tilt Offset

The LNB axis is not parallel to the antenna main axis, the feed horn is skewed. There is an angle between converged beam direction and LNB receiving aperture, raising coupling loss and introducing extra polarization loss.

2. How Feed Offset Degrades SNR

The core function of dish antenna is to collect weak satellite signals and focus energy to the feed horn at focal point. SNR = Received Signal Power / System Noise Power.

Feed displacement does not change the inherent noise figure of LNB, but greatly reduces received signal power. Concentrated RF energy leaks out of feed horn and coupling loss increases. Noise floor remains nearly unchanged while signal power drops, resulting in degraded SNR.

Larger displacement brings higher coupling loss and lower SNR. Once offset exceeds threshold, carrier will lose lock directly. Tilt offset also breaks polarization matching, reduces H/V polarization isolation and further degrades reception performance.

3. Typical Fault Phenomena

1. Antenna azimuth & elevation locked successfully, but SNR is obviously lower than theoretical expectation;
2. Fine-tuning antenna azimuth/elevation leads to gentle SNR variation without sharp peak;
3. Replacing with brand-new LNB yields no obvious SNR improvement;
4. Large SNR difference exists between horizontal and vertical polarization during polarization switching;
5. Margin barely sufficient under clear sky, easy to lose lock in rain.

4. Common Engineering Misjudgments

Misjudgment 1: Low SNR means defective LNB
Analysis: If SNR does not improve after LNB replacement, LNB fault can be ruled out; inspect feed position first.

Misjudgment 2: Antenna alignment equals feed matching
Analysis: Azimuth & elevation alignment only directs satellite signal onto reflector. Decentered feed or wrong focal length still prevents maximum gain.

Misjudgment 3: Feed calibration is only required for large dishes
Analysis: Small antennas have wider beamwidth and are more sensitive to focal length and decenter. Loss induced by feed offset accounts for higher proportion.

5. Field Commissioning & Correction Steps

Prerequisite: Finish coarse alignment of antenna azimuth, elevation and polarization angle, monitor SNR reading in real time on receiver.

① Transverse decenter correction: Adjust feed bracket slowly left/right, up/down to find SNR peak then lock;
② Axial focal length correction: Slide LNB forward/backward along central axis and record SNR peak position;
③ Tilt correction: Adjust LNB angle to eliminate skew and balance SNR between H and V polarization;
④ Recheck: Scan in small range repeatedly to confirm global SNR maximum and fasten all bracket screws;
⑤ Waterproof reinforcement: Seal connectors after locking to prevent position shift caused by vibration.

6. Comparison Table of Offset Impacts

7. Summary

Feed displacement is a hidden fault at antenna installation level. Azimuth and elevation alignment does not guarantee proper feed matching. Axial, transverse and tilt offset introduce RF coupling loss, reduce received signal power and degrade SNR without damaging LNB hardware. If SNR falls below expectation after antenna pointing, inspect feed position first. 3-dimensional fine adjustment to locate SNR peak and lock bracket can restore theoretical antenna gain and improve rain fade margin. Small antennas are more sensitive to feed position, feed optimization and calibration are mandatory during field installation.

Quick Judgment Formula