What is the off-axis EIRP density limit for a VSAT terminal and how does it affect antenna selection?
VSAT Off-Axis EIRP Density Compliance
Off-axis EIRP density compliance is a fundamental constraint in VSAT system design. It sets the minimum antenna size for a given transmit power level and determines the maximum uplink data rate achievable by the terminal.
| Parameter | GEO | MEO | LEO |
|---|---|---|---|
| Altitude | 35,786 km | 2,000-35,786 km | 200-2,000 km |
| Latency (one-way) | ~270 ms | 50-150 ms | 1-20 ms |
| Coverage per Sat | Full hemisphere | Regional | Local footprint |
| Handover | None | Periodic | Frequent |
| Path Loss (Ku-band) | ~206 dB | 190-206 dB | 170-190 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Frequently Asked Questions
How does antenna size affect the maximum allowed transmit power?
Larger antennas have lower sidelobe levels (higher sidelobe suppression), allowing higher transmit power while staying within the off-axis limits. At Ka-band (30 GHz) with 2-degree satellite spacing: 0.75 m antenna: maximum approximately 1-2 W to comply. 1.2 m antenna: maximum approximately 4-8 W. 1.8 m antenna: maximum approximately 10-20 W. 2.4 m antenna: maximum approximately 25-50 W. The relationship is roughly: P_max scales as D^4 (the antenna gain G scales as D^2, and the sidelobe level also improves as D^2).
What if my antenna has better sidelobes than the reference pattern?
If your antenna's measured sidelobe levels are below the regulatory reference pattern (which good antennas typically achieve by 5-15 dB), you can use the measured pattern for the compliance analysis instead of the reference. This allows higher transmit power or a smaller antenna. This requires submitting the antenna's measured pattern (from an accredited test range) to the regulatory authority as part of the license application.
How do I handle cross-polarization off-axis limits?
Cross-polarization off-axis limits are typically 10 dB more relaxed than co-polarization limits (the cross-pol interference is less harmful because it is rejected by the adjacent satellite's receive antenna polarizer). FCC cross-pol limit: 19 - 25 log(theta) dBW/4kHz. Good antenna design (clean polarizer, symmetric reflector) achieves > 25 dB cross-pol isolation on boresight and > 15-20 dB off-axis, easily meeting the cross-pol requirements if the co-pol requirements are met.