What is the G/T figure of merit for a receiving system and how do I calculate it?
G/T Engineering and Optimization
G/T is the most important single specification for a satellite or radar receiving terminal because it encapsulates both the antenna performance and the noise environment in one number that directly enters the link equation.
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
Margin Allocation
G/T is measured by observing a celestial radio source with known flux density: (1) Hot source method (radio stars): point the antenna at a strong radio source (e.g., Cassiopeia A at C-band) and measure the increase in received noise power (delta_Y in dB) when the source enters the beam. G/T = delta_Y / S(f) × 8×pi×k / lambda^2, where S(f) is the flux density of the source in Janskys (1 Jy = 10^-26 W/m^2/Hz). Cas A at 4 GHz: S ≈ 1100 Jy (decreasing 0.7% per year). (2) Sun method: the sun is a strong thermal radio source (T_sun ≈ 10,000-100,000K depending on frequency and solar activity). Measure the Y-factor when pointing at the sun vs cold sky. Calculate T_sys from Y and T_sun. This method is simpler but less precise due to solar variability. (3) Satellite beacon method: measure the received beacon power from a satellite with known EIRP. Calculate G/T from the link equation: G/T = C/N_measured + 10×log10(kBW) + FSPL + A_atm - EIRP_beacon. This method is practical for operational terminals but requires accurate knowledge of the satellite beacon EIRP.
- 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
Propagation Modeling
To maximize G/T: (1) Maximize antenna gain: larger aperture, higher surface accuracy, lower sidelobe level. For a parabolic dish: G = eta × (pi × D / lambda)^2. eta = aperture efficiency (typically 0.55-0.75). Surface RMS error must be < lambda/20 for less than 1 dB gain loss. (2) Minimize system noise temperature: primary factors: feed loss (place LNA at the feed, not at the bottom of the antenna tower). LNA noise figure (use the lowest-NF LNA available: 0.3-0.5 dB for uncooled GaAs pHEMT at Ku-band, 0.1-0.2 dB for cryogenically cooled). Antenna noise temperature (minimize sidelobe level to reduce ground noise pickup, design feed for minimal spillover). (3) The interaction between gain and noise: increasing antenna gain by enlarging the dish also reduces the fraction of the beam that sees the warm ground (narrower main beam = less ground pickup through sidelobes), reducing T_ant. Both effects improve G/T. However: larger antennas have narrower beams, requiring more precise pointing (pointing loss partially offsets the gain improvement if tracking is inadequate).
Frequently Asked Questions
How does G/T change with elevation angle?
G/T decreases at lower elevation angles for two reasons: (1) Antenna gain (G) typically does not change with elevation for a tracking antenna (the beam stays pointed at the satellite). However, for electronically steered phased arrays: scan loss reduces gain by cos(theta) to cos^1.5(theta) at low elevations. For a flat-panel phased array at 20° elevation: scan loss ≈ 2-3 dB. (2) System noise temperature (T_sys) increases at low elevation because: (a) more of the antenna pattern intersects the warm ground (increasing T_ant), (b) the atmospheric path length increases (increasing atmospheric noise contribution), and (c) the antenna looks through more atmosphere (increasing tropospheric emission). Typical T_sys variation: 100K at 90° elevation rising to 150-200K at 10° elevation at Ku-band. Combined G/T degradation at 10° elevation: 3-6 dB compared to zenith.
Can I improve G/T by cooling the LNA?
Yes, but the benefit depends on the relative magnitudes of T_LNA and T_ant: If T_ant >> T_LNA: cooling the LNA provides minimal improvement (the antenna noise dominates). For a mobile phone (T_ant ≈ 200K): cooling the LNA from 50K to 5K reduces T_sys from 250K to 205K (0.9 dB improvement, not worth the complexity). If T_ant << T_LNA: cooling is highly effective. For a satellite ground station pointing at cold sky (T_ant ≈ 10K): cooling the LNA from 50K to 5K reduces T_sys from 60K to 15K (6 dB improvement, very significant). Cryogenic LNA cooling (to 15-20K physical temperature) is standard for: radio astronomy (every dB of sensitivity matters), deep-space communications (DSN), and some military applications.
How does rain affect G/T?
Rain degrades G/T in two ways: (1) Rain attenuation reduces the received signal by A_rain dB. This directly reduces C/N but does not change G/T (G/T is a receiver property, not a propagation property). However: (2) Rain along the antenna beam path emits thermal noise (the rain acts as a warm attenuator at approximately 260-280K). This increases the antenna noise temperature: T_ant_rain = T_ant_clear + T_rain × (1 - 10^(-A_rain/10)). For 5 dB rain attenuation at 260K rain temperature: T_rain_contribution = 260 × (1 - 10^(-0.5)) = 260 × 0.684 = 178K. If T_ant_clear = 40K: T_ant_rain = 40 + 178 = 218K. T_sys increases from 120K to 298K (4 dB degradation in 10×log(T_sys)). So rain causes both signal attenuation (5 dB) AND noise increase (4 dB equivalent), for a total C/N degradation of approximately 9 dB, not just 5 dB.