An Absorbing Layer: Loss and Emission
- Question: what reaches the antenna?
- Incoming signal + background noise () → absorbing layer (0.1 dB loss, 270 K physical, thermal emission) → antenna input: signal lower, noise = ?
- The layer attenuates incoming radiation and generates its own thermal noise
- Isothermal layer model (NRAO, Essential Radio Astronomy, Ch. 1; DSN 810-005)
Noise Seen Through an Absorbing Layer
- With layer loss in dB:
- First term: transmitted background; second term: layer emission
Atmosphere at 270 K
- Signal ledger:
- Transmitted background:
- Layer emission:
- Total noise seen by the antenna:
- Same physics as a room-temperature feed: emitted at the output (feed loss at two planes)
- Clarified Oct. 19: an absorbing atmosphere weakens the signal and emits thermal noise
Atmospheric Absorption
- Gas and cloud opacity vs. frequency (Green Bank summer example: zenith opacity, 1.0 cm precipitable water vapor, 55% cloud cover; curves for total, hydrosols, oxygen, dry air, water vapor):
- Water vapor has a feature near 22 GHz
- Oxygen absorbs strongly around 60 GHz
- Cloud liquid water adds loss
- Dry, high sites reduce water along the atmospheric path
- Attenuation in dB opacity (Condon & Ransom, Fig. 1.2)
WARNING
- This curve is gas/cloud opacity, not rain attenuation
Rain Attenuation
- Specific attenuation at the same rain rate, 25 mm/h (ITU-R P.838-3, Table 5, horizontal polarization):
| Frequency | 8 GHz | 12 GHz | 20 GHz | 30 GHz |
|---|---|---|---|---|
| Specific attenuation | 0.4 dB/km | 1.1 dB/km | 2.8 dB/km | 5.1 dB/km |
- For these microwave bands, higher frequency gives much more rain attenuation
- Total fade also depends on rain-path length and elevation
- Ka-band generally needs more rain margin than X-band (frequency choices)
- (Connected model samples, not station fade predictions)
Rain Margin and Site Selection
- Rain margin: extra allowance for weather fades at a chosen availability
- Choose it using frequency, local rain statistics, elevation, and path geometry
- Dry climates and high sites often reduce atmospheric attenuation and noise
- Higher elevation shortens the path through the lower atmosphere
- Site diversity can keep service available when rain affects one station
- Weather reduces and can raise antenna noise, lowering G/T
- Reference: ITU-R P.618-14 (Earth-space propagation); no universal rain margin is assigned here
Weather in the Link Budget
- Adding atmospheric loss changes both sides of the S/N₀ ledger:
- Update both the atmospheric signal loss and the antenna noise — atmospheric emission can change G/T (budget changes)