What Changes When Frequency Changes?
- Start from the full received-power equation:
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- Free-space path loss increases with frequency
- For fixed apertures, antenna gains also increase
- We must say what stays fixed before comparing links
- The comparisons below hold range, transmit power, and other losses constant
Doubling Frequency: Three Comparisons
| What stays fixed? | | | | |
|---|
| Both antenna gains | 0 | 0 | +6 dB | −6 dB |
| Tx gain and Rx aperture | 0 | +6 dB | +6 dB | 0 |
| Both physical apertures | +6 dB | +6 dB | +6 dB | +6 dB |
- For fixed apertures, assume unchanged efficiency and accurate pointing
- Rounded changes; free-space comparison with constant transmit power and other losses
Fixed Apertures at Two Frequencies
1 m Transmit, 3 m Receive, 2000 km
| Quantity | 2.1 GHz | 8.4 GHz |
|---|
| Tx gain: 1 m aperture | 24.25 dBi | 36.30 dBi |
| Rx gain: 3 m aperture | 33.80 dBi | 45.84 dBi |
| Path loss: 2,000,000 m | 164.91 dB | 176.96 dB |
| Received power: 10 W Tx | −96.86 dBW | −84.82 dBW |
- Four times the frequency: about 12 dB more received power here
- ; gain constant 17.81; path-loss constant 32.45 with in m, in GHz
Frequency Choices in a Real Mission
- The free-space calculation isolates only one part of the trade:
- Pointing and surface accuracy become more demanding as frequency increases (narrower beams, pointing loss)
- Atmospheric attenuation depends on frequency and weather (Atmospheric Loss and Noise; ITU-R P.840-9, Attenuation Due to Clouds and Fog)
- Available spectrum and compatible ground equipment also constrain the choice
Homework 3: Why Gain Alone Is Insufficient
- Homework 3, Problem 1(c): doubling changes both antenna gain and free-space loss
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| What stays fixed? | Changes at twice the frequency | Received power |
|---|
| Ground antenna gain | +6.0 + 0.0 − 6.0 dB | 0.0 dB change |
| Both physical apertures | +6.0 + 6.0 − 6.0 dB | +6.0 dB change |
- We must know the ground-antenna assumption and any other frequency-dependent losses
Frequency Bands and Spectrum Access
- S, X, Ku, and Ka are useful space-communications bands; L, VHF, and UHF also serve particular missions
- Available bandwidth depends on the allocated service, direction, region, and authorization (NTIA, United States Frequency Allocation Chart)
- Higher-frequency links can offer wider allocations while facing tighter pointing and weather trades
- A band name alone does not guarantee a usable bandwidth or non-line-of-sight coverage