Question E2A08:
Why should effective radiated power (ERP) be limited to a satellite that uses a linear transponder?  

Choices:  
A: To prevent creating errors in the satellite telemetry  
B: To avoid reducing the downlink power to all other users  
C: To prevent the satellite from emitting out-of-band signals  
D: To avoid interfering with terrestrial QSOs    

Correct Answer is  B: To avoid reducing the downlink power to all other users  

Correct answer: B — To avoid reducing the downlink power to all other users

1) Why B is correct
- Linear (bent-pipe) transponders have a fixed total downlink power budget. Many use AGC to keep the total output power constant.
- If one station uses excessive uplink ERP, the transponder reduces its gain to maintain that constant output. That lowers the downlink level for everyone else, making their signals weaker.
- In other words, one “loud” uplink forces the transponder to turn down the overall gain, hurting all other users.

A useful model:
- Let the transponder maintain a constant total downlink power, $P_{\text{out,total}}$.
- With AGC, the gain becomes
  $$G = \frac{P_{\text{out,total}}}{\sum_i P_{\text{in},i}}$$
- Each user’s downlink power is
  $$P_{\text{out},i} = G \cdot P_{\text{in},i} = \frac{P_{\text{out,total}} \, P_{\text{in},i}}{\sum_i P_{\text{in},i}}$$
- If one user increases $P_{\text{in}}$ a lot, $G$ drops and everyone else’s $P_{\text{out}}$ falls.

2) Why the other choices are incorrect
- A: Telemetry is usually on a separate, robust downlink channel. Excess uplink ERP into the linear transponder primarily affects other users via AGC/saturation, not the spacecraft telemetry.
- C: Overdriving can create intermod and splatter within the transponder passband, but well-designed satellites filter out-of-band emissions. The exam’s key reason is the shared power budget and AGC effect on other users.
- D: Satellite uplinks are in satellite sub-bands aimed at the spacecraft, not terrestrial QSOs. The principal concern is impact on other satellite users, not ground-based QSOs.

3) Useful formulas and rules of thumb
- ERP (relative to a dipole):
  $$\text{ERP}_{\text{lin}} = P_T \times G_{\text{ant(dBd)}} \times \eta_{\text{line}}$$
  In dB:
  $$\text{ERP}_{\text{dBW}} = P_{T,\text{dBW}} + G_{\text{dBd}} - L_{\text{dB}}$$
- EIRP uses gain relative to isotropic: $G_{\text{dBi}} = G_{\text{dBd}} + 2.15$ dB.
- Operating rule of thumb for linear transponders:
  - Use the minimum uplink power that yields a clean, copyable downlink.
  - Keep your downlink no stronger than the transponder beacon or the average user level.
  - If increasing your power makes the overall transponder noise floor jump or other signals drop, you’re too loud—reduce ERP.