Question E2A11:
What type of antenna can be used to minimize the effects of spin modulation and Faraday rotation?  

Choices:  
A: A linearly polarized antenna  
B: A circularly polarized antenna  
C: An isotropic antenna  
D: A log-periodic dipole array    

Correct Answer is  B: A circularly polarized antenna  

Correct answer: B — A circularly polarized antenna

Why B is correct:
- Faraday rotation rotates the plane of linear polarization as a signal passes through the ionosphere. With linear antennas this can cause large fades when the received polarization no longer matches.
- A circularly polarized antenna is insensitive to rotation of the polarization plane, so signal strength remains essentially constant.
- Satellite spin modulation rotates the transmitted linear polarization vector; a circularly polarized receive antenna converts that to a constant level rather than a fading signal.

Key polarization loss facts:
- Linear-to-linear mismatch (angle $\theta$):
  $$\text{PLF} = \cos^2\theta,\quad L_p(\text{dB}) = -10\log_{10}(\cos^2\theta)$$
  At $\theta = 90^\circ$, loss is infinite (complete fade).
- Linear-to-circular:
  $$L_p \approx 3\ \text{dB}$$
  Constant, independent of $\theta$ — this removes spin/Faraday-induced fading.
- Circular-to-circular:
  - Same sense (RHCP↔RHCP or LHCP↔LHCP): $L_p \approx 0$ dB
  - Opposite sense: very high isolation (often 20–30 dB or more in practice)

Rule of thumb on Faraday rotation:
- Rotation angle increases as frequency decreases:
  $$\psi \propto \frac{\text{TEC}}{f^2}$$
  More severe on HF/VHF; circular polarization mitigates the fade.

Why the other choices are incorrect:
- A: A linearly polarized antenna — susceptible to both Faraday rotation and spin modulation; can experience deep fades up to complete signal loss.
- C: An isotropic antenna — purely theoretical; has no defined polarization and cannot be built.
- D: A log-periodic dipole array — it is linearly polarized; does not address polarization rotation or spin modulation by itself.

Practical tip:
- For satellites, match the expected sense (often RHCP). Using circular on at least one end eliminates rotation-induced fading; using circular on both ends (same sense) also avoids the 3 dB linear-to-circular loss.