*Question T3A04:
What happens when antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization?*  

*Correct Answer is  B: Received signal strength is reduced*  

### AI Response: (may contain errors)  

When antennas at opposite ends of a VHF or UHF line of sight radio link are not using the same polarization, the correct answer is:

**B: Received signal strength is reduced.**

### Explanation:

1. **Why the correct answer is correct:**
   - Antennas are designed to transmit and receive signals in specific polarizations (horizontal, vertical, or circular). When the transmitting and receiving antennas are not aligned in the same polarization, the efficiency of the signal transfer decreases. This results in a reduction of the received signal strength. The mismatch in polarization can lead to significant losses, often quantified as a reduction in gain.

2. **Why the other choices are incorrect:**
   - **A: The modulation sidebands might become inverted**  
     This is not true. The modulation sidebands are related to the modulation scheme used (like AM or FM) and are not affected by polarization mismatches.
   
   - **C: Signals have an echo effect**  
     Echo effects are typically caused by multipath propagation, where signals reflect off surfaces and arrive at the receiver at different times. This is unrelated to polarization.
   
   - **D: Nothing significant will happen**  
     This is incorrect because a significant reduction in signal strength occurs when there is a polarization mismatch, which can affect communication quality.

### Useful Concepts:
- **Polarization Mismatch Loss:** A rule of thumb is that a polarization mismatch can result in a loss of about 20 dB if the polarizations are completely orthogonal (e.g., one antenna is vertical and the other is horizontal).
- **Antenna Gain:** The gain of an antenna in the direction of its polarization is maximized when the receiving antenna matches that polarization.

Understanding the importance of polarization in antenna design and placement is crucial for effective communication in VHF and UHF radio links.