Question 3-72K6

Aircraft transponder systems receive 1030 MHz interrogation signals, which fall within the L-band frequency range (1-2 GHz). To reliably detect these signals from ATCRBS ground stations, which can be located in any direction relative to the aircraft, an **omnidirectional** antenna is required. **B) An L-band monopole blade-type omnidirectional antenna** is correct. A monopole antenna, often designed as a vertical blade on an aircraft's fuselage, provides the necessary omnidirectional reception pattern and is physically robust for aviation environments. Its size can be optimized for L-band frequencies. **A) An electronically steerable phased-array directional antenna** is incorrect. These are complex, expensive, and typically used for advanced radar or communications, not for simple transponder interrogation reception. Their directional nature would also be detrimental for receiving interrogations from all angles. **C) A folded dipole reception antenna** is incorrect. While a valid antenna type, folded dipoles are not the standard or most practical choice for external aircraft transponder reception; monopoles are more common and robust. **D) An internally mounted, mechanically rotatable loop antenna** is incorrect. Aircraft transponder antennas are externally mounted for optimal performance. Mechanically rotatable loop antennas are primarily used for lower frequency direction finding and are not suitable for high-frequency transponder reception or the required omnidirectional pattern.