Question E9H08:
What is the function of a sense antenna?  

Choices:  
A: It modifies the pattern of a DF antenna to provide a null in only one direction  
B: It increases the sensitivity of a DF antenna array  
C: It allows DF antennas to receive signals at different vertical angles  
D: It provides diversity reception that cancels multipath signals    

Correct Answer is  A: It modifies the pattern of a DF antenna to provide a null in only one direction  

Correct answer: A — It modifies the pattern of a DF antenna to provide a null in only one direction.

1) Why A is correct
- A loop or Adcock DF array by itself has a bidirectional “figure‑8” pattern with two equal nulls 180° apart, creating a bearing ambiguity.
- A sense antenna (an omnidirectional element) is added and phase/level-adjusted so that the sum of the loop (directional) and sense (omni) responses produces a cardioid pattern with only one null.
- This removes the 180° ambiguity and lets you read a unique bearing.

2) Why the other choices are incorrect
- B: It increases the sensitivity of a DF antenna array — Not the purpose. The sense antenna is used to resolve ambiguity, not to boost sensitivity; in practice, levels are adjusted for pattern shaping, not gain.
- C: It allows DF antennas to receive signals at different vertical angles — No. Elevation response is set by the array geometry (e.g., Adcock reduces skywave), not by the sense antenna.
- D: It provides diversity reception that cancels multipath signals — No. Diversity requires independent channels/antennas and combining methods. The sense antenna is not used for diversity or multipath cancellation.

3) Useful formulas and rules of thumb
- Loop/Adcock (figure‑8) amplitude vs azimuth: $A_L(\theta) = L\cos\theta$ (two nulls at $\theta = \pm 90^\circ$ relative to loop axis).
- Sense antenna (omni) amplitude: $A_S(\theta) = S$ (constant with azimuth).
- Combined (cardioid) amplitude:
  $$A(\theta) = S + L\cos\theta$$
  - With proper phasing and $S \approx L$, this yields a single deep null at $\theta = 180^\circ$ (since $\cos 180^\circ = -1$), eliminating the 180° ambiguity.
- Rule of thumb: The sense antenna’s job is ambiguity resolution by creating a cardioid pattern with one null, not increasing sensitivity.