Question E9H09:
What type of radiation pattern is created by a single-turn, terminated loop such as a pennant antenna?  

Choices:  
A: Cardioid  
B: Bidirectional  
C: Omnidirectional  
D: Hyperbolic    

Correct Answer is  A: Cardioid  

Correct answer: Cardioid

1) Why “Cardioid” is correct
- A single-turn, terminated loop (pennant, flag, K9AY) is a traveling-wave loop. The resistive termination breaks the symmetry and absorbs energy from one side, creating a strong rear null and a single broad forward lobe.
- This produces a classic cardioid pattern: one main lobe with a deep null to the rear (high front-to-back ratio).
- A simple model of a cardioid field pattern is:
  $$E(\theta) = E_0\,[1+\cos\theta]$$
  and the power pattern:
  $$P(\theta) \propto [1+\cos\theta]^2$$
  showing a maximum at $\theta=0^\circ$ and a null at $\theta=180^\circ$.

2) Why the other choices are incorrect
- Bidirectional: An unterminated small loop has a figure-8 bidirectional pattern broadside to the plane of the loop. Termination suppresses one lobe, making it unidirectional, not bidirectional.
- Omnidirectional: Small verticals can be omni in azimuth; loops are not. A terminated loop has a pronounced null and a forward lobe, so it’s not omni.
- Hyperbolic: Not a standard descriptor of antenna radiation patterns in this context; a distractor.

3) Useful formulas and rules of thumb
- Cardioid model:
  $$E(\theta) = E_0\,(1+\cos\theta),\quad P(\theta)\propto (1+\cos\theta)^2$$
- Rule of thumb:
  - Unterminated small loop → bidirectional figure-8.
  - Add a termination → unidirectional cardioid with a deep null toward the terminated end.
  - Expect high F/B (often 20–30 dB) and broad forward beam; commonly used for receiving (low noise).