*Question G9C11:
What is a beta or hairpin match?*  

*Correct Answer is  A: A shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching*  

### AI Response: (may contain errors)  

A beta or hairpin match is a specific type of impedance matching technique used in antenna systems, particularly with Yagi antennas. 

### Correct Answer Explanation:
- **Correct Answer (A)**: A beta or hairpin match is indeed a shorted transmission line stub placed at the feed point of a Yagi antenna. This stub is designed to provide impedance matching by creating a reactive component that cancels out the unwanted reactance of the antenna. The length of the stub is typically a quarter wavelength ($\lambda/4$) at the operating frequency, which allows it to effectively match the impedance of the antenna to the feed line.

### Incorrect Answer Explanations:
- **Choice B**: A 1/4 wavelength section of 75-ohm coax in series with the feed point of a Yagi to provide impedance matching is incorrect because a beta match specifically refers to a shorted stub, not a coaxial cable. Coaxial cables can be used for matching, but they do not define a beta match.

- **Choice C**: A series capacitor selected to cancel the inductive reactance of a folded dipole antenna is incorrect because this describes a different matching technique. While capacitors can be used for impedance matching, they do not represent a beta or hairpin match.

- **Choice D**: A section of 300-ohm twin-lead transmission line used to match a folded dipole antenna is also incorrect. While twin-lead can be used for matching, it does not specifically refer to the beta match technique, which involves a shorted stub.

### Useful Formulas and Rules of Thumb:
- The length of a quarter-wavelength stub can be calculated using the formula:
  
  $$ L = \frac{c}{4f} $$

  where:
  - $L$ = length of the stub in meters
  - $c$ = speed of light in meters per second (approximately $3 \times 10^8$ m/s)
  - $f$ = frequency in hertz

- For effective impedance matching, the goal is to make the input impedance of the antenna equal to the characteristic impedance of the feed line (often 50 ohms or 75 ohms).

Understanding these concepts will help you grasp the purpose and application of a beta or hairpin match in antenna systems.