Question E9E09:
Which of the following is used to shunt feed a grounded tower at its base?  

Choices:  
A: Double-bazooka match  
B: Beta or hairpin match  
C: Gamma match  
D: All these choices are correct    

Correct Answer is  C: Gamma match  

The correct answer is: Gamma match.

Why it’s correct:
- A gamma match is specifically designed to shunt-feed a conductive element that is grounded at the feedpoint (like a grounded tower).
- It uses a parallel “gamma rod” connected near the base and a series capacitor to feed the tower while keeping it DC-grounded for safety and lightning protection.
- It works well with unbalanced coax and does not require insulating (splitting) the driven element.
- Commonly used for AM broadcast and HF shunt-fed towers.

Why the others are incorrect:
- Beta or hairpin match: This is a shunt inductor across a center-fed, insulated driven element (typical on Yagis). It requires the element to be split and isolated from ground, which a grounded base tower is not.
- Double-bazooka match: The “double bazooka” is a type of coaxial dipole antenna, not a base shunt-feeding method for a grounded tower.
- All choices are correct: Incorrect because only the gamma match applies to shunt-feeding a grounded tower at its base.

Useful formulas and rules of thumb:
- A grounded tower fed at the base usually has low resistance with inductive reactance. The gamma match provides a series capacitance to cancel that inductive reactance:
  - Choose series capacitance so that:
    $$X_C \approx -X_{\text{tower}}$$
  - Then:
    $$C = \frac{1}{2\pi f |X_C|}$$
- The gamma match acts like an L-network and an autotransformer; adjusting the gamma rod spacing/length changes the impedance step-up to reach 50 Ω.
- Rule of thumb:
  - Use a gamma match when the element is grounded or cannot be split.
  - Use a beta/hairpin when the driven element is center-fed, insulated, and you need an inductive shunt to cancel capacitive reactance.