Question E9E02:
What antenna matching system matches coaxial cable to an antenna by connecting the shield to the center of the antenna and the conductor a fraction of a wavelength to one side?  

Choices:  
A: Gamma match  
B: Delta match  
C: T-match  
D: Stub match    

Correct Answer is  A: Gamma match  

Correct answer: Gamma match

Why A (Gamma match) is correct:
- A gamma match feeds a center-fed element (e.g., a Yagi driven element) with unbalanced coax by:
  - Connecting the coax shield to the electrical center of the driven element (often grounded to the boom).
  - Connecting the coax center conductor to a tap point a short distance (a fraction of a wavelength) along the element via a parallel “gamma rod” and a series capacitor.
- This matches a low feedpoint impedance (often 20–30 Ω on a Yagi) up to 50 Ω and provides an unbalanced-to-balanced transition without a separate balun.

Why the others are incorrect:
- B: Delta match
  - Uses a triangular connection to both sides of a split (balanced) driven element with a balanced feed. It does not connect the coax shield at the center and tap only one side.
- C: T-match
  - A symmetric “double gamma” feeding both sides of a split element with two parallel arms and usually two capacitors. It is balanced and does not use the shield at the center.
- D: Stub match
  - Uses a transmission-line stub for impedance transformation. It is not characterized by connecting the shield at the center and the hot conductor to an offset tap on the element.

Useful formulas and rules of thumb:
- Series capacitor used in a gamma match to cancel inductive reactance:
  $$X_C = \frac{1}{2\pi f C}$$
- Current division factor (from element and gamma rod geometry):
  $$\alpha = \frac{\ln\!\left(\frac{s}{a}\right)}{\ln\!\left(\frac{s}{b}\right)}$$
  where:
  - $a$ = radius of driven element
  - $b$ = radius of gamma rod
  - $s$ = center-to-center spacing between element and gamma rod
- Approximate impedance step-up ratio:
  $$R \approx (1+\alpha)^2$$
  The gamma match typically raises the element’s feedpoint resistance toward 50 Ω; the series capacitor is adjusted to cancel residual reactance.
- Practical tap distance along the element is often on the order of $0.05\lambda$ to $0.15\lambda$, then fine-tune position and capacitance for best SWR.