Question E9C06:
What is the effect of adding a terminating resistor to a rhombic or long-wire antenna?  

Choices:  
A: It reflects the standing waves on the antenna elements back to the transmitter  
B: It changes the radiation pattern from bidirectional to unidirectional  
C: It changes the radiation pattern from horizontal to vertical polarization  
D: It decreases the ground loss    

Correct Answer is  B: It changes the radiation pattern from bidirectional to unidirectional  

Correct answer: It changes the radiation pattern from bidirectional to unidirectional.

1) Why this is correct
- Adding a terminating resistor at the far end of a rhombic or long-wire antenna makes it a traveling-wave antenna. The resistor absorbs the wave that reaches the end, preventing it from reflecting back.
- With no reflection, the current distribution is predominantly a single traveling wave, so the main radiation is in the end-fire direction toward the terminated end. This converts the pattern from bidirectional to unidirectional.

2) Why the other choices are incorrect
- A: “It reflects the standing waves back to the transmitter” — The termination does the opposite: it absorbs energy and minimizes reflections, reducing standing waves.
- C: “It changes polarization from horizontal to vertical” — Polarization is set by the orientation of the wire. A termination does not change polarization.
- D: “It decreases the ground loss” — The terminating resistor does not reduce ground loss; it adds intentional loss (heat in the resistor). Ground loss depends mainly on height, ground conductivity, and geometry.

3) Useful formulas and rules of thumb
- Match the termination to the antenna’s characteristic impedance to suppress reflections:
  $$\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}, \quad SWR = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$
  Where $Z_L$ is the terminating resistor and $Z_0$ is the antenna’s (line) characteristic impedance.
  - If $Z_L \approx Z_0$, then $\Gamma \approx 0$ and SWR is near 1:1 along the wire, yielding a clean unidirectional pattern.
- Typical terminating values:
  - Rhombic: $Z_0$ often in the 600–800 Ω range.
  - Long-wire/Beverage: several hundred to ~1000 Ω, depending on height and surroundings.
- Trade-offs:
  - Terminated (unidirectional): broader bandwidth, better match, cleaner pattern, but some power is dissipated in the resistor (lower radiated power than unterminated).
  - Unterminated (bidirectional): higher efficiency but radiation in both directions and more standing waves.