Question E9A05:
What does the term “ground gain” mean?  

Choices:  
A: The change in signal strength caused by grounding the antenna  
B: The gain of the antenna with respect to a dipole at ground level  
C: To force net gain to 0 dB by grounding part of the antenna  
D: An increase in signal strength from ground reflections in the environment of the antenna    

Correct Answer is  D: An increase in signal strength from ground reflections in the environment of the antenna  

Correct answer: D — An increase in signal strength from ground reflections in the environment of the antenna

1) Why D is correct
- Ground gain is the extra gain you get when the direct wave from your antenna and a wave reflected off the ground add constructively in a given direction. This coherent addition can boost the field strength and thus the received signal in that direction.

2) Why the other choices are incorrect
- A: Grounding the antenna (bonding to earth) does not define “ground gain.” Ground gain is about propagation and interference, not the act of connecting to ground.
- B: It’s not a comparison “with respect to a dipole at ground level.” Ground gain refers to the increase from ground-reflected energy, regardless of the reference antenna.
- C: There is no concept of “forcing net gain to 0 dB by grounding part of the antenna.” That’s unrelated to how ground reflections affect radiation patterns.

3) Useful formulas and rules of thumb
- Two-ray (direct + ground-reflected) model:
  - Total field roughly: $E_{\text{tot}} = E_d + \Gamma E_r e^{-j\Delta\phi}$
  - Phase difference (far field, elevation angle $\theta$ above the horizon): $\Delta\phi \approx 2 k h \sin\theta + \phi_\Gamma$, where $k = 2\pi/\lambda$, $h$ = antenna height, and $\Gamma$ = complex ground reflection coefficient.
- When the reflected ray has nearly the same magnitude as the direct ray and they add in phase, the field doubles, giving a power increase of up to about $+6$ dB.
- Real ground (finite conductivity, roughness) and non-ideal polarization reduce this, so practical “ground gain” is often in the +3 to +6 dB range at low takeoff angles, especially over very good ground (e.g., saltwater).
- Antenna height in wavelengths controls which elevation angles see constructive interference. Adjusting $h$ shifts the low-angle lobe(s), which is key for DX at HF.