Question E3B12:
What is chordal-hop propagation?  

Choices:  
A: Propagation away from the great circle bearing between stations  
B: Successive ionospheric refractions without an intermediate reflection from the ground  
C: Propagation across the geomagnetic equator  
D: Signals reflected back toward the transmitting station    

Correct Answer is  B: Successive ionospheric refractions without an intermediate reflection from the ground  

Correct answer: B — Successive ionospheric refractions without an intermediate reflection from the ground

Why B is correct:
- Chordal-hop propagation occurs when a signal is refracted by the ionosphere (usually the F region), then continues to another region of high ionization and is refracted again, traveling along a “chord” above the Earth. There are multiple ionospheric refractions with no ground bounces between them.
- Because it stays high in the ionosphere, it avoids repeated D/E-layer absorption and ground-reflection losses, often producing stronger, more stable long-distance signals.

Why the other choices are incorrect:
- A: Propagation away from the great circle bearing describes skewed-path propagation due to ionospheric tilts, not chordal hops.
- C: Propagation across the geomagnetic equator is transequatorial propagation (TEP), a different mechanism.
- D: Signals reflected back toward the transmitting station describe ionospheric backscatter, not chordal-hop.

Useful formulas and rules of thumb:
- Secant law (MUF for a given layer and incidence angle):
  $$ \text{MUF} = f_\text{critical} \sec \chi $$
  where $f_\text{critical}$ is the layer’s critical frequency and $\chi$ is the angle of incidence at the ionosphere (from the normal).
- Ground reflection typically adds several dB of loss per hop and each ground hit forces another pass through the absorbing D layer. Chordal-hop paths avoid these, so signals can be stronger by roughly one or more “ground-bounce equivalents” (often 6–20 dB saved over multiple hops).
- Typical single-hop distances:
  - F2 single hop: roughly 2000–3000 km
  - Es single hop: roughly 800–2200 km
  - Chordal hops can string together long segments without touching ground, enabling very long paths with fewer losses.