Question E3B01:
Where is transequatorial propagation (TEP) most likely to occur?  

Choices:  
A: Between points separated by 2,000 miles to 3,000 miles over a path perpendicular to the geomagnetic equator  
B: Between points located 1,500 miles to 2,000 miles apart on the geomagnetic equator  
C: Between points located at each other’s antipode  
D: Through the region where the terminator crosses the geographic equator    

Correct Answer is  A: Between points separated by 2,000 miles to 3,000 miles over a path perpendicular to the geomagnetic equator  

Transequatorial propagation (TEP) is a VHF-mode that occurs between stations on opposite sides of the geomagnetic equator, using enhanced ionization near the magnetic equator.

Why A is correct:
- TEP paths are typically oriented nearly north–south, crossing the geomagnetic equator at right angles (i.e., perpendicular).
- Usual station separation is about 2,000–3,000 miles ($\approx 3{,}200$–$4{,}800$ km), matching observed single-TEP path lengths.

Why the other choices are incorrect:
- B: “Along the geomagnetic equator” is wrong—TEP is not along the equator but across it, and 1,500–2,000 miles is shorter than typical TEP distances.
- C: Antipodal paths (~12,500 miles) are unrelated to TEP; that’s a different, rare propagation scenario.
- D: The “terminator” refers to gray-line propagation (HF), not TEP. TEP is tied to the geomagnetic equator, not the day–night line.

Useful rules of thumb:
- Orientation: Path crosses the geomagnetic equator roughly perpendicularly; endpoints are usually within about 15°–25° geomagnetic latitude of the equator.
- Distance: $L \approx 2{,}000$–$3{,}000$ miles ($L \text{[km]} \approx L \text{[mi]} \times 1.609$).
- Bands and timing: Most common on 50 MHz and sometimes 144 MHz, often late afternoon into evening, especially near the equinoxes.
- Midpoint: The path midpoint lies near the geomagnetic equator where ionization peaks enable the mode.