Question E2C10:
Why do DX stations often transmit and receive on different frequencies?  

Choices:  
A: Because the DX station may be transmitting on a frequency that is prohibited to some responding stations  
B: To separate the calling stations from the DX station  
C: To improve operating efficiency by reducing interference  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

Correct answer: D — All these choices are correct

1) Why D is correct
DX operations often use “split” (transmit and receive on different frequencies) for multiple reasons that all apply:
- Regulatory/sub-band limits (Choice A): Some responders can’t legally transmit on the DX’s transmit frequency due to region or license-class band segments. Split lets them respond where they are authorized.
- Managing pileups (Choice B): Separating calling stations from the DX station keeps the DX’s transmit frequency clear so the DX can hear return calls more easily.
- Reducing interference (Choice C): Moving the callers off the DX’s frequency reduces QRM, AGC pumping, and splatter, improving rate and copy for everyone.

2) Why A, B, and C are “incorrect” alone
- A, B, and C are each true statements, but each covers only one reason. The exam expects the most complete answer, which is D.

3) Useful formulas and rules of thumb
- Split notation and offsets:
  - If the DX says “UP n,” then callers transmit at:
    $$f_{\text{callers}} = f_{\text{DX TX}} + n\ \text{kHz}$$
    while listening to the DX at $f_{\text{DX TX}}$.
  - If the DX says “DOWN n,” then:
    $$f_{\text{callers}} = f_{\text{DX TX}} - n\ \text{kHz}$$
- Variable windows:
  - Sometimes the DX announces a range, e.g., “UP 5–10,” meaning callers spread:
    $$f_{\text{callers}} \in \left[f_{\text{DX TX}} + 5,\ f_{\text{DX TX}} + 10\right]\ \text{kHz}$$
- Typical offsets (rule of thumb):
  - CW/RTTY: UP 1–3 kHz
  - SSB: UP 5–20 kHz depending on pileup size
- Operating tip:
  - Never transmit on the DX’s TX frequency if the DX is operating split. Tune where the DX is listening and time your call when you hear whom the DX just worked.