Question E9H02:
Which is generally true for 160- and 80-meter receiving antennas?  

Choices:  
A: Atmospheric noise is so high that directivity is much more important than losses  
B: They must be erected at least 1/2 wavelength above the ground to attain good directivity  
C: Low loss coax transmission line is essential for good performance  
D: All these choices are correct    

Correct Answer is  A: Atmospheric noise is so high that directivity is much more important than losses  

Correct choice: A — Atmospheric noise is so high that directivity is much more important than losses

1) Why A is correct
- On 160 m and 80 m, external noise (atmospheric and man-made) usually dominates the receiver noise by 10–30 dB or more. Improving signal-to-noise ratio (SNR) comes primarily from rejecting noise via antenna directivity (e.g., Beverages, K9AY/KAZ, small magnetic loops, phased verticals).
- When external noise dominates, even a lossy but highly directive receive antenna can yield better SNR than an efficient but non-directive one.

Key idea:
- If external noise power density $N_e$ dominates, a passive loss $L$ (linear) before the receiver reduces both signal and noise equally:
  $$SNR' \approx \frac{S/L}{N_e/L} = \frac{S}{N_e} = SNR$$
  So pattern/directivity matters far more than small efficiency or feedline loss.

2) Why the other choices are incorrect
- B: “They must be erected at least 1/2 wavelength above ground…”
  - Not true for receiving on these bands. Many excellent low-band receive antennas work close to ground (Beverage at ~2–3 m high, small loops, phased short verticals). Also, 1/2 wavelength is impractically high:
    - 80 m band ($f \approx 3.5$ MHz): $\lambda \approx 86$ m, so $\lambda/2 \approx 43$ m.
    - 160 m band ($f \approx 1.8$ MHz): $\lambda \approx 167$ m, so $\lambda/2 \approx 83$ m.
- C: “Low loss coax transmission line is essential…”
  - For receiving on 160/80 m, it usually isn’t. With external noise dominating, a few dB of line/antenna loss scarcely changes SNR (see relation above). Low-loss line is nice to have, but not “essential” for good receive performance on these bands.
- D: “All these choices are correct”
  - False because B and C are not correct.

3) Useful formulas and rules of thumb
- Wavelength:
  $$\lambda = \frac{c}{f}$$
- Half-wavelength heights (impractical for RX on these bands):
  - 80 m: $\lambda/2 \approx 43$ m
  - 160 m: $\lambda/2 \approx 83$ m
- Effect of passive loss when external noise dominates:
  $$SNR' \approx SNR$$
- Directivity (helps reject noise from other directions):
  $$D = \frac{G_{\max}}{G_{\text{avg}}} \quad\text{and}\quad D \approx \frac{4\pi}{\Omega_A}$$
  Higher $D$ → better receive SNR on noisy low bands.