Question E9H04:
What is the purpose of placing an electrostatic shield around a small-loop direction-finding antenna?  

Choices:  
A: It adds capacitive loading, increasing the bandwidth of the antenna  
B: It eliminates unbalanced capacitive coupling to the antenna’s surroundings, improving the depth of its nulls  
C: It eliminates tracking errors caused by strong out-of-band signals  
D: It increases signal strength by providing a better match to the feed line    

Correct Answer is  B: It eliminates unbalanced capacitive coupling to the antenna’s surroundings, improving the depth of its nulls  

Purpose of the electrostatic (Faraday) shield:
- A small DF loop should respond mainly to the magnetic field (H-field). Unwanted pickup of the electric field (E-field) through stray capacitance creates a “monopole-like” component that fills in the loop’s deep null.
- A slotted, grounded shield surrounding the loop intercepts the E-field and equalizes potential around the loop, greatly reducing unbalanced capacitive coupling to the surroundings. The H-field still penetrates, so the loop’s magnetic response (and its sharp null) is preserved.
- Result: deeper, cleaner nulls for accurate direction finding.

Why the correct answer (B) is correct:
- The shield reduces E-field coupling, so the loop’s response is dominated by H-field. This minimizes pattern distortion and “fills” in the null less, improving null depth and bearing accuracy.

Why the other choices are incorrect:
- A: “Adds capacitive loading, increasing bandwidth” — Bandwidth of a tuned loop is set by its Q. Extra capacitance does not purposefully increase bandwidth; changes in C mainly shift resonance. The shield’s role is E-field suppression, not bandwidth broadening.
- C: “Eliminates tracking errors from strong out-of-band signals” — That is a receiver selectivity/linearity issue (use preselectors/filters). A Faraday shield does not provide frequency-selective rejection of out-of-band signals.
- D: “Increases signal strength via better feed-line match” — Matching is done with coupling loops/transformers. The shield does not improve matching; if anything, it can slightly reduce signal due to added losses while improving pattern purity.

Useful formulas and rules of thumb:
- Magnetic pickup (for a small loop in a sinusoidal field):
  $$V_{\text{H}} \propto \omega N A B$$
  where $N$ is turns, $A$ is loop area, $B$ is magnetic flux density, and $\omega = 2\pi f$.
- Stray E-field pickup behaves like a capacitive feed:
  $$V_{\text{E}} \propto E \cdot \frac{1}{\omega C_{\text{stray}} Z_{\text{in}}} \quad \text{(qualitative)}$$
  Reducing $C_{\text{stray}}$ via shielding reduces $V_{\text{E}}$ and preserves the null.
- Loop bandwidth:
  $$BW \approx \frac{f_0}{Q}, \quad Q \approx \frac{X_L}{R} = \frac{\omega L}{R}$$
  The shield’s purpose is not to change BW; it’s to improve pattern by suppressing E-field coupling.
- Rule of thumb for the shield: include a single longitudinal slot (to avoid a shorted turn) and bond the shield to ground at one point; keep the loop conductor insulated from the shield.