The directional antenna RF is modulated at _____ Hz.

The directional antenna RF is modulated at _____ Hz.

• A. 55 Hz
• B. 255 Hz
• C. 155 Hz
• D. 105 Hz

Answer: (C)

The key idea is that a directional antenna’s RF signal is given a deliberate low-frequency modulation so the receiver can lock onto a known signature and measure bearing reliably. The modulation frequency should sit in a range that the demodulator can handle with good signal-to-noise, while staying well away from common interference like power-line hum and higher RF clutter. 155 Hz is chosen because it lands in a sweet spot: high enough to avoid mains noise and slow drifts, yet low enough for the receiver’s detection and filtering stages to process accurately and with good dynamic response. This facilitates a clean, trackable envelope or phase reference used to determine direction. The other options would either bring the signal closer to troublesome interference bands or require more demanding filtering and processing, making the bearing measurement less robust in typical operating conditions.

The key idea is that a directional antenna’s RF signal is given a deliberate low-frequency modulation so the receiver can lock onto a known signature and measure bearing reliably. The modulation frequency should sit in a range that the demodulator can handle with good signal-to-noise, while staying well away from common interference like power-line hum and higher RF clutter.

155 Hz is chosen because it lands in a sweet spot: high enough to avoid mains noise and slow drifts, yet low enough for the receiver’s detection and filtering stages to process accurately and with good dynamic response. This facilitates a clean, trackable envelope or phase reference used to determine direction.

The other options would either bring the signal closer to troublesome interference bands or require more demanding filtering and processing, making the bearing measurement less robust in typical operating conditions.