*Question G9D08:
How does a “screwdriver” mobile antenna adjust its feed point impedance?*  

*Correct Answer is  B: By varying the base loading inductance*  

### AI Response: (may contain errors)  

A "screwdriver" mobile antenna adjusts its feed point impedance by varying the base loading inductance. Here's why:

1. **Correct Answer: B - By varying the base loading inductance**
   - A screwdriver antenna has a motorized mechanism that adjusts the position of a coil (inductor) at the base of the antenna. By moving the coil, the inductance changes, which in turn adjusts the antenna's resonant frequency and feed point impedance. This allows the antenna to be tuned for different frequencies without physically changing its length.

2. **Why the other choices are incorrect:**
   - **A: By varying its body capacitance**
     - Body capacitance refers to the capacitance between the antenna and the vehicle body. While this can affect the antenna's performance, it is not the primary method used by screwdriver antennas to adjust impedance.
   - **C: By extending and retracting the whip**
     - Although changing the physical length of an antenna can adjust its resonant frequency, screwdriver antennas specifically use inductance variation for tuning. Some antennas do use whip length adjustment, but that is not the defining feature of a screwdriver antenna.
   - **D: By deploying a capacitance hat**
     - A capacitance hat is a structure added to the top of an antenna to increase its effective electrical length. While it can affect impedance, screwdriver antennas do not use this method for tuning.

3. **Useful Concepts:**
   - The resonant frequency of an antenna is determined by its inductance ($L$) and capacitance ($C$). By changing the inductance, the resonant frequency and impedance can be adjusted:
     $$ f = \frac{1}{2\pi\sqrt{LC}} $$
   - Adjusting the inductance allows the antenna to be resonant at different frequencies, which is essential for mobile operations across various bands.