Question
How does a “screwdriver” mobile antenna adjust its feed point impedance?
Answer Options
- A) By varying its body capacitance
- B) By varying the base loading inductance
- C) By extending and retracting the whip
- D) By deploying a capacitance hat
Correct Answer: B
Explanation
A screwdriver antenna is a type of motorized, multi-band mobile antenna. Since HF mobile antennas are physically short, they require a large loading coil to achieve resonance. The screwdriver mechanism allows the operator to change the electrical length of the coil for any desired band.
This adjustment is achieved by varying the base loading inductance. A motor drives a contact block up and down the coil, changing the number of turns used. This allows the operator to select the precise inductance needed to resonate the antenna on any frequency, thus presenting a low, resistive feed point impedance.
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