Question
Which of the following is a disadvantage of multiband antennas?
Answer Options
- A) They present low impedance on all design frequencies
- B) They must be used with an antenna tuner
- C) They must be fed with open wire line
- D) They have poor harmonic rejection
Correct Answer: D
Explanation
Multiband antennas, particularly those that rely on traps (parallel LC circuits), introduce resonant elements into the antenna structure to achieve operation on multiple bands. While convenient, the traps themselves are resonant circuits at high frequencies.
When transmitting on a lower band, the entire antenna length is used, and the trap acts as a simple inductor. However, if the transmitter generates a strong harmonic (a multiple of the fundamental frequency), this harmonic may fall on a frequency where the trap is resonant. This can lead to the trap acting as a short circuit or a highly efficient radiator, causing poor harmonic rejection and potentially violating FCC rules.
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