At a lower frequency the trap allows the full length of the element to be employed, albeit with a shifted resonant frequency due to the inclusion of the trap's net reactance at that lower frequency.
2.
For ever shorter antennas ( requiring greater " electrical lengthening " ) the radiation resistance plummets ( approximately according to the square of the antenna length ), so that the mismatch due to a net reactance away from the electrical resonance worsens.
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