Four-sided, each side of a different physical length; traps at each vertex for changing resonance according to band usage 

Four-sided; an LC network at each vertex except for the transmission connection; tuned to resonate at the operating frequency

Bidirectional; four-sided, each side approximately one wavelength long; open at the end opposite the transmission line connection

Unidirectional; four-sided, each side a half-wavelength long; terminated in a resistance equal to its characteristic impedance



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