The flatness or frequency sensitivity of an RF coupler is the measure of how the coupling value can vary over a given frequency range. For a quarter-wavelength coupler operating over an octave band, typical coupling flatness is within ± 0.75 dB of the nominal value.

All things being equal, stronger coupling factors (3, 6 and 10 dB) show greater flatness than weaker coupling factors (20 through 50 dB). When operating over frequency bands greater than an octave, the flatness tolerance may need to be lessened due to the inherent characteristics of coupling roll-off.

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