Adjacent antenna elements must be less than 1/2 wavelength apart. Deviation and hence sensitivity increase with spacing up to the point where the phase difference between the signals from adjacent elements begins to approach 180 degrees. A phase of 180 degrees does not, however, correspond to 1/2 wavelength because the mutual impedance between the antennas causes the phase to increase over the free space (uncoupled) condition. We recommend that the spacing between the elements be 1/4 wavelength at the nominal operating frequency. This spacing can then be used over a full octave bandwidth. That is: