My experiments and those of others have shown that about 0.22 wavelength on a side is optimum. Closer spacing increases mutual coupling between whips, which in turn increases undesirable amplitude directivity effects. Wider spacing increases doppler tone deviation, which can be detrimental on some receivers, depending on their IF filter characteristics. An 18-inch array like the one in the book covers 121 through 174 MHz. Bearing accuracy is unacceptable outside this range.
Sensitivity is maximum near the resonant frequency of the quarter wavelength vertical whips.
Civil Air Patrol volunteers, Coast Guard Auxiliary members, and others interested in search and rescue can use this array on the 120 MHz aircraft band and the 160 MHz marine band. Keep in mind that Doppler processing works only with FM receivers. A scanner or marine set receives narrowband FM from 138 to 174 MHz, but most scanners and all transceivers have only AM detectors in the 120 Mhz aircraft band. They won't work with a doppler RDF set. Instead, you must use a receiver that features selectable narrowband FM mode for the aircraft band Inductor and capacitor values in the switcher are not nearly as critical as whip length and spacing, so long as these components have self-resonant frequencies near the high end of the array operating range.