Bi-Yagi technical detail

Brief mention of the Bi-Yagi antenna has prompted some questions due to its unusual design; most are familiar with a Yagi that is directive ONE way, but two?


Early on it was decided that something other than an omnidirectional antenna would allow for some control and optimizing of the path being worked. The requirement is for greater than unity gain, horizontal polarization, broadband nature, and ease of adaptation to the aircraft. Power directed vertically in this application is nearly totally wasted, and that which is directed toward the horizon is desired.

When modeled as vertical quarter-wave monopoles against an infinite ground plane, the five element Bi-Yagi radiates close to a figure eight pattern. Consisting of simply one one-inch diameter driven element matched omega fashion, with two directors at each side, operation is pretty straightforward. Gain is near four DBd in TWO directions. It can be thought of as simply a half, or single ended Yagi operating against a ground plane, turned horizontal.

Pattern of BI-YAGI antenna A conventional Yagi-Uda array operates similar to a phased driven array, with directional cancellation or reinforcement achieved by a combination of element spacing, element current ratios and phases. The only difference is, with a parasitic array, currents are induced, and phase is controlled by tuning parasitic elements reactively above or below resonance to generate the current phase lead or lag. The unit was mounted inside the fiberglass wingtip, which makes a nice radome. The BI-YAGI

Based upon on-air reports during flight testing it was observed that indeed a fore-aft pattern appears, but nulls at the 90 degree points seem asymmetrical, as expected with wingtip mounting. The antenna is fed directly with 20 feet of LMR coaxial cable. Normal operation is horizontally polarized - occasional pilot-induced vertical polarization is limited by co-pilot complaints.