W8BI 421.25 MHz Dual-Slot Antenna Far-Field Radiation Pattern
An analysis of the 421.25 MHz ATV transmitting antenna was conducted using full-wave electromagnetic analysis software. This antenna was constructed by KB8OFF, W8RVH and others, and installed at approximately 140 feet atop the W8BI ATV tower at the Beyers Road site. An excellent description of this antenna can be found on the WK8L website. The W8BI Slot antenna is modified for operation on 421.25 MHz. Note the lobing effect that occurs in the direction of the slot. This lobing is fairly minor, with approximately 2 dB of gain relative to the rear of the slot opening. For this reason, the slot is aimed Northward towards the greater Dayton area. Click here for a coverage map.
It is important to note that the gain of this antenna is primarily gain in the elevation plane. In this plane, the modelled gain is approximately 8-10 dB, depending on the azimuth angle of radiation. Note that these results do not take into account the parasitic effects of the tower. We plan to take gain measurements from an aircraft in the future to validate this model. We acknowledge that the 8-10 dB gain figure is slightly higher than that reported by the ATCO Dual Slot antenna which operates on 427.25 MHz. Further installed testing is warranted to resolve this discrepancy.
It is important to note that the slot antenna maintains a good impedance match across a wide range of frequencies, a desirable characteristic of an antenna designed for ATV operation. The chart below shows the fairly wide VSWR bandwidth possible with this construct. Note that the slot antenna was simulated for operation at 427.25 MHz in order to compare to gain figures reported by the ATCO repeater group.