An automatically tuned 7-30MHz mobile antenna

Presents the design and construction of an automatically tuned 7-30 MHz mobile HF vertical antenna, originally published in QEX / Communication Quarterly in 2003. The resource details a base-loaded vertical antenna system that mounts on a vehicle's roof, incorporating a variable inductor as its loading coil. A three-legged chariot, driven by a modified model airplane servo, travels inside the coil to adjust inductance. The control unit, featuring a Basic Stamp microcontroller and SWR sensor, emulates a Kenwood AT-50 tuner for seamless integration with a Kenwood TS-50 transceiver, allowing automatic tuning across all ham bands from 40 to 10 meters.

The project emphasizes practical application, providing a solution to the narrow-banded nature of mobile HF antennas and the inconvenience of manual band changes. It achieves a maximum SWR of 1.3:1 across its operating range. The mechanical design is thoroughly documented with detailed drawings, including a full-resolution GIF and AutoCAD R14 DWG files, illustrating components like the stainless steel whip, PVC coil tube, and the servo-driven chariot mechanism. Construction requires a lathe, but the author notes it can be accomplished with a hobby lathe, making it accessible to those with moderate mechanical skills.