Effective HF Mobile Antennas
Understanding and building efficient HF mobile antennas for lower bands, including base-loaded and center-loaded designs.
Description
WB2VUO presents a practical examination of effective HF mobile antennas, focusing on the inherent efficiency challenges encountered on the lower bands, specifically 160, 80, and 40 Meters. The resource delves into the necessity of loading coils for mobile operation below 21 MHz, where full-sized antennas are impractical. It contrasts base-loaded and center-loaded designs, noting that base-loaded antennas are simpler for the average ham to construct but offer lower efficiency compared to center-loaded configurations.
The author provides specific data for an 8-foot whip, detailing its electrical length and radiation resistance across various HF bands, from 0.08 ohms on 160 Meters to 16.1 ohms on 12 Meters. This data highlights the extremely low radiation resistance on lower frequencies, which significantly impacts feedpoint impedance due to ground and feedline losses. The discussion includes practical considerations for feedpoint impedance, noting that a typical 8-foot whip on 10 Meters might present 30-45 ohms, allowing for acceptable SWR without an ATU.
Construction sketches illustrate both base-loaded and center-loaded mobile antennas, with advice on material selection like galvanized steel for rugged bottom sections. The article also includes coil value charts from the ARRL Mobile Manual for both base and center loading, emphasizing the importance of using large diameter wire to minimize losses and suggesting capacity hats to reduce coil inductance and improve performance on 160-40 Meters.