40m small loop antenna featuring 3 foot diameter copper loop and DX Engineering construction components. All of this sits atop my favorite antenna foundation… the standard speaker stand.
The two rectangular black plates are the same as used in the AHVD antenna.
The loop is fed with a toroid transformer where the loop is one turn and the feed is over 20 turns. This is as close as I could get to the theoretical 50+:1 needed to match 50 ohms to a dozen or so milli-ohms.
Measured S11 results are quite satisfactory on 40m with a return loss greater than 20 dB and bandwidth about 4 kHz.
A DC gear motor drives the 5-500 pF vacuum variable capacitor through isolating torque components. A custom DC motor controller provides the operator with center off rotating control with variable speed and indicator lights. A custom PicAxe program controls an LMD18201 motor control IC.
The loop mounts to the vertical mast that, in this prototype, is aluminum. An extensive study reveals this mast does not interfere with the operation of the antenna. The DC motor control cable, however, did require choking with a toroid to mitigate observable influence.
