AT-11

The AT-11 is a full featured auto or semi automatic antenna tuner designed for HF (1.8 to 30 MHz) transceivers using 2 to 100 watts. The tuner uses a switched "L" configuration with 256 capacitor, 256 inductor and Hi/Lo-Z settings to provide over a quarter-million tuning combinations (figure 1)(figures appear at the end of text). The "L" network works great with just about any coax fed antenna (dipole, vertical,beam, ect). Tuning time is between 0.1 and 6.2 seconds.

Operation of the tuner is auto or semi automatic. In auto mode, the tuner will seek a 1.5 match anytime the SWR is above 3.0. In semi mode, the tuner will seek a match when the tune push button is pressed. Both modes require that more than 2 watts of RF power be present. Up and down buttons are used for fine tuning the inductors and capacitors and can be used in either mode.

Three LEDs provide an indication of SWR. Green indicates SWR of less than 1.5, Green/Yellow is 1.5-2.0, Yellow is 2.0-2.5, Yellow/Red is 2.5-3.0 and Red indicates more than 3.0. The fourth LED is a tuning indicator. It is lit when the tuner is trying to find a match.

The AT-11 is a small to medium sized project. It should take the average builder an evening or two to complete (we averaged 6 hours on the prototypes). Besides the normal building tools needed (soldering iron or soldering pencil, wire cutters, screw drivers, ect), the only test equipment needed is an HF transceiver,dummy load (or resonant antenna) and voltmeter.

Before starting, you may want to get a copy of the January 1996 QST article where the AT-11 first appeared. Although not needed for construction of the kit, it contains a little more theory about the AT-11 along with some pictures. If you don't have or can't find the article, a reprint may be available from QST, 225 Main St., Newington CT 06111.