The AC-31 is an HF mobile centre-loaded whip antenna which may be tuned remotely from a control position on any frequency within its two octave tuning range. The system consists of three units: the antenna unit proper, the SWR sensor unit, and the control head with interconnecting cables. The antenna unit proper consists of three main sections: the whip, the body, and the base. The body consists of a strong outside fiberglass tube and a thin-walled inner tube on which the tuning coil is wound. A core composed of powered iron and copper tuning slugs is moved within this coil by a DC motor and gear drive in the base tube. The m* tor drive consists of a permanent magnet DC motor and two nylon reduction gears iurni > a threaded rod. A block riding on the threaded rod is coupled to the tuning slug by an insulated pushrod. Travel of the block is limited to seven inches by light switches which break the power to relays in the control unit, which control the power to the motor. A loop of nichrome wire is mounted so that as the block moves on the threaded rod a shorting contact changes the resistance of the circuit. The motor drive and coil assembly slides into the base tube and body ot the antenna and is scaled and anchored by tightening three screws which expand a rubber gland at the bottom of the base tube. : “ The control unit has a frequency scale for approximate tuning and an SWR scale for fine tuning. The variable resistance in the antenna forms one arm of a bridge circuit in the control unit. The circuit provides a reading related to the tuning slug position, so the meter can be calibrated directly in frequency. Fine tuning is accomplished by switching to an SWR scale on the same meter. The meter is then operated by an external SWR detector placed in the coax line to the antenna. This detector has constant sensitivity over the antenna's frequency range and once set-up for an installation requires no further adjustments. Tie control unit has a transistorized motor speed control circuit. The initial or low rector speed is set by R-13. a rter about half a second Cl has charged through constant current diode D3 to a point w'.are D2 conducts, and the motor rapidly reaches full speed. When the ANT. TUNE switch is released, Cl discharges rapidly so it is possible to get sustained low speed for fine tuning by tapping the ANT. TUNE switch. Sen the antenna is used to receive only, or monitor the receive frequency of a cross-bond channel, the antenna may be tuned for maximum noise or signal or for mqximum recwi. er "S" meter reading.