Building a magnetic loop antenna: control box

Small Loop Control Box
AA & A Loop control boxThe inside of the AA&A control box.  Power in/out is via loudspeaker sockets!

The narrow bandwidth of a tuned magnetic loop means you have to have some method of easily changing the tuning capacitor. For a 6m loop of about 1m diameter you can decide if you want FM or ssb or cw coverage, set the capacitor and you have enough bandwidth to operate without further ado. HF loops however unless you ant a 'spot' frequency coverage need a remote means of adjustment. For portable work a long non-conductive tuning rod might do, but the real answer is to use a geared electrical motor.

AA&A loop control



Schematic of how you might control a magloop motor witgh two buttons and a variable resistor

The CapCo or AA&A loop, very popular a few years ago, and my main HF antennas have a simple push switch control box.
By using a DC voltage of only 3 - 4 v and a variable resistor to control the tuning speed the system effectively compensates for variations in the length of control cable- but much below 'half way' the motor stalls. A full voltage but variable pulse control box would have allowed for finer control. As it is, the up/down (or High/Low as printed on the box) are stabbed at for fine tuning. With familiarity, this is quite a quick process. The motor tends to generate some electrical noise which conveniently gets louder as the antenna reaches the tuned point! The narrowness of the antennas tuning simulating the effect of a noise bridge.

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