DIY air coil tuner

[Mickster]

Came across a pretty neat DIY solution yesterday.
An empty glue stick body, with a slug attached to the glue carrier, to provide nulling in a BFO reference coil.
Skip ahead to 11:03 here:

 

[unclejed613]

when i worked at NAD in the 1980s, we had a tool for tuning the air core coils in FM tuners that was made out of a plastic tuning tool (that was worn out). we would glue a piece of copper foil on one end about 2 or 3 mm long, and would glue a piece from a broken ferrite core (choosing a piece about the same size as the piece of copper foil) to the other end. you would feed a sweep generator into the tuner input, and have a voltage doubler probe at the output of the RF strip (basically the probe is an AM detector of sorts). the probe is connected to an oscilloscope vertical (Y axis), and the sweep source for the sweep generator is connected to the horizontal input (X axis) and the scope in XY mode. the tuner is tuned to the center frequency of the sweep generator. the deviation of the sweep generator would be 100khz iirc. to tune the air core coils, you would first bring the copper end of the tool in between turns of the air core coil. if the signal level on the scope increased, the turns needed to be spread apart (the piece of foil acts as two series capacitances between the adjacent turns, so the tuned circuit 's resonant frequency goes up). if the signal doesn't change, or decreases, then trying the ferrite end in the same way, will indicate whether or not to squeeze the turns closer.. when aligning one of these air coil tuners, it's very helpful to use a hair dryer to melt the wax off the board (try to save it so you can dribble it back onto the board to prevent accidental de-tuning)... so, copper foil= stretch the coil. and ferrite=squeeze the coil. try to use plastic tools to move the coil turns, you don't want your body capacitance to skew the tuning. with enough practice, you can know how much movement will be needed based on how close the foil or ferrite is to the coil. and because the coil IS a coil (i.e. in the shape of a spring), expect some backlash in the tuning adjustments as the wire tries to spring bact to it's original position. again, practice will give a feel for how much backlash to expect, and how to compensate for it.