The Ferrite Ferret

       Analog Dial         Ferrite Ferret   Construction #1   Construction #2   Notes             

This is the "power supply" Notice the protecting diode connected to the on/off switch. This
prevents any damage in the event the battery is accidentally put in backward.

I didn't want to ruin my TA7642 by hooking it up wrong,
so I made a diagram of how it should be connected.
The result.

The ferrite rod is mounted away from any other parts of the radio on plastic stands.
The stands were made from a plastic 8MM tape box. The top was sliced off, then the sides of the top were cut off and holes drilled. Note, this box is made of a slightly flexible plastic.

Making the coil
The coil is wrapped around a form that can be slid over the ferrite rod. This one is 62 turns of #30 magnet wire.
The thinner the wire, the less turns you'll need and the more selective the receiver.

You'll need about 64 turns of #24 wire if that's what you have. It's only about six feet of wire, so you won't lose much by experimenting. Make extra turns that you can take off. if you don't have enough turns you'll have to make another coil.


The ferrite rod has a "sweet spot" at each end. The coil must be able to move back and forth to find the optimum location. This will also affect where the stations appear on the dial. Move the tuning capacitor to where you want the lowest station to be found, then slide the coil till it's tuned in.
Insert a paper shim to hold it in place.
Don't glue it, you will want to mess with it later.
While making the coil, I used masking tape to hold the wire in
place, then applied a drop of polyurethane to the windings.

 DON'T drill holes in the coil form and pass the wire through.
The ferrite rod will scrape off the insulation and short it out.


                                      Wait! Don't solder your coil in yet.

Keep your broadcast coil aside and try a six turn coil. You'll hear shortwave broadcasts up to 41 meters and beyond. The TA7642 isn't supposed to be able to do that! It should cut off at 3 megahertz but it works at 7 megahertz. My theory is that the early chips actually did only go to 3 Mhz but modern manufacturing techniques have extended it. NOTE: You'll need a longwire antenna connected to one side of the coil to pick up shortwave.