Re: Ferrite antenna design in older portable radios

Paul S. in CT

The older and later 'pocket radio' designs were both made with a mix of price and quality. Older designs had weaker transistors/tubes and needed a high quality ferrite coil winding. A pocket model of the 60's had better transistors and could be made physically smaller, the circuit would compensate for the smaller ferrite rod.

Nonetheless, both designs are not optimum, as the most recent studies have shown. I refer to Ben Tounge's (of Blonder-Tounge Radio/TV) Article #29 at his namesakes website. I also note that there were mathematical errors in popularly published catalogs of ferrite material.

So what is a near optimum design one might ask?

1.) Keep the ferrite bar at least 10 times its diameter for nulling properties.
2.) Try to keep the coil winding between 1/2 and 1/3 of the ferrite bar length.
3.) Space the coil winding 3x the wire diameter (in typical designs its usually 2x the diameter). For example #30 gauge wire of 0.01" diameter is wound at 0.03" instead of the typical 0.02". This removes some "proximity effect" especially when many turns are involved.
4.) The wire diameter itself has to be minimized to reduce the "Skin Effect". This seems counter-intuitive because thin wire has more resistance per foot (meter). But, thats DC resistance... we are looking to reduce AC (AM radio signal) resistance. Such AC resistance gets larger when very little current flows in the center of the wire: the smaller diameter wire "saturates" the wire better, with more AM radio signal current flowing in the center of the wire.
5.) Because of #3 and #4 above, a single wire of small gauge (say #30 or better yet #32) can be wound upon the ferrite bar. However, the best choice is still Litz wire based upon #46 gauge strands. Its more expensive, and you do get what you pay for. I will also say that one would be surprised at how good a single #32 wire (0.008") spaced at 0.024" is.
6.) Using a single wire, one can wind directly on the ferrite bar (most of this 'magnet wire' is insulated), but most prefer using a layer of heat-shrink tubing over the ferrite bar.

So, in recap, the older radios got the wide spacing right, and the newer ones got the small gauge wire right, but neither got both right. One thing not mentioned in most commercial radio designs is the actual inductance of this ferrite bar antenna. I do know that some designs are intentionally high up to 700uH for Sony's in the 60's-70's. Modern DSP designs need 350-500uH, and can use short bar lengths (re: small wire diameter).

Paul S. in CT FN31nl

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