Ferrite antenna design in older portable radios


gary_thorburn <gwt@...>
 

I've accumulated portable radios dating from the 1950s to the present. I've noticed that in general, older radios were usually wound with the turns widely spaced.  I have an old 8-inch ferrite antenna with turns spread out nearly its entire length.  Modern radios typically have the turns closely spaced on a paper form positioned on the rod.  Anyone have ideas about what the difference would be?  I would expect the wider spaced turns would result in less capacitance in the coil, which I think would be a good thing.  But a closely wound coil on a paper form permits sliding the coil on the rod, positioning it for correct inductance. This permits some optimization at assembly time.  Incidentally, I have a working Philco portable, model B-631, from about 1954.  It has a 7-inch long, 17mm diameter ferrite, wound widely. (NOT an ultralight!)  A Philco ad for this radio claimed that it was the first commercially available portable using a ferrite antenna, which they called the "Magnecor".

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