One thing I forgot to mention: In Olivier F5LVG's DC receiver article back in SPRAT 100, he casually mentioned building a transformer for his mixer by taking two inductors of the appropriate values and GLUING THEM TOGETHER. What a great idea! I had to try it. I did. Picture above. It worked in my Ceramic DC receiver, but the trifilar transformer from Farhan in India worked better. Perhaps the coupling was tighter. But hey, it worked. Three cheers for Olivier.
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ReplyDeleteTransformers is another subject that mystifies many. The simple fact is that any 2 wires in close proximity will share the same magnetic field and act as a "transformer". The closer the wires are, the tighter the coupling. In the physical limit, coupling would be 100% if the wires occupied the same space (an impossibility) - with no core! This is why we often twist the wires into a 'bifilar' or 'trifilar' winding - to enhance the proximity effect. You can actually build a respectable transformer with 2 wires of any arbitrarily short length. The problem with this would be that there would be virtually no inductance in the primary, and so the transformer would be a short circuit to the driving signal. We solve this problem by making the wires longer and usually by winding on a magnetic core. The core not only increases the inductance, it helps couple any magnetic flux that wasn't coupled by proximity of the wires. In the original BITx, Farhan made all his transformers by winding his twisted wires into the shape of a toroid, using rubber faucet washers as the 'core'.
Joe
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