Chokes really should be wound nice and tight against the toroid core and given that coax has a minimum bend radius (25 mm for RG58), to do it right I’d want to use something with a tight bending radius, which only really leaves RG-174 and RG-316, none of which I had on hand. So the simple answer is that I just used what I had lying about. While my testing has shown a slightly wider bandwidth of bifilar over coax wound CMCs, it’s not enough to make me choose one over the other.<\/p>\n\n\n\n
Supplies:<\/strong><\/p>\n\n\n\n The process:<\/strong><\/p>\n\n\n\n For the windings, I use CAT 5\/6 cable. The wire pairs are HDPE coated with a characteristic impedance of 100 \u03a9.<\/p>\n\n\n\n At this point you will have a bifilar winding with a characteristic impedance of 100 \u03a9, not quite what we need. By adding a second winding, which we can later wire up in parallel, we can achieve that magic characteristic impedance of 50 \u03a9.<\/p>\n\n\n\n You will have observed that CAT cable has four pairs, and we’ve just used two. My experience has shown that storing the other two pairs for later use is but folly, as they will have long disappeared when the day of need dawns. As such, while you’re at it, just wind another choke.<\/p>\n\n\n\n With the chokes wound, the next step is to verify their insertion loss and common mode attenuation. I’ve already written an article on effectively measuring CMC performance<\/a>.<\/p>\n\n\n\n\n
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