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FT200-2 RF Toroid RF Choke
On Feb 9, 2:24*pm, Jim Lux wrote:
Dave Platt wrote: I used off-the-shelf (surplus-store) ferrite tubes - very probably a 43 mix or something close to it, based on the simple inductance measurements I did with an MFJ analyzer. *They're roughly 1 inch long, and large enough to allow three through-the-center passes of RG-8X coax.. I glued somewhere around six of them, end-to-end, to create a long tube, and then ran the RG-8X through... creating a long three-turn choke. *Added N connectors to the ends of the coax and stuffed the whole thing into a chunk of PVC tube with end-caps. You might do better breaking the thing up into multiple cores and multiple windings. *That is, rather than 3 turns through 6 cores, do 3 turns through 1 core, then 3 turns through another core, then 3 turns, etc. The logic here is that the impedance scales as the number of cores, but doing them all in one shot means that the input coax is very close to the output coax, so you have a lot more capacitance coupling the input to the output. I think Jim K9YC actually did some comparisons using #31 2.4" cores comparing stacking to stringing em out. Jim's comment is a good one from another standpoint he didn't mention explicitly too: if you put a single choke right at the antenna feedpoint, and the feedline is in some sense resonant (e.g. a quarter- wave long with the far end grounded), the fields from the antenna may very well excite currents on the outside of the feedline anyway, even with a "perfect" choke. By putting the separate chokes spaced out along the line, say a quarter-wave apart at the highest operating frequency, you break up resonances that might otherwise occur in the line. By the way, for a single-band choke on a band (10 meters) that was extremely troublesome for me in one installation many years ago, I wound (approx., from memory) 5 turns of the feedline coax into a solenoid coil about 12cm diameter, secured so they were stable. That made a coil of roughly 3 microhenries inductance, self-resonant just a bit above 30MHz. A pure 3uH would only be a little over 500 ohms reactance, but resonated with a wee bit of capacitance across the coil, the result was a very high impedance that solved the problem nicely. I've since found the technique valuable and easy to apply to RG-58C/U type line used at 147MHz--much smaller coils, secured on sections of PVC water pipe. Cheers, Tom |
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