Hi Gert, What you are suggesting, *** at 1.8 MHz ***, even with 34 of those cores on a 1 meter length coax would have only a few hundred ohms blocking, not nearly enough blocking to do anything serious. It might somewhat knock down back-feed of noise from the house running out to the antenna, but you could always do much better. If you are trying to keep the transmitted RF at your antenna feedpoint from using your feedline as its primary counterpoise, it's far short of what's needed.
For one of my chokes I have six separated turns of RG400 wound through a stack of five FT240 form #31 toroidal cores taped together. That's to keep 1.8 MHz from using a run of feed coax as a very lossy untended parasitic element. The RG400 is a much smaller outside diameter than 393, but RG400 is rated at 7 kW. It is not necessary to use RG 393. If you're worried about the beads heating up and destroying the coax, you're already designed wrong. A well designed choke at ham power levels will not heat up, especially not to the point of destroying ordinary coax. I chose the RG400 because of 7 kW, silvered fine copper wire in shield and center conductor, Teflon dielectric and jacket, it's small, it was designed to be bent (center conductor is stranded, for near right angle turns in aircraft wiring harnesses, it won't cut through the dielectric at a bend), and it is still being manufactured new. RG400 lasts forever, so these longish surplus jumpers with connectors on eBay will always do the job for shack wiring, especially the ones with a male BNC on one end which saves me the hated job of affixing BNC connectors. You need to read and digest K9YC's work: http://audiosystemsgroup.com/RFI-Ham.pdf It's quite more general in scope than just RFI. It's just that RFI is usually making a ham station unworkable, blowing up contests and missing that once in a lifetime DXpedition. It's a hair-on-fire emergency that can finally force a ham to address common mode issues. So Jim stuck that in the title. Best of luck on your project and a merry holiday season. 73, Guy K2AV On Sat, Dec 23, 2017 at 1:27 PM, ON4GPE <[email protected]> wrote: > Hi All, > > I would like to build a good high power common mode choke for 160m. > I have a piece of RG393/U ptfe 10mm coax. (1 meter) > I was thinking of using this and make me a "W2DU style" choke. > Searching for the ferrite cable cores, i found on the Mouser website 2 > types which would fit: > > Fair-rite 31 material : https://www.mouser.be/ProductDetail/623-2631626402 > Laird LF material : https://www.mouser.be/ProductDetail/875-LFB187102-000 > > Laird got a higher impedance at 1.8 Mhz. > > I think a lot of ham's on this topband reflector got more experience, so > what's your advice ? > > Thanks, Gert ON4GPE > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband _________________ Topband Reflector Archives - http://www.contesting.com/_topband
