Re: Topband: Fabricated common mode chokes - Sourcing
- Original Message - From: "Jim Brown" To: Sent: Wednesday, August 31, 2011 10:34 AM Subject: Re: Topband: Fabricated common mode chokes - Sourcing > On 8/30/2011 10:30 AM, ZR wrote: >> Did you open up a Cushcraft balun to discover that method? I believe some >> other companies do similar and Teflon wire is used so it doesnt char as >> does >> THNN. > > "These methods" are well documented in the literature, especially Jerry > Sevick's work (W2FMI). In one of his later books, he discussed some > chokes wound with THHN, noting that their Zo was on the order of 100 > ohms. With Sevick you had to wait an edition or two until he made corrections to some of his "assumptions". There was also a series in CQ that were often questionable. There would often be long discussions on the air with folks taking him to task. What I have NOT found in the literature are discussions of the > significance of choking impedance, nor of measurements of choking > impedance. I may have been the first to do that for common mode chokes > outside of the military. Far from it, you havent been looking hard enough. As far back as the 80's it was published in ham journals, particularly the NCJ and some individual club contest journals as the subject was important to contesters. When I was selling large 43 mix beads in 85 I was sent a breakdown of impedance by band. I later did some work on my own of the FT-240 43 and 72/77 cores. At some point in the mid-late 80's it was a presentation at Dayton by one of the top contesters with a strong RF engineering backround; I dont remember who for certain so wont do guesswork but several are on this reflector. A few years ago, after I published my work, > someone sent me a military research document that did so in the 70s (and > reached the same conclusions that I did). I had documentation from Mitre published around 81 when I worked at Wang Labs RF Group and earlier at Sanders Associates (late 70's) who was involved in the Tempest program which include EMI/RFI interference from a wide range of equipment.Lots of ferrites were involved and Fair-Rite was just a small player then. > As to charring -- that's an issue if the device in question is seeing a > lot of dissipation. Properly designed and applied, chokes should NOT > see a lot of dissipation unless they are being used in series/parallel > combinations to do impedance transformation. A strict sleeve balun with no impedance transformation can get very hot when the antenna VSWR increases. Sevick didnt understand the real world when he wrote his assumptions on power handling capabilities based strictly on 50 Ohm lab results. Unfortunately many ham antennas are not 50+J0 over the full band. For this reason Teflon wire or coax is often used when the capability of many readily available amps is involved. Carl KM1H ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
Re: Topband: Fabricated common mode chokes - Sourcing
> As to charring -- that's an issue if the device in question is seeing a > lot of dissipation. Properly designed and applied, chokes should NOT > see a lot of dissipation unless they are being used in series/parallel > combinations to do impedance transformation. > > 73, Jim Brown K9YC OR, if the choking need location NECESSARILY has a lot of voltage to block, and then perhaps a bifilar-isolation transformer wound on a low-mu powdered iron toroid (#2 ideally) would be better. I have had applications where use of a high mu toroid was blocking so much voltage (necessarily so) that it just cracked and burned up the toroid. A careful re-examination of the model, with added long runs of balanced feed lines LITERALLY entered as wires uncovered the voltage, which in one case put close to thirteen hundred volts RF across a MEASURED 4k ohms RESISTIVE choke. One can easily figure out the burned toast factor there. Pop goes the toroid. And lossy, lossy, lossy. Really need to know the magnitude of what you are blocking. 73, Guy ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
Re: Topband: Fabricated common mode chokes - Sourcing
On 8/30/2011 10:30 AM, ZR wrote: > Did you open up a Cushcraft balun to discover that method? I believe some > other companies do similar and Teflon wire is used so it doesnt char as does > THNN. "These methods" are well documented in the literature, especially Jerry Sevick's work (W2FMI). In one of his later books, he discussed some chokes wound with THHN, noting that their Zo was on the order of 100 ohms. What I have NOT found in the literature are discussions of the significance of choking impedance, nor of measurements of choking impedance. I may have been the first to do that for common mode chokes outside of the military. A few years ago, after I published my work, someone sent me a military research document that did so in the 70s (and reached the same conclusions that I did). As to charring -- that's an issue if the device in question is seeing a lot of dissipation. Properly designed and applied, chokes should NOT see a lot of dissipation unless they are being used in series/parallel combinations to do impedance transformation. 73, Jim Brown K9YC ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
Re: Topband: Fabricated common mode chokes - Sourcing
Paul N1BUG said : Mouser has the Fair-Rite 2.4" #31 core for $7 each. That is the lowest price I have found for small quantities. -- Last week I purchased the 2.4" O.D. 31 mix cores (Fair-Rite part number 2631803802) from Arrow Electronics, and the single quantity pricing was $5.32 per part. Shipping was $8.00, but the low piece price more than makes up for the shipping cost (I purchased 5 pieces). Arrow Electronics is a Fair-Rite distributor, and they were the lowest price I could find from a Fair-Rite distributor. Arrow Electronics currently has 473 pieces in stock. 73's Don (WD8DSB) = ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
