On Monday, September 06, 2010 02:36:01 pm Chuck Hutton wrote: > I liked your explanation except for the part below. Considering the choke > impedance to be part of a voltage divider along with the ground > resistance, you surely need a choke with at least several thousand Ohms of > impedance. With a poor ground connection (sandy soil), you can need even > more. I won't bore everyone with the explanation. > > Bottom line: I don't think you can ever get even close to that with the > over-the-cable chokes. I can recommend only the chokes made with mini-coax > and high permeability (at least 2000) material.
Ground isn't a part of the issue with common-mode currents on the coaxial cable's outer shield surface. This isn't a matching transformer, or other circuit that may have a connection to the local ground. The choke impedance places itself in series with the currents flowing on the outer surface of the coaxial cable's shield surface. There is no ground connection. Using #31 core material, if you stacked maybe 5 or 6 toroid cores, and assuming that you wound the feedline through the cores so as to minimize capacitance coupling between turns, i.e. very large loops not wound tight on the cores surface and separating the turns radially from each other, you can get in the neighborhood of 3k to 4k of series impedance to common mode currents on the coax shield at 1 MHz with 7 or 8 turns or so. That'll drop as you approach the bottom of the mediumwave band and be worse at longwave. I haven't played with other core materials that might work better there. But you do have good impedance values up to 10 MHz or so. So it's helpful on the tropical bands too. If you need better high frequency performance above 10 MHz, adding more cores and dropping the number of turns is the way to go. But the low frequency performance suffers then. Probably 7 cores and maybe 3 turns might be a good starting point for that. The impedance of a choke core, i.e. the core and one turn, is multiplied by the square of the turns ratio. If you need more impedance, wind more turns. But you have to balance that against loss at the higher end of the frequency range because of the various capacity coupling effects on the choke design, most of which are dependent on the winding style and cable used too. If you have an impedance bridge testing the various configurations and surplus cores and testing stacked cores and various winding ideas and wiring is worth doing. And don't forget that you can series up these chokes along the ends of the cable, and the impedances will add. (series inductive and capacitive reactances, because they are of opposite sign cancel each other), but if you design with this in mind you can still have good wideband performance. For example to get the performance you want you might need one choke to deal with longwave and another for mediumwave through the tropical bands. Just wind 2 separate chokes designed for the frequencies you need and separate them at each end of the cable a little. You'd have 2 chokes at each end of the feedline. Same goes for tubular cores. These can be series placed to increase the impedance. Rarely is one core going to be enough at mediumwave. But again core material is important here. If you are using #31 material, you might see around 1k of series impedance at 1 MHz from 40 1/2-inch long tight fitting beads. Chuck makes an excellent point, it takes a lot more ferrite material in tubular form to get usable impedance than it does when winding turns through a toroidal ferrite core. The good thing is that if you are testing the chokes with the feedline connected to a shielded dummy load, you'll know with testing how things are going. Your station receiver will be plenty of tool to tell you how it's going. Having an impedance bridge is fun, but in the end, it's how it works in place that matters. And a little cutting and trying goes a long way here. Sweep the receiver across the range of frequencies of interest when signals should be there and see what you do or don't hear leaking in. One other point is that you can place turns of Ethernet cable through toroid cores too, creating chokes to reduce the leakage of crud from an Ethernet device like a cable modem or DSL router, an Ethernet switch, or a wireless access point. I actually think this works better than using shielded CAT cable, as the enclosures and connectors (RJ-45) are not designed to be RF tight anyway. And remember the power cords of these devices too. Anything leaving the device can act as a radiating antenna. But again, you need to have enough impedance in the choke you are placing over the cable at the frequency you are trying to suppress, to make the thing work. The best thing is just to start somewhere and see what happens. Experiment with several configurations and work your way to the desired level of suppression that you need. Rick Kunath _______________________________________________ IRCA mailing list [email protected] http://montreal.kotalampi.com/mailman/listinfo/irca Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers For more information: http://www.ircaonline.org To Post a message: [email protected]
