Hi Victor, > Using them a lot between VHF and UHF boxes to break ground RF paths.
Beads and common mode chokes only modify common mode impedance on the outside of cable shield. They do this by allowing a voltage difference to appear across the bead or choke. This means the cable shield going in floats at a different RF potential than the cable shield exiting. It is almost never a good thing to have potential differences across cable shields on a desk full of lower-level lines. If any of us have bothersome or significant RF currents in our shacks on the outside of cables, we really should learn why the RF is there and fix the real problem. The last thing we should be doing is floating ground connections (which is what the outside of the shield provides) between different equipment on a desk. The most we can expect from isolating the outer shield path is moving equalizing currents to other cables and wires between gear. Beads over transmitting cables between major equipment just isn't a good idea at all as a general practice. > Really helps to prevents all kinds of unpredictable oscillation effects. If the equipment system changes by moving wires around, or changing the impedance of the shield outside, there must be a problem with connector mounting and shield impedance, or poor cabinet design. As for equipment, there are a few cases of terrible cabinets. At least one commercial antenna tuner has an intentionally insulated cover, and another tuner uses a single core 4:1 current balun that forces output lines into significant voltage unbalance. No one can dispute there are occasional serious errors in equipment design. The correction for those and other errors is not in grounding connections to a ground buss, or isolators, or strings of beads. The fix is correcting the defect in the gear. Otherwise, when we throw an isolator or beads on cables, we simply move the problem someplace else in the wiring where it waits to cause problems in the future. Isolators have a place in the world of antennas and feedlines outside the ham shack. They can be important when a system is neither perfectly balanced nor perfectly unbalanced. Marconi verticals with less than perfect grounds are examples of systems operating in the netherworld between perfectly unbalanced and perfectly balanced. But in cases like that, we want the isolation that allows voltages to be different along a section of coaxial cable OUTSIDE the house, not inside the house where noise and devices sensitive to RF live. Inside the house, we really want all cabinets and device chassis to have the same RF potential. We do not want to isolate cabinets from each other, allowing them to float to different potentials to ground. If throwing beads or isolators on an RF line inside the shack changes something, that line has a problem that needs fixed or at least needs understood. The only beads in my shacks on any RF cables are on receiver leads that have phono plugs, because the layout of the plugs and the shield connections are less than ideal. I live with those connections because that is the type of connector used. I understand what the problem is and choose to work around it, and I keep an eye on it. I can't imagine having connections like that on a 100-watt transmitting line, let alone 1500 watts. Transmitters deserve real connectors mounted properly, and cabinets with RF integrity at joints. This is all measureable and rationally explainable. It isn't voodoo science, like stringing beads everywhere to "make signals stay inside cables". Beads don't make anything stay inside something, they allow a larger longitudinal voltage difference to occur along a small length of conductor. We want that between radials and a coax feedline shield outside the house, but why would we intentionally want two shield ends of a shielded RF cable between two cabinets to have greater potential difference on our desks? We really should never want shields to be that way, if we have any choice at all. 73 Tom _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK