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In article ,
"Dave M" wrote: Meindert -- I generally agree with your statement regarding shields. The question I have, though, is as follows Given that the ground for an SSB antenna carries a lot of RF current, and given that all grounds end up tied together at some point aboard a vessel, by leaving the shields ungrounded at one end, isn't there a risk of creating a parasitic antenna out of your signal shields. I have heard reports (not my experience) of the AC safety grounds being capacitively tied back to ground at their other end to avoid this problem. I would think this would only be a problem where the cable approximated a quarter wavelength - which might explain why I have only heard this problem reported on AC grounds, which could grow to be pretty long. Has anyone else in the group experienced such a problem, and if so, what did they do to correct it? Dave Morschhauser RF Grounds should NEVER be used as DC Grounds. RF Grounds should ALWAYS be as short as possible. RF Grounds are not all created equal, and never will be as good as you want, or think they should be. Ground is not Ground, the world around....RF Grounds are different than other Grounds, and should not be confused with these other Grounds. Bruce in alaska more explainations, require more serious study.... -- add a 2 before @ |
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I guess that explains how Aircraft are able to transmit and receive, too.
Thanks Larry, Meindert, and Bruce for your comments. Dave. "Larry" wrote in message ... Bruce in Alaska wrote in news:bruceg- : Ground is not Ground, the world around....RF Grounds are different than other Grounds, and should not be confused with these other Grounds. 1/4 wavelength down the wire from "ground", a voltage reference to which other points are measured, is an OPEN. At this point, induced voltage in the "ground wire" or "shield" is at maximum, and can be high enough to kill around high powered transmitters. As you approach 1/2 wavelength from "ground", once again you find a null of voltage. Again at 3/4 wavelength from "ground", we once again find a maximum voltage lobe that may burn you. We don't HAVE to connect anything in RF to "ground ground", Mother Earth, to get a "ground". If we lay out a piece of wire and keep the end from touching anything conductive, usually by putting an insulator there or using tubing at higher frequencies which requires no insulator and support structures to hold it up...we get an "artificial ground". Look at any common CB "ground plane" antenna and note the "radials" sticking out from the base where the coaxial shield is connected to these "radials" that are 1/4 wavelength long. The end of the radials is open, at least a very high impedance in common air. 1/4 wavelength back from that "open" is the artificial ground, where the coax shield is connected. This point performs at some high altitude just as if the shield were connected to "ground ground", as far as the RF is concerned (but NOT the lightning!). A 1/4 wavelength "sleeve" (beer cans with the ends cut out welded together end to end work great for hams), with the coax going up inside the sleeve but not touching its open end, also makes an artificial ground 1/4 wavelength away from the open end. -- Larry This jerk called my cellphone and was nasty. Continental Warranty -- MCG Enterprises -- Mepco- 24955 Pacific Coast HWY Suite C303 Malibu California 90265 888-244-0925 Fax: 310-456-8844 Email: Read about them he http://www.ripoffreport.com/view.asp...3&view=printer |
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