Richard, WD8AJD asked: Question Ron. I have seen/heard of a grounding system that uses coax. The inner conductor and shield are shorted together at the rig(I think). The inner conductor is attached to the grounding rod. The shield is floated at this end. Would this accomplish what you are describing??
-------------------------------------------------- Hi, Richard - I've not seen that myself, but it should work. All that's needed for an RF ground using a 1/4 wave length of wire is to have the conductor 1/4 wave long and floating (insulated) at the far end. That shield would accomplish that just like a separate wire would provided: 1) The far end is insulated and 2) It is electrically 1/4 wave long. The second item means you have to take into consideration the velocity factor of the coax. Physically, it'll be a bit shorter than a "free space" 1/4 wavelength. For coaxial lines that is typically somewhere in the range of 85% to 95% of the "free space" length. I'm not sure whether distributed capacitance between the shield and center conductor will also effect the length or whether that's part of the velocity factor compensation, but the length is usually not really critical. So you'd cut the shield off at a length of 1/4 wavelength from the rig and the center conductor would continue on as needed to the earth ground. The fact is that a wire running around the shack baseboards or somewhere else close to other objects will be electrically 1/4 wave long when slightly shorter than the expected "free space" 1/4 wavelength, thanks to the loading effect of the nearby objects. Either sort of ground can be pruned for optimum length by monitoring the current flowing into it at the rig. That requires an RF ammeter, but it can be as simple a small flashlight bulb in series with the wire right where it attaches to the rig. Keeping output power constant, prune the wire for maximum brilliance. Maximum brilliance indicates maximum current which results when the impedance and RF voltage is at minimum. However, in most cases such fine tuning isn't really needed to keep the RF voltage on the rig down to a level that doesn't cause trouble. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
