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Jerry wrote:
"I`m having trouble visualizing doing this grounding without affecting antenna performance." Shortwave radio antennas I`ve used were all made from Copperweld wire to withstand lightning and weather. Also, copper wire can stretch and fatigue. Copperweld`s steel core prevents this. Signal Corps rhombic kits use (3) No.12 Copperweld wires twisted together to make a cable used for antenna and transmission line. A special Wihd Turbine Company insulator is included to space the line for 600-ohm impedance. These bolt atop short tower secttons used as transmission line supports. Unless military surplus is available, substitutions would be necessary. But, open-wire line is rugged and withstands the challenges. Pick a place outside your shack to drive ground rods to serve as a ground bed for your antenna system to dump your lightning strikes to. Place the rods at about the length of your ground rods away from each other. The more rods, the better. Cost will prevent too many rods. Interconnect all the ground rods and connect this ground system to your electric service ground system. It`s the law in most jurisdictions. Run your open-wire line from your antenna to a point above your ground bed. You need arc-gaps between each transmission line cable and the earth. Form copper vees to make arc-gaps. The vertex of one Vee is going to face another to make a pair. Connect one Vee firectly to the earth. Connect the other of the pair directly to the transmission line cable. Do the sane for the other transmission line cable. When the gaps are completed, adjust the space between them until they flash over from your transmitter power, then back off until they just don`t flash over. You should now be ready for lightning on the transmission line. Connect your ladder line, twin lead, coax or whatever you will use to complete the connection to your radio to your open-wire line here above your ground bed. Isolate the radio from the powerline through a brute-force filter with MOV`s added for lightning suppression. Audio, control, and any other wires connected to the radio also need filters with MOV`s added but the current carrying capacity of the filters can be lower than that required for the power wires in most cases. A common ground point is required for all these filters. If you don`t use coax somewhere between your radio and antenna, you will lose some of the protection it provides. Its close internal spacing couples its conductors tightly. We found even solid-state receiver front-ends weren`t endangered by lightning because of grounded antennas and the coax. It would flash over before it let lightning through. Best regards, Richard Harrison, KB5WZI |
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