-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Larry Dodson Sent: Wednesday, February 20, 2008 10:26 AM To: elecraft@mailman.qth.net Subject: [Elecraft] Re: Best way to secure RF ground from 2nd floor
I recently tried this on my multi band offest center dipole. 300 ohn twin feed to near ground; box there contailing 4:1 balum which connects via RG8 to rig in second floor room. In the box ( one foot above ground) I connected outer sheild of coxial to horizontal copper earth pipes below ground. Result is that I get good SWR across 80M band and no RF in shack. I am getting good signal reports, but have not had the opportunity to check _______________________________________________ That's a setup that often works well. The balun decouples the twin lead from the coax, so there's little RF current flowing on the outside of the shield (lots of RF INSIDE, but the RF doesn't penetrate the shield). The coax is far enough from the antenna itself that it's not picking up too much RF on the outside by induction. So, even if a high impedance voltage loop happens to appear at the rig end, the high RF voltages are safely shielded inside the coaxial line. The dangers of such an arrangement, as your comment suggests you're aware, are: 1) The behavior of the balun can be quite unpredictable with potentially high losses when the SWR on the line is high, as it probably is on most bands. 2) The coaxial line will have relatively high losses, also due to a high SWR. Your 300 ohm ribbon will also show losses but they are much lower because of its higher impedance. For example, if you terminate the 300 ohm line in a 2000 ohm load it will see an SWR of about 7:1. Terminate the 50 ohm coax in the same 2000 ohm load, and it will see an SWR of 40:1! The ideal solution is a remote antenna tuner, in your case where the balun is located. Then the 50 ohm coax would always have a very low SWR. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
