Don makes an excellent point. RG8X is supposed to be very low loss cable, but it's still 50 ohm transmission line and all low-impedance lines have relatively high losses at high SWRs.
My data shows that RG8X has a loss of 1.0 dB/100 feet at 10 MHz when properly matched. That's quite good, and Roy's installation with 70 feet of line would show only 0.7 dB loss on 30 meters *if matched*. The fact that changing the length of the transmission line changes the impedance at the rig demonstrates that his line is not matched: it's terminated in something other than 50 ohms resistive. The SWR at the rig end is not an accurate indication of the actual SWR on the line, even if the SWR meter is put at the end of the coax after the tuner. That's because the SWR at the rig end will always indicate lower than the actual SWR at the antenna end of the feedline due to the losses in the feeder. To assess losses accurately, the SWR must be measured at the antenna end of the feeder. That's usually not so convenient to do. If that 70 foot length of RG8X is seeing an SWR as low as 3:1 at the antenna, the loss in the feedline will be nominal, about 1.1 dB. But if the SWR is, say, 20:1 at the antenna, the losses in that 70 feet of RG8X will be more than 5 dB! In that case, putting 100 watts into the feedline at the transmitter would result in about 30 watts reaching the radiator. That's still enough two "work the world" when the band is open, so on-air reports are pretty meaningless. Still, it's a huge drop from 100 watts. Ron AC7AC -----Original Message----- Roy, That goes to prove once again that "my feedline tunes may antenna" due to the transformer effects of the feedline length. The major problem with a multi-band antenna is finding a feedline length that provides an adequate match for the tuner on all bands - but I wonder just how high the SWR is on the RG-8X and how much signal is getting lost in that feedline. 73, Don W3FPR _______________________________________________ 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
