Vic wrote: Right now I'm using an end fire array antenna that I also can use as a 52 ft. circumference loop. Given the very high SWR on the feedline, especially when running the loop on 40M and 80M, I recently swapped out the 18 feet of 450 ohm "window" ladder line between my tuner and the loop, and replaced it with a homemade ladder line made from 3/8" outside diameter copper refrigerator tubing... I immediately noticed much stronger signals after doing the feedline swap. That had me wondering just how much loss I was experiencing with the ladder line, in comparison to my current feedline.
I was thinking about how I could measure this... using the XG1, attaching it to a 10 ft. dipole made of insulated wire, then taping the dipole to the center of the loop opposite the feedline, using the XG1 to generate a weak signal onto the antenna. Then I would measure the signal strength at the K2 speaker output, going through the tuner,(after having first tuned up the antenna to 7.040 without the XG1 attached). Then I would repeat the same experiment with the different feedline and then compare the difference. One of my concerns would be the impact on the XG1 by hooking it up to a 10 ft. or so dipole, and the fact that it would not be transmitting into a 50 ohm load. Is there an easier way to do this? ---------------------------------------- That sounds like a good idea, Vic. One good preliminary test might be to disconnect the loop at the feedline, tie the ends of the feeders together and see if you can detect any signal from the XG1 after it is in place. That's to be sure you aren't picking up anything by any means other than the loop. Turn off the K2's AGC and make sure you aren't overloading the K2 at any point. If you have one, a step attenuator at the antenna connection to the K2 would give you actual dB measurements, based on comparisons, but if you aren't overloading anything and the K2's AGC is off, you can calculate the dB difference by comparing the voltages. I'm a long-time user of open wire line, and I've slipped and called it "lossless" more than once. Compared to coax, it seems so, but it really isn't. And ladder line has a much higher potential for losses than true open wire line because of its tiny conductors. My open wire line feeding my doublet is made of #12 copper wire, and I've occasionally thought of switching to 1/8" or 1/4" copper tubing instead. The issue is, of course, ohmic losses at the high current points. The currents can get really high - many, many amperes of current flowing in the wire - and that leads to high resistive losses in the wires. With true open wire line (mine is spaced with dog-bone insulators at about 3-foot intervals) the insulation losses are miniscule, but the resistive losses can still cause a lot of trouble with high SWR's. One real advantage of open wire (or ladder) line is the relatively high impedance. When 50 ohm coax is looking into, say a 2,000 ohm impedance the SWR is 40:1. A 450 ohm open wire line looking into the same 2000 ohms has an SWR of only 4.5:1. In the other direction if the antenna presents an impedance of, say, 10 ohms, the 50 ohm line shows 4:1 while the 450 ohm line shows 9:1. In either case the 450 ohm line isn't going to show anything like the SWR, or RF current extremes 50 ohm line can experience. So, depending upon the SWRs you have on that feedline, I can readily imagine that you will see an improvement of several dB by switching to a more efficient line. 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
