For clarity: every line has loss. Every single line. What should have been better implied is that coax exhibits a significantly higher loss per foot than window line; while both have losses. This is well proven over decades of use.
High SWR affects the losses in a negative way (the losses increase). Since coax already has a higher loss, it can be substantially higher with the same high SWR than window line. So, when a mixed feed system is used, it makes sense to minimize the losses through the coax to reduce the overall losses; keep the coax short. Side notes: since coax is often more subject to bending than window/ladder line, the internal spacing between elements will vary, potentially increasing the risk of failure while handling high RF voltages. On the flip side, precip or nearby objects (rain gutter) affect window/ladder line, sometimes dramatically. Result: Use "the good stuff" and keep everything as isolated from unrelated objects as possible. As I've said many times; every station is a collection of compromises. In making any choice, one reduces the potential options. In my case, operational ability and efficiency were brought to an acceptable (to me) level through these choices and limitations. I started with 340' feet of wire, center fed (opposing 5/8 wave at 75 meters) and 70' of window line. Changing to 100' of window line had no significant effect (it changed the tuner values but not where I could operate with an acceptable SWR). Changing from 10' of coax from rig to the CMC (common mode choke, typically incorrectly called a current balun, used at the shack window) to ~35' has a significant change in tuning and band use ability. It allows me to operate above 300W on the bottom end of 80 meters when 10' makes it dicey (tuner matches, but can't handle/maintain the match above 300 watts). So it isn't impossible, it's empirically determined. Changing the 1:1 CMC to a 4:1 CMC (same everything otherwise) added low power (<200 watts) 160 meter ability (2:1 or worse, tuned) and substantially lowered the unmatched (bypassed) SWR on all bands. If what I read is accurate, the 4:1 also has a slightly higher loss in efficiency than the 1:1. It conveniently converts window line to coax (my preference in a shack) in this model. [I'm still planning on adding 12 meters more wire (17.5' each leg) to the antenna to lower the resonant frequency on 80 meters. It should also help match on 160, but may cost me elsewhere.] Ideally the KAT500 would be balanced feed capable with a CMC built in. I have a couple capable balanced feed tuners, but they're manual only, not useful for remote ops (about a quarter of my radio time). But then also ideally, I'd be wealthy, not live in an HOA and CCR infested development, not near an airport runway, in a ham tower friendly county with tall trees and salt water nearby. ;-) choices:options I do really need to get a good antenna analyzer, but for now my signal is heard and I hear with minimal noise, thanks in part to the CMC (the DX Eng. 10 KW model, which should be bullet proof for the KPA500 and high SWR). The rest is all Elecraft, it really is "the good stuff". ;-) 73, Rick wa6nhc Tiny iPhone 5 keypad, typos are inevitable > On May 17, 2014, at 2:29 PM, "Wes (N7WS)" <[email protected]> wrote: > > Rick claims that when using a series connection of two different transmission > lines, one coaxial, one window line, changing the length of one (coax) > affects something or the other, while changing the length of the other > portion (window line) has no effect whatsoever. > > This is clearly impossible. I suggested earlier that one possibility is > common-mode current on the coax that is confusing the instrumentation. Just > because a choke is applied doesn't mean that it is actually effective. > > Rick should not be faulted for assuming that the window line is "low loss" > while the coax is not. I have been trying to debunk this myth since 1993 > when, "The Lure of the Ladder Line" was published in QST. At that time, I > had lots of (snail mail!) correspondence with then Antenna Book editor, Dean > Straw, about this. I pointed out that the chart of line loss vs. frequency > that had appeared in every ARRL Handbook and Antenna Book since antiquity was > wrong (easily noted by inspection). We collaborated on a revision of this > chart and I was invited to write something about balanced line use. See: > http://k6mhe.com/n7ws/Ladder_Line.pdf published in the ARRL Antenna > Compendium Vol 6. > > I should mention that during this correspondence I pointed out that tuner and > balun losses should not be ignored because they could be as detrimental as > line loss. I had access to Touchstone, an early professional circuit analysis > program, and gave examples of (IMHO) excessively high tuner losses that > resulted from low Q components and the misadjustment of Tee type tuners. > (Regrettably, I don't have copies of these letters any more.) After doing so, > a QST favorite author, Frank Witt had a two-part article about tuner losses > published in the April and May 1995, QST. (I wasn't invited to write this > one) This month's "Hands-On Radio" column in QST brings up these losses > again. > > Straw wrote the program TLW, bundled with the ARRL Antenna Book, which > (again) underestimated window line loss. ARRL has just admitted this and > offered a new version that allegedly fixes the problem. (I don't know, I > don't have a new enough version to qualify for the update, and I wouldn't use > it anyway. AC6LA's programs (ac6la.com) are superior and highly recommended.) > > Sorry to ramble on this somewhat off-topic thread but I think this is worth > mentioning. > > Wes N7WS > >> On 5/15/2014 12:54 PM, Rick Bates, WA6NHC wrote: >> [snip] >> >> The coax portion of the feed should be as SHORT as possible, in my case it >> is currently about 10' (2.8 meters). The losses are highest there and the >> extreme SWR makes it MUCH worse, keep it short; use the best stuff, not the >> cheap stuff. Changing the length of the coax portion has a HUGE impact on >> where (or if) the system tunes; the window line, none. > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
