Al, I don’t disagree with you at all, but I do want to make a comment. After having used coax, “open wire” line, and window line in several locations I think I can add to the discussion. Many folks have antenna tuners that “remember” the L/C combination for “segments” of each band they operate on. Those memories are quite handy when one is moving about the bands - change frequency and the tuner dutifully sets the L/C combination to match what was used the last time at that spot.
Unfortunately, there is a big difference in those settings between wet and dry window line. If you’ve “trained” your tuner for a bazillion segments across each ham band in dry conditions, you can rest assured that when it is pouring-ass raining outside those settings go out the window (no pun intended). It is for that reason that, back at my previous QTH in Northern California, I removed 110 feet of window line feeder and replaced it with open wire line on my 88 foot long doublet. I had put the antenna up with window line in the summer and all was fine - until the rainy season started. After the rains started I could forget using the stored memories in my various “auto tuners”. So, yes - window line is OK but be prepared to re “tune” your match when the window line gets wet. I, for one, will not use the stuff again. Jim Bennett / K7TXA Eagle, ID > On Jan 14, 2022, at 11:00 AM, Al Lorona <[email protected]> wrote: > > Whenever somebody mentions "window line" here, one of the first objections > raised is that "when it gets wet, it has very high loss". > > The landmark paper cited is by Wes Stewart, N7WS, which can be found here: > https://www.sadxa.org/n7ws/Ladder_Line.pdf . > > I'm not trying to discredit that paper in any way, but I'd like to point out > two things to keep in mind the next time you hear someone completely and > summarily dismiss window line on the grounds that it has "high loss when > wet", and citing that paper: > > 1. You should note that Wes took measurements at 50 MHz. Window line is > rarely used at that frequency, and anyway the loss has to be extrapolated > back down to the HF region. For instance, if wet window line has a loss of > 5.2 dB at 50 MHz, the loss at 10 MHz is only 2.3 dB and at 3.5 MHz is 1.4 dB. > (See the paper for the equation to extrapolate to another frequency.) 1.4 dB > sounds a heck of a lot better than 5.2, and is on par with 100 feet of the > best coax you can buy. (We are talking only about matched line loss here, and > ignoring additional loss due to SWR.) So that's the first thing. > > 2. Wes's method involved the now legendary use of a "wetting agent" (I assume > soap of some kind) added to the water he sprayed onto his 12-foot sample of > transmission line to simulate rainy conditions on weathered line. To be fair, > Wes himself cautioned that this probably created a worst-case scenario but > judging from other references on the web this wetting agent has been blamed > for excessively inflating the loss to the point of questioning the relevance > of the experiment. DJ0IP and G3TXQ have both claimed that (see > http://www.karinya.net/g3txq/wet_ll/ ). In G3TXQ's case, he was able to > replicate Wes's results by using "a few drops of washing-up liquid" but he > states, "It is not clear whether the high losses associated with using the > wetting agent were due to the more complete wetting of the line or to some > electrical properties of the agent. Nor is it clear how well this test > represents conditions that will be experienced in the 'real world'." > > What's happening to the line when it gets wet, and why should it's > characteristics change at all? To answer this, understand that in balanced > line, the energy is carried in the field between the conductors. Window line > has mostly polyethylene (PE) between the wires which has a dielectric > constant of 2.3, as opposed to air's which is 1.0. Spraying water-- which has > a dielectric constant of 80-- onto PE insulation changes the material > properties between the conductors and, thus, the capacitance. As capacitance > changes, so does everything else: velocity factor, impedance, and loss. One > thing saving us from a catastrophic change in properties is that water tends > to bead up and remain as droplets, and droplets don't like to stay connected > with other droplets. So if you looked at a path perpendicularly across the > surface of the line, you'd see individual, sparse drops rather than a > continuous film of water. > > I couldn't find the dielectric constant of soapy water anywhere on the web -- > but let's assume that the number is close to that of pure water: 80. If you > could somehow give the field a *continuous* path of water from one side to > the other on the window line, you'd have significantly altered the dielectric > constant of the PE; you'd essentially have added a new "path" in "parallel" > with the PE with a dielectric constant some 35 times higher. Although it > would be a very thin layer, it nonetheless could have a significant effect, > as N7WS and G3TXQ have shown. > > We also have an important data point from Bob, K4TAX, showing that the loss > of Bob's window line is pretty much unaffected by rain, although the > electrical length does change. I'll address why this might be later on, but > the point is that this is credible, measured data. Why are these data so > different? > > I did discover that soapy water's pH is above 10 -- possibly as high as 12-- > indicating that there are abundant salts present, probably sodium and/or > potassium. These dissolved salts strongly ionize the water, greatly > increasing its conductivity (and perhaps changing its other electromagnetic > characteristics). On the other hand, rain water is essentially pure, much > closer to distilled water, with relatively few contaminants capable of being > ionized. (See > https://www.waterboards.ca.gov/water_issues/programs/swamp/docs/cwt/guidance/3130en.pdf). > > > Having said all of this, when you study the G3TXQ paper I mentioned earlier, > there's a glaring data point: Steve measured nearly constant, high loss, > using soapy water, *from 7 through 30 MHz*. This confirms that an effect of > the soap is swamping out everything else. In other words, there's something > *in the soap itself* that is skewing the measurements beyond its intended > purpose. He just wanted the soap to make the water cling better, but it ended > up perturbing the very thing he was trying to measure (loss) way too much. > It's a little bit like taking a lot of vitamin D for health, which causes you > to become constipated! > > My conclusion is that using soap to make the water cling to the window line > was a brilliant idea but may have introduced much more error into the > measurement. I believe the trend is correct -- that clinging water will > increase loss-- but the absolute numbers aren't as realistic. G3TXQ's data > also shows loss he measured during moderate rainfall, but the increase is > from about 0.2 dB to 0.35 dB per 100 feet at 10 MHz. This is inconsequential > at the other end of the QSO, and is still far better than the best coax you > can buy. > > I guess one question is, as window line ages and gets attacked by smog, > becomes dirtier and more sun-baked, will it in fact support a continuous film > of water in a rainstorm like soapy water does? Perhaps, in the Arizona desert > or in hot and smoggy Los Angeles? And possibly not in another place that gets > many times more rain? Do hams have to start waxing their transmission lines?! > I forgot to ask Bob the age of his line, but if it's relatively new, maybe > it's still able to shed water effectively? Maybe Bob lives in an area where > stuff left outside stays relatively clean? Maybe all window line isn't > equal... maybe some PE is better than others? I don't know the answers to > any of these questions. > > We may prefer open-wire line (or coax) to window line for good reasons, but I > believe it's rash to dismiss window line on the basis of "its loss goes way > up when wet". Up, yes. Way up? Probably not. High enough to make a big > difference? I doubt it. In planning a new antenna installation, hams should > choose the most convenient transmission line for their needs, and should not > feel guilty nor apprehensive for choosing window line if they see fit to do > so. > > > Al W6LX/4 > > ______________________________________________________________ > 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]
