[Elecraft] [OT] Loss in window line [OPINION] [LONG]

[Al Lorona]

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