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Author Topic: Testing Integrity of Window line?  (Read 296 times)
KK7KZ
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Posts: 464




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« on: October 17, 2009, 04:08:13 AM »

Is there a proper way to test the integrity of a long length of 450 window line for leakage? Say, for instance, one has a 200' run of window line that has been up for five years. Is there is way to measure the leakage/losses and how does one know when it is time to replace it with new?

Thanks!

Ron
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W4BQF
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« Reply #1 on: October 17, 2009, 05:14:42 AM »

Leakage? What is that? If the window line has been up for a long time, probably the best you can do is clean the line with a wet cloth or some very mild solution of cleaner that will not damage the cover.

If you mean by 'leakage', radiation, it doesn't work that way. To ensure your window line does not radiate, you have to make sure that phase of the signal on each side of the window line is exactly 180 degrees out of phase. The most simple way to do that is by making sure each side of your antenna is EXACTLY the same physical length.

Tom - W4BQF
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KG4RUL
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« Reply #2 on: October 17, 2009, 05:35:37 AM »

There is an article regarding 'window line' in the November QST.  Their experience seems to indicate that losses in older line are fairly small compared to new.
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KK7KZ
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« Reply #3 on: October 17, 2009, 07:14:36 AM »

I am concerned about two performance issues using window line:

One, the aging factor and possible losses/leakage caused by dirt, contamination and the general problems associated with weathering.

Two, for a window line fed system, how does one determine the remaining power available/efficiency at the antenna when virtually all the wattmeters are designed for 50 ohm systems?

I can easily determine the power loss in coax by simply using a wattmeter and dummy load at the end of the feedline. With known power in and measured power at the antenna feedpoint I know what losses I have in my system.

How is this done in a window line system?

Ron
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KK7KZ
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« Reply #4 on: October 17, 2009, 07:21:04 AM »

One other point: I recall reading somewhere (I think it was in ON4UN's book but I can't seem to locate it) that the integrity of a window line system could be tested by breaking the connection at the antenna and then using the tuner to try and load the line. One would normally expect the SWR to be infinity but leakage factors and losses would bring that down to some real number like 5 or 6 to 1 which would suggest the system is usable. Less than that would mean it is time to change out the feedline.

Anyone else read this or know about this?

Ron
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N3OX
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« Reply #5 on: October 17, 2009, 08:35:03 AM »

"How is this done in a window line system? "

You could get a 450 ohm power resistor and dunk it in a known mass of mineral oil in a well insulated container and apply 100W to the other end of the line using your tuner to match the transmitter end of the line to 50 ohms.

Then you measure the temperature rise over time and calculate the power dissipated in the 450 ohm resistor (power dissipated = mass  * specific heat capacity * temperature rise / time of measurement).

You can even compare with the results you get when you only use a foot or two of line into that setup, just to have a second opinion regarding your wattmeter :-) .  Then compare with the published matched line loss.  

Of course, an easier way if you have a couple cores lying around might be to build a pair of 9:1 baluns and measure the difference in power throughput for the pair direct connected and with the line in between and compare to the matched line loss.

Unfortunately, neither of these things will tell you if you have problems with losses in wet grime in a very high electric field situation with a mismatch ... but that sort of problem should go away if the line is clean.

True conductive leakage (which, by the way, I don't think is very likely) and most other problems would theoretically show up in a matched line test.  However, matched line loss is SO low that measurement inaccuracy could easily swamp the line loss.  

Eventually I'd probably conclude that it's too much work vs. buying new ladder line if it was keeping me up at night :-)

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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