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Function of a airline
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on Apr 15, 2011
on Apr 15, 2011 by Dr_joel
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Can someone help me and explain what the heck is the function of an airline. Is it a precision termination, vswr, precise distance, or what.
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Apr 15, 2011 9:41 AM
There are several functions of an airline (listed in reverse order of usefulness):
1. They help you fly from point A to point B! (but I don't think this is what you are talking about)
2. They are sometimes used just as a low-loss physical extension
3. They are used to add line length to a measurement (helps to add ripple on reflection measurements)
4. They are a
impedance reference for the length of the airline.
Item 4 is the most important and is the whole purpose of the airline...unless you need to travel! They can be made with great precision to be exactly 50 ohms impedance (typically). They also have very low loss. These characteristics are used to help measure/determine specifications at higher frequencies where a fixed load is inadequate. Because of the great precision and stability of well-made airlines, they can be used as 50 ohm transfer standards.
While airlines typically have a precise electrical length specified, this is not a major concern for most applications.
One common application (#3 above) is to use an airline to determine source match. A reflection measurement is made with an airline attached. Some known mismatch is connected to the end of the airline. This will produce ripple in the resulting measurement and, based upon the peak-peak amplitude of the ripple, the source match can be determined.
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Apr 15, 2011 6:42 PM
As Jvall said, the airline is a precision impedance transmission line. As such, it's S11 is zero, and S21 is nearly 1. So, it provides phase change with no amplitude change. If you do a 2 port cal, then re-measure the thru, it's always flat (unless you use unknown thru, then it's the thru loss). If you measure an airline, it should also be perfectly flat, but it will always show ripple. The ripple is a measure of the badness of the calibration.
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