Window line is great at HF.? But remember that it consists of tandem sections of transmission lines that have two different characteristic impedances.? When the wavelength is so long that a large number of such sections in tandem will approximate a continuous line, the line will work well.
Another example of such a structure is at voice frequencies.? A telephone cable pair's voice frequency transmission characteristics can be improved significantly by adding series inductances, usually in the old Bell System 88 mH every 6000 feet.? Up to about 3300 Hz, the transmission characteristics are made more uniform and the attenuation is reduced.? But beyond 3300 Hz (approximately) the structure looks like a low pass filter with parallel capacitance and series inductance and the cutoff is suprisingly sharp and abrupt.
Although I've never used window line except at HF, I suspect that there will be some frequency above which the window line will begin looking like a series of different lines of two different characteristic impedances with all sorts of interesting reflections.? Maybe this won't be important below UHF, but it might be something to keep in mind.
73,
Maynard
W6PAP
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On 7/29/21 1:18 PM, Fred Moore wrote:
Interesting questions. How about the same scenario for the 300ohm balanced
windowed line. How would you measure for the 1/2WL of that cable
Fred - N4CLA
On Thu, Jul 29, 2021 at 1:22 PM ward harriman <ward.harriman@...>
wrote:
I'm not exactly sure what you want to measure. I assume you'd like to
measure the velocity factor and the characteristic impedance.
Might I suggest the following:
Build a test fixture to connect the two leads of your 450 ohm line to
BOTH ports of the nanoVNA. This could be just a couple of coax cables whose
shields are tied together and the conductors are 1" (or so) apart.
Calibrate this arrangement.
Connect your piece of 450 ohm line to the center conductors.
Connect a 200 ohm (or something about that) load to the other end.
Measure the resulting impedance across a range of frequencies well
below to well above the desired operating point (See below for 'well
below' and 'well above').
Compute the resulting impedance. (There are two ways to do that, one
using S11 and one using S21. I would recommend the S21 approach)
From that sweep you can determine:
1) The length of the line: The frequency at which the impedance
is 200 ohms tells you the frequency at which the line is 1/2 wave and
therefore the electrical length of the line.
2) The impedance of the line: At twice that frequency the line is
1/4 wave. The 200 ohms will be converted to something like 1k. The
characteristic impedance is just Sqrt(Zmea * 200). (Again, this would be
better measured using the S21 because 1k is a bit high.)
If all you are interested in is the correct length for 1/4 wave, you
can make life a bit easier... no math necessary. Leave the line 'open' and
simply measure S21 and S11. The frequency at which the line is 1/4 wave
will cause the 1/4 wave to be a 'short'. At that frequency, S11 will be
'0' and S21 will be '1'.
If someone knows of any pitfalls to this approach, please let us know!!!
