Measure Cable: Balanced Line?

[ae4cw]

Bob,

Is the 4170 capable of measuring balanced line (the Measure Cable function)? I have been attempting to do so using an Elecraft BL-2 balun set for 1:1 and using a calibration file that calibrates through the output terminals of the balun. I have 9 inches of RG-58x between the 4170 and the balun. The test case is 20 feet of 300 ohm (nominal) ladder line.

I have been getting very inconsistent results:
1) sometimes the results look almost reasonable, e.g. wavelength and velocity factor close but not mutually correct vs. frequency, Z of 267.570 + j 2.062, loss about right.
2) a repeat of a reasonable result test sometimes yields VF's of .500, Z in the 160 range and high loss.
3) at other time the impedances bounce above the nominal 300 ohms.
4) the function does not seem to recognize starting with an open line; it must be shorted at the beginning to get reasonable results (sometimes).

I tried creating a custom cal but the measure cable function would not work at all with that.

Perhaps I'm doing something completely wrong or the 4170 just doesn't support measurement of balanced line, and figured it was time to ask the expert.

[w0qe]

I have used unbalanced VNAs to measure the characteristic impedance and loss of balanced lines many times. Calibrate the AIM4170 at the unit (no cable) in either standard or custom mode. Solder the 300? ohm line across a male BNC connector and plug it into the AIM4170. Drape the balanced line or hang it so it is not in contact with anything. Use string or fishing line. You may want to sweep a range of frequencies as the best answers occur when the line is not a multiple a quarter wavelength. It is possible that the characteristic impedance is a function of frequency but it changes very little of the HF range for normal lines.

Test#1: Short the far end of the balanced line and read the impedance at the AIM4170. Call the answer A=jB.

Test#2: Open the far end of the balanced line and read the impedance at the AIM4170. Call the answer C=jD.

The characteristic impedance of the open wire line is the square root of the product of (A+jB) times (C+jD), or SQRT( (AC - BD) +j(AD + BC) ). You must to the math with the complex numbers for this to work. Generally the product is almost totally real as the characteristic impedance of most transmission lines is not very reactive. This formula works no matter how much loss the line has. To determine the line loss use a program like WinSmith or TLW (free in ARRL handbook) once the characteristic impedance of the line is known. This would be a good validation of the Measure Cable function. 73, Larry, W0QE

[dl4raj]

Hi,

the best line length for that measurement is lambda/8.

73
Clemens
DL4RAJ

[ae4cw]

Larry (W0QE),

thanks for your procedure for measuring the impedance of balanced lines. I'll be away from home for the next week, so can't test the procedure until then. However, regarding the VF and loss: I've used TLW but don't see a way to determine the loss and/or velocity factor for an unknown line. The line in question is the 18 gauge 300 ohm ladder line supplied by The Wireman and not listed in the TLW cable drop-down. Wireman lists the VF as .91 but has no information on the loss. I'm interested in measuring both if possible. Can you clarify this, please?

Clemens, I'll start with 1/8 lambda for the impedance test.

Thanks to both!!

[w0qe]

AE4CW,

Let's start with what you know which is the physical length of the cable. By doing the test I mentioned above you can determine the feedline characteristic impedance and by looking at the plot of Xs with either a short or open at the far end can determine what frequency is an electrical quarter or half wavelength (and of course multiples). From this you can determine VF. Now you have 2 of the basic 3 parameters of transmission lines with only loss remaining to calculate. By the way at HF and low VHF loss is nearly exactly proportional to the square root of frequency so you can extrapolate if needed when done.

Now you can load TLW and enter a "custom" transmission line which has the proper length, characteristic impedance, and velocity factor and make a couple of guesses at the loss and with the line shorted or open compare the TLW predicted impedance to what you measured with the AIM4170. A couple of guesses at the loss will get you a very good idea what the real loss is. This sounds harder than it really is. Wes Stewart N7WS had measured many open wire lines and a search will find his work. He published a controversial paper showing a dramatic increase in window ladder line and solid covered twin lead when wet a few years ago. His results were probably worst case but they demonstrate the varaiability in loss with weather. True open wire line and of course coax do not have these problems. If the procedure isn't clear enough you can contact me directly when you return home. 73, Larry, W0QE

[ae4cw]

Larry, got it. Thanks for taking the explanation a bit further. I'll reply here in about 10 day with my results.

73, Chuck