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Post by digital1955 on Jan 3, 2012 20:54:24 GMT -5
I have a 4170C. I need to measure the Zo of some low impedance balanced line that I made up. I hooked up some short clip leads to a PL-259 which I am directly hooking on to the balanced line under test. The balanced line is around six feet long and unterminated. The AIM reports the Zo is very close to what I would expect it in my case, around 90 Ohms. If I test RG-8 coax, it also usually comes out within a few ohms of 50. I tested some commercial 300 Ohm line I had and its only reporting that its 209 Ohms. In all cases, the balanced line is suspended between to points with open air about 4 feet around it.
Is there anything wrong with my hookup or technique? I am shooting for the most accurate measurement possible, so the 300 Ohm commercial line reading 209 ohms puzzles me. Can I am improve my technique or is it really possible the 300 Ohm line is just off that much?
-Scott
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k9fv
New Member
Posts: 37
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Post by k9fv on Jan 4, 2012 18:46:04 GMT -5
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Post by digital1955 on Jan 4, 2012 19:46:18 GMT -5
Ok thanks Ken. I found that post through google once long ago, but I was using the forum search and didn't find it. I will make up a proper balun, calibrate and retry the 300 ohm line. Thanks -Scott. |
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Post by Bob on Jan 4, 2012 21:57:46 GMT -5
Hi Scott,
Looks like you're cooking now.
I'll take this opportunity to mention that putting the calibration loads (short,open,resistor) after the balun will compensate for imperfections in the balun itself. In the HF range, a resistor with short leads will be okay. It doesn't have to be 50 ohms. A good quality RF resistor (even a 1/4W carbon comp) will be ok. Measure it with an accurate ohmmeter and enter that value at the prompt during calibration.
To minimize stray capacitance to ground, the whole test system should be as compact as possible. Operating the AIM and the computer on battery power is best. Use cables as short as possible for interconnections.
Putting toroids on the power and communication cables may help.
--73/Bob
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Post by digital1955 on Jan 4, 2012 23:09:26 GMT -5
Do you have any recommendation on how long the test piece should be? I was using 6 feet, but this stuff will be used in the HF range (but it will be very very short, like 2 feet long max). Thanks, Scott Hi Scott, Looks like you're cooking now. I'll take this opportunity to mention that putting the calibration loads (short,open,resistor) after the balun will compensate for imperfections in the balun itself. In the HF range, a resistor with short leads will be okay. It doesn't have to be 50 ohms. A good quality RF resistor (even a 1/4W carbon comp) will be ok. Measure it with an accurate ohmmeter and enter that value at the prompt during calibration. To minimize stray capacitance to ground, the whole test system should be as compact as possible. Operating the AIM and the computer on battery power is best. Use cables as short as possible for interconnections. Putting toroids on the power and communication cables may help. --73/Bob |
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Post by Bob on Jan 5, 2012 11:57:53 GMT -5
Hi Scott,
6 feet should be ok.
For another check, you can put a resistor (Rload) of around 200 ohms at the end of the line and see what the swr is. Z0 approx = SWR*Rload.
-- 73/Bob
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