Electronics World Cover,TOC,and list of posted Popular Electronics articles QST Radio & TV News Radio-Craft Radio-Electronics Short Wave Craft Wireless World About RF Cafe RF Cafe Homepage RF Cafe in Morse Code Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs Twitter LinkedIn Crosswords Engineering Humor Kirt's Cogitations Engineering Event Calendar RF Engineering Quizzes AN/MPN-14 Radar 5CCG Notable Quotes App Notes Calculators Education Magazines Software,T-Shirts,Coffee Mugs Articles - submitted by RF Cafe visitors Simulators Technical Writings RF Cafe Archives Test Notes Wireless System Designer RF Stencils for Visio Shapes for Word Search RF Cafe Sitemap Advertising Facebook RF Cafe Forums Thank you for visiting RF Cafe!

Measurement of Cable Impedance - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

 Post subject: Measurement of cable impedance
Posted: Mon Jan 15, 2007 6:23 am 

Joined: Mon Jun 19, 2006 10:27 am
Posts: 13

characteristic impedance of a finite length of line at a given frequency can be derived by taking two impedance measurements, one with the end open circuit and the other with the end short circuited. From these, Zo is calculated as follows:

Z_0 = SQRT ( Z_short * Z_open )

Anyone knows how to derive this equation ?

 Post subject: Measurement of Cable Impedance
Posted: Tue Jan 16, 2007 7:10 am 
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 268
Location: London UK
Most standard textbooks on line transmission cover this. It is derived from the T equivalent circuit of the transmission line, having total series impedance of Z1 and shunt impedance z2, correctly terminated in Z0.

Looking from the left end as the input with the right end short circuited, the input impedance is

Zsc = Z1/2 + Z1*Z2/(Z1 + 2*Z2) .............1

Open circuited, the input impedance at left is

Zoc = Z1/2 + Z2 ......................................2

The Characteristic Impedance is given by

Z0 = SQRT( Z1*Z2 + Z1^2/4) ..................3

which needs a separate proof.

If equations 1 and 2 are multiplied together you get

Zoc * Zsc = Z1^2/4 + Z1 * Z2 = Z0^2

Thus Z0 = SQRT (Zsc * Zoc)

My textbook is an ancient Everitt "Communication Engineering" written with a quill pen on parchment. :-)

 Post subject:
Posted: Tue Jan 30, 2007 12:57 pm 
User avatar

Joined: Tue Dec 26, 2006 5:39 pm
Posts: 25
Here is a quick way to test.

Connect to your network analyzer or impedance meter. Find the frequency at which the cable is 1/4 or 1/2 wavelength. Then adjust your network analyzer or impedance meter to a frequency so that your cable is 1/8 wavelength.

Note the reactance when you short or open the far end will be equal and opposite. The magnitude of reactance you measure equals the characteristic impedance. Example, you measure 0.5 +j42 and 0.5 -j42 you now know that the cable Zo is 42.

On a vector network analyzer you can do this really quickly without need to KNOW your frequency. Set the analyzer to 0 Hz sweep and polar display. Keep alternating between shorted and open at the far end and watch the display. Vary the frequency until you see that the capacitive reactance is the same as the inductive reactance (the 'dots' on the CRT will be exactly vertically aligned) as you switch from short to open. Then read the reactance and you have the answer.

For example I have found rolls of RG58 as low as 42 ohm and as high as 62 ohm. It is a lesson to never pay attention to "RG" numbers ever again.

 Post subject:
Posted: Fri Mar 09, 2007 5:34 am 

Joined: Mon Jun 19, 2006 10:27 am
Posts: 13
thank you nubbage and wb9jtk !

Posted  11/12/2012

RF Cafe Software

   Wireless System Designer - RF Cafe
Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chartâ„¢ for Visio
Smith Chartâ„¢ for Excel

About RF Cafe

Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2022
Kirt Blattenberger,

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:

Try Using SEARCH
to Find What You Need. 
There are 1,000s of Pages Indexed on RF Cafe !