Impedance on BNC Cable

Papabravo

Joined Feb 24, 2006
16,509
Originally posted by windoze killa@Apr 9 2006, 06:52 PM
Firstly, my previous comment was meant in humour.

I taught transmission lines and RF fundamentals for a few years. One thing I must point out about the quotes above is that they refer to the "perfect" transmission line. Of course the is no such thing but we need a starting point.

A point about wave guides. They are more or less the same as any other transmission line. Because of the frequency that they are used at they rely totally on skin effect, that wonderful thing that stops solid conductor coax from working well at really high frequencies.

Another thing a lot of people forget about in RF design is velocity factor and propagation delay. Due to phase shifts through the imaginary LC circuits in a transmission line there is a delay over the length of the line.
[post=16014]Quoted post[/post]​
WK

No worries, I took your comment entirely in that vein. There are lots of things about this stuff that are less than obvious. The first time I saw an impeadance plot on a Smith Chart display I was blown away.
 

pebe

Joined Oct 11, 2004
626
Originally posted by windoze killa@Apr 9 2006, 10:52 PM
Firstly, my previous comment was meant in humour.
I guessed it was.
...Another thing a lot of people forget about in RF design is velocity factor and propagation delay. Due to phase shifts through the imaginary LC circuits in a transmission line there is a delay over the length of the line.
[post=16014]Quoted post[/post]​
Then there's transfer impedance...!
 

CATV

Joined Apr 17, 2007
11
Another method of measuring transmission line impedance is to use a return loss bridge and a sweep generator and detector. Not as expensive as a network analyzer. BTW good network analyzers that can do this job can be had for under USD $10,000. Look up HP 8712. Won't do smith chart though.
 
Hi,

I have been trying to measure the impedance of a normal 50Ohm BNC cable but i get an open reading.

I probed the positive lead to the center connector while the negative lead to the ground (outer surface) of coaxial cable.

Can someone pls advice what is the proper way of measuring them.
Appreciate some feedback

Thanks.
U can measure its return loss or reflection coefficient and u can calculate the impedance of not only BNC cable but all other co-axial cable.But U need some costlier equipments to measure it.In our lab i have done this and i have got the impedance of BNC cables.
 

Ron H

Joined Apr 14, 2005
7,014
There is a way to measure the impedance of a transmission line with reasonable accuracy using a scope and signal generator. Connect the signal generator to the transmission line. Connect a termination load across the opposite end of the transmission line. With the scope, measure the voltage across the signal generator output when connected to the transmission line & load. Then, measure the voltage across the load. Now, subtact the "load voltage" from the "generator voltage". The result is the voltage across the transmission line. Now, work the problem like a proportion;

TML means transmission line,

load voltage / load resistance = TML voltage / TML impedance

so,

TML impedance = (load impedance X TML voltage) / load voltage

The transmission line & load connected across the generator constitutes a series circuit. In this mannor we can use Kirchoff's voltage law that states, "the sum of all voltage drops in a series circuit must equal the applied voltage". In other words, the resistances are proportional to the voltages across them. With this method, We can "treat" the transmission line as a simple resistor. All we are interested in is the level of impedance. The characteristic impedance of the feedline is that level of impedance that will be exhibited by the feedline at any frequency within the specified range of the feedline. If the feedline is designated for use at VHF/UHF frequencies for instance, then the feedline will exhibit a characteristic impedance of rated-ohms throughout that entire range of frequencies.
Huh? A transmission line terminated in Z0 will have the same voltage amplitude at both ends of the line (ignoring losses), with the only difference being delay (phase shift).
 
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