I have just completed a lab wherein I successfully connected two coaxial cables with different impedances, without any attenuation at the connection. That is I worked out the impedances of both coaxial cables which gave zero attenuation (I used a CRO to observe the reflected wave and when there was no reflected wave there was no attenuation) and then calculated the resistor which, when inserted between the coaxial cables, 'fooled' the input signal into thinking the second coaxial cable had the same impedance and hence had matching impedances!

The question is: How does the mere act of inserting a simple resistor (the right one obviously) 'fool' the signal into thinking a cable with the wrong impedance has the right impedance. I find this quite fascinating.

And another question: How are the electric and magnetic fields inside a coaxial cable related to V and I?

oxymoron,

And another question: How are the electric and magnetic fields inside a coaxial cable related to V and I?

It is all a matter of the response of current and voltage to capacitance, resistance and inductance.

In the case of a capacitor Voltage lags current. In the case of an inductor current lags voltage.

A simple phrase helps one remember the functions.

ELI the ICE man.

E (Voltage) leads (COMES BEFORE) I (Current) and I (Current) leads E(Voltage); where "L" is inductance and "C" is Capacitance.

The formula for Impedance is Z = (R^2 + (Xl - Xc)^2)^.5 where R is passive resistance in Ohms, Xl is inductance in henry's and Xc is capacitance in Farads.

You can see that closing a switch to put battery voltage across a capacitor there is initially a large surge of current but no voltage at the terminals. As the capacitor takes on a static charge it builds a counter emf which then causes the voltage across the terminal to build. That is ICE. The opposite is true for an inductor.

R affects the rate at which current flows and hence how fast the capacitor charges which shifts the Impedance. Impedance is dynamic resistance vs R which is passive resistance.

Does that answer your question? If not I'm not sure howelse to present the issue.

Nice pneumonic MacM. The baluns we use to match antenna leads, don't they have a resistor and a small coil? From my electronic classes I understand that impedance is mainly considered as a "dynamic" resistance so some simple matching of effective ohmage can be brought about with just adding a resistor within certain parameters.

Mr Chips,

The baluns we use to match antenna leads, don't they have a resistor and a small coil? From my electronic classes I understand that impedance is mainly considered as a "dynamic" resistance so some simple matching of effective ohmage can be brought about with just adding a resistor within certain parameters.

Basically true but depending on the function of the circuit you might do better to actually measure capacitance and inductance in the frequency range of concern and fine tune those as opposed to just adding resistance. It might be that you would need to remove resistance and that is not as easily done. You would have to tune the dynamic part of the circuit.

Just adding resistance limits current flow and power capacity for example.

Thankyou MacM for taking the time to respond. Unfortunately what you said doesn't quite answer my question.

Interestingly, "ELI the ICE man" is exactly the same pneumonic that my dad taught me! (very handy in ELEC1300).

Anyway, I just need to know why adding a simple resistor in between two coaxial cables with different characteristic impedences makes it look like... wait I think I have it!! The resistor makes the 'finite' coax look like it is infinitely long. Maybe. ?

I will think of this some more.

irresistable pedantry attack

It's mnemonic!

"Pneumonic" means "relating to the lungs".

ahh... that's better