Help! please explain impedance

[pollop]

Hi
I would like to know what impedance is. I know it is similar to resistance but has a complex term.
Also, how would one interpret an impedance graph with seperate lines for real and imaginary?
THANK YOU

 

[A.Anand Srinivasan]

it is the equivalent value of resistance if that particular network is to be modelled as a resistive one... it is dependent on frequency....
on the graph the y component is the imaginary component and doesn't result in power dissipation while the x axis component is the real component and results in power dissipation.... refer about power factor you would get the picture....

 

[pollop]

How about if the x-axis is frequency and the two y-axes are Re and Im? How would you look for the ideal/best frequency of that antenna/system?

 

[A.Anand Srinivasan]

i dont get you.... what do you mean by two y-axes being real and imaginary??? are you talking about the y-axis and z-axis...

assuming that you are asking the second question in terms of attenuation.... you can just use a single y-axis which is marked in terms of impedance.,....

 

[zeeshanzia84]

How about if the x-axis is frequency and the two y-axes are Re and Im? How would you look for the ideal/best frequency of that antenna/system?

The real part of impedance refers to its purely resistive component, which is INDEPENDENT of frequency, so there would be straight line parallel to x-axis for the y-axis labelled ' Re'; whereas the imaginary component is the one dependent on frequency - it refers to the Reactive (inductive/capacitive) component of impedance.

 

[xxargs]

How about if the x-axis is frequency and the two y-axes are Re and Im? How would you look for the ideal/best frequency of that antenna/system?

Look at Smithchart ???

This form of grafical chart is invented around 1930 for make easier calculate on impedance and make more human friendly show of impedance behavior if measure enough sample in different frequency - and now in real time uppdate in VNA is very helpfull i matching and tuning process.

Look also after hp old but famous application note '95'

 

[pollop]

i am talking about a graph something like this

 

[A.Anand Srinivasan]

now i get what you are talking about.... here the real part gives the attenuation that the signal is going to face and the imaginary part gives the phase change that the signal is going to face.... so there might be a frequency where both the attenuation and phase change might be least and in that case it is taken.... else there is a tradeoff involved between phase change and attenuation based on our need.....

 

[pollop]

so in this case, the blue line represents the real part and the red line represents Im. The best frequency here would be ~0.375GHz or ~0.85GHz because the attenuation is least at these frequencies?

 

[A.Anand Srinivasan]

please put the unit for the two axes....
if the real part axis denotes attenuation then you are right.... if the imaginary part axis denotes phase delay then the minimum occuring in the red line can be selected if we require minimum phase delay....

 

[pollop]

I can't seem to find the units in HFSS, I am assuming they are Ohms? How would I find out?
Thanks

Added after 1 minutes:

Look at Smithchart ???

Is there an option to do this using HFSS?