Dear Aarti,

if you have a single antenna, with a single port, the scattering parameter that you have to consider is the return loss S11, that is nothing else that the reflection coefficient. It gives you informations concerning the antenna matching. Usually a satisfactory match is for S11

Hello,

The Z parameter is used to determine the quality factor of an antenna which can give you an insight about the attainable bandwidth. Z(ant)=R+jX, where R=R(rad)+R(Loss), so you can predict somehow the losses and the efficiency. It could also be useful for determining an equivalent circuit model of the antenna.

Dear Massimo,

The explanation provided by you on S-parameters is completely acceptable. But I am confused about the role of Z-parameters.

Dear Montaha,

Can you refer some article or book on this? Also if I have the Z-parameter plot, how to interpret it?

Dear Aarti,

I would recommend the book Antenna Theory for Balanis. In the 2nd chapter, he tackled the fundamentals of an antenna. Otherwise, I found an interesting chapter on Internet. You can download via the link: http://my.ece.ucsb.edu/York/Bobsclass/201C/Handouts/Chap3.pdf

To make the interpretation of the Z plot simple, you can pass to the smith chart:

Negative or positive values of the antenna's reactance means a capacitive or inductive behaviour.

The peak in the resistance is associated to a resonance (not necessary a matching).

A resistance value of Z0(reference impedance) with tiny value of reactance= matching. (near the SC center).

Knowing these values, you can design the convenient matching network.

The reactance values concern the power that is stored in the antenna(not radiated): magnetic or electric.

Usually, hypothesis for lossless antennas are made, and the Rant=Rrad.

Thus, you can estimate the antenna's efficiency.

Secondly, the variations of the reactance versus the frequency 'translate' the quality factor of the antenna, considering the values of the resistance. You can find the right formula on the paper with the cited link.

I hope the above information will be helpful.

Dear Aarti,

Ifrom the scattering parameter S you can pass through the Z matrix more intuitive from an electromagnetic point of view, however you can consider the other matricial representation namely the Z, Y , S and change they with simple formula. I'll try to better explane the role of the Impedance matrix Z, the Z matrix is an NxN matrix for a single port antenna the single element represent the antenna impedance and it is composed by a real part plus an imaginary term (reactive terms) Z=R +jX. You must keep the X near to zero, and R that is the radiation resistance, it must be equal to the characteristic impedance of the line or the device connected to the antenna to obtain a perfect match. Take a look at the Balanis "Antenna theory ". If you have a multi ports antenna like the example that I provided for the scattering matrix consisting in a TX and a RX antenna you have a 2x2 impedance matrix. The terms along the diagonal represent the so called self impedance terms and they give you information related to the antenna impedance. The terms outsid ethe diagonal represent the mutual impedance terms and they represent the mutual electromagnetic coupling between tha two antenna ports. To obtain the element Zij =Vi / Ij this mean that you must measure the open port voltage at port i, when a current generator is connected toward port j. The other ports must be keep open. Take a look at the following book "microwave engineering" by D. Pozar, it provides a good explanation. I hope that this informations will be useful

Regards

Massimo

Dear Aarti,

As I understand from your question, you are getting two plots.

Most probably you have a frequency dependent plot of real part of Z (R - resistence) and imaginary part of Z (X -reactance).

Z is the ratio between voltage and current at antenna connector, at different frequencies, in complex formalism. That means thaf Z=U(as phasor)/I(as phasor).

Since Z=R+jX, you will have at some frequency on your plot some value. The main thing you need to know is that, in order to have maximum power transfer (either transmitted or received), you have to apply a generator (or receiver) with comlex conjugate impedance (Z*=R-jX) to the antenna connector.

Of course, if the antenna is having negative reactance at some frequency then the antenna reactance is capacitive, and if it is positive then it is inductive, and the adapted impedance has to be with the opposite character.

If you have any other impedance than Z* into the antenna connector, there will be power losses by reflection (see explanation of Massimo about the reflection coefficient). The most desirable situation is when X=0 (no reactance, only resistence), an usually, for most antennas Z=50 ohms, which nowadays is the standard RF cable impedance in most applications (notable exception: CATV and TV aerial reception antenna circuts are using 75 ohm)..

So, the Z parameter - aka the antenna impedance is important because tells you about the quality of the power transfer form antenna to receiver or form transmitter to the antenna whitout an impedance transform, and tells what kind of impedance transform you have to perform in order to achieve maximum power transfer.

If you are talking about cuadripole (two-port network) parameters, maybe you are pointing to this: http://en.wikipedia.org/wiki/Two-port_network. In antenna arrays, it is used a multipole analysis for mutual coupling and impedance, using the Z parameters, but then you need to be more specific in your question. Otherwise, I hope you will find my ansfer usefull.

The Z (or Y) parameter gives you an idea on how to model your antenna. It seems that you are using a resonant antenna. In that case, it can be modeled as a parallel RLC circuit or a Series RLC Circuit. As you know from Network Theory, the impedance of these circuits are frequency dependent. At the resonant frequency, the electric & magnetic energies are equal and hence the reactive part is zero. In terms of impedance, the imaginary part corresponds to the value of reactive element. So at resonance, the value of reactive element is zero, i.e. the imaginary part of the Z-Parameter Plot will cross the frequency axis at resonance. Similarly, either the current (in case of series) or the voltage (in case of parallel) will be maximum (through) across the resistor at the resonant frequency. Accordingly, the real part of the Z-Parameter (i.e. the resistance of the circuit) will show you a dip or peak. A sharp peak (dip) indicates a high Q-circuit & hence a narrow bandwidth. A broader peak (dip) indicates a low Q-circuit & hence a broad bandwidth.

As it seems, you are working with Dr. Pujara. My advice is you should consult with him on these fundamental issues. You may participate in good conferences/workshops and discuss with senior people.

Best wishes.

Prof. Mishra,

Thank you for your answer. I will definitely consider your suggestion.

From Z parameter, you can calculate the antenna input impedance.

The antenna has a resonance at that frequency when the imaginary part of the antenna impedance crosses zero.

At resonance, we should have real part of the antenna impedance only and for proper impedance matching with the feed line which is of 50 Ohm, the antenna impedance should also be zero.

The imaginary part should be zero. And the real part is close to 50