Antenna impedance Za=Rr+RL+jxa

Where:

Rr= Radiation resistance , the losses across Rr is the losses that appears as radiation (useful power)

Average radiated power Pr=0.5*|Iantenna|^2 *Rr

From the equation mention above you must notes that the actually radiated power depends only on Rr .so, I dont think in antenna there is only imaginary part.

Regards.

Hamzah Al-Azzawi

Hamzah,

Thanks. I am not familiar with the antenna definitions. So based on your explanations, I wont be able to reduce the Rr by adding negative resistance and also I wont be able to use the antenna as an inductor. Correct me if I am wrong.

Mehdi,

for ex. If you want cancel the JXl you can use capacitor(-Jxc) to perform this operation because there is phase difference 90° & -90° , but the resistance 0° so can not cancel resistance .

For more details about antenna impedance and matching of antenna referring to the link below.

http://www.antenna-theory.com/tutorial/smith/smithchart5.php

Regards.

Hamzah Al-Azzawi

If you put your antenna in a conducting box so it can't radiate, then it will get rid of the radiation resistance. The box may change the inductance a bit. There will still be resistive losses.

Malcolm, Thanks for your points. It seems reasonable what you have told. I should give a try and simulate it to see how it works. I can put shield around the antenna in Cadence and see how different it is.

Regards,

Mehdi

An antenna inside a conductive box still radiates and/ior couples energy. You can never make an antenna with zero radiation resistance if power is absorbed/coupled. Think about it. Really put on your thinking cap here. What does a zero radiation resistance imply? An open or short does not absorb power. Anytime power is absorbed into a complex load there is a real part associated with the impedance.

The question I have to ask is what are you trying to do? If you need a low value inductor for use at high frequencies, just use a piece of shorted coax of the appropriate length.

George,

Thanks for your response. Generally, I want to know if I can use the same on-chip antenna as an inductor for the oscillator that I have in my circuit. Actually in different period of times, the antenna and oscillator operation are independent, that's why I want to know if I can use it or not?

Thank you. That helps considerably. Not sure how you would go about routing the antenna for dual operation. Would it not be easier to use a stripline inductor for the purpose = shorted length of transmission line?

I mentioned coax in previous response not knowing your application. Any type of transmission line would work configured as an inductor. GH

George Hnatiuk lacks basic understanding of radiation, and I agree with Malcolm White, assuming that

1. Mehdi means that imaginary part Xin of his antenna's input impedance is positive

2. one still has Xin > 0 inside the metallic box under consideration

Your inductive antenna will need to be part of a resonant circuit to radiate effectively. As it is, its impedance is probably so low that you can't drive enough current through it to radiate, without losing much more power in the driver. If it is resonated with a capacitor, you get a high current in the inductor with a higher input impedance so that it radiates more and can be driven more efficiently - provided the capacitor is not too lossy.

If you are going to use the inductor as part of a resonant circuit and don't want it to radiate you will need some kind of shield to stop it radiating, unless the resonant frequency is a lot lower that when it is used as an antenna, in which case the radiation will be much less efficient, and may not be a problem. For small antennas the radiation efficiency is proportional to the ratio of the volume of the antenna to a cubic wavelength.

George - if you put a loop antenna in a large metal box the whole thing becomes a coupled cavity resonator, not an antenna, and does not radiate.

Depending on the size of the box and the frequency the input may see a load, a nearly total reflection (open or short circuit) or anything in between. If the box is small compared to a wavelength then the loop will look like an inductor. Look, for instance, at loop coupling into waveguide.

Malcolm,

That is true. I was trying to give a short answer. I appreciate you taking the time to explain in more detail to our student friend. You gave a good reply. It just seemed to me that if an inductor is needed, a resonance cavity was an unnecessary complication when one could easily just build a simple stripline inductor. It is so hard to answer these questions that often appear in these forums because they are ill posed when written as a one liner.

Upon reading my previous answer I was trying to address our student and not you directly but it did not come across that way - sorry. was in a hurry and did not reread for content. I will go back and correct that.

GH

Thanks George - no problem.