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Design >> RF Design >> circuit representation of antenna as a source
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Message started by aaron_do on Apr 8^th, 2010, 7:14am

Title: circuit representation of antenna as a source
Post by aaron_do on Apr 8^th, 2010, 7:14am

Hi all,

in order to represent the antenna as a source, we normally use a Thevenin equivalent model. If we are only interested in the effect on the load, we could just as equally use a Norton equivalent model.

My question is, if we are interested in the power dissipation in the source, is it more accurate to use the Thevenin model, the Norton model or a mixed model like the attachment. Note that in the Thevenin model, when the load is infinite (open circuit), the power dissipated in the source is zero. In the Norton model, when the load is zero (short circuit), the power dissipated in the source is zero. In the mixed model, if Is = Vs/Rs then the power is always the same and is equal to the available power (I think).

I know that the answer depends on what kind of source we are talking about, so here I am asking about a typical antenna. Does it also depend on the type of antenna? My thinking is that since radiation involves transmission of power, the antenna must be capturing power. Therefore it is more correct to use the model below.

thanks,
Aaron

Title: Re: circuit representation of antenna as a source
Post by loose-electron on Apr 9^th, 2010, 4:04pm

Monsier Thevienen = Mister Norton when on the outside. What's the big deal?

Title: Re: circuit representation of antenna as a source
Post by aaron_do on Apr 9^th, 2010, 7:47pm

Hi,

I understand that, but to satisfy my own curiousity, which representation is more physically correct for an antenna. i.e. does the antenna receive power, or a voltage, or a current?

cheers,
Aaron

Title: Re: circuit representation of antenna as a source
Post by RFICDUDE on Apr 10^th, 2010, 5:25am

I think the way to think about an antenna is that it transforms electromagnetic wave power into electric (or EM) wave power on a conductor(s) which we want to flow between a port(s) connected to our circuits.

The EM wave in freespace can be described equivalently as a power density (dBm/m^2), electric field density or magnetic field density. They are all directly related through the freespace impedance, so any of these is valid (Thevenin and Norton work for freespace too).

The important thing to remember when modeling antennas is that there is an lossless impedance transformation from freespace to our electrical port that is related to how well the antenna is coupling to the EM wave (radiation resistance). The radiation resistance looks like an internal source impedance in series (Thevenin) with any resistive losses in the antenna, but the radiation resistance is not a thermal resistance (no noise contribution if you are worried about modeling antenna SNR). The antenna resistive losses do add kT thermal noise.

An ideal antenna would have 100% effeciency in converting the EM wave to the electrical wave measured at the port, so there would be just a radiation resistance in series with the source (or shunt for Norton). If there are losses, then those need to be added.

Here is a link to page that has a good description of the model for LF applications. It has a good SPICE model showing all the different parts of the model. I have tried it out and it seems to work well.

http://sidstation.lionelloudet.homedns.org/antenna-theory-en.xhtml

Notice that they have a conversion between electric and magnetic field strength (since they are considering the loop antenna to convert a B field into an EMF at the antenna port). But you could just as easily plug in a power density and calculate the E or B field.

I hope others have comments too, I'm trying to understand this better myself. ;)

Title: Re: circuit representation of antenna as a source
Post by aaron_do on Apr 11^th, 2010, 4:27am

Thanks for the reply. I guess it just completely depends on what kind of antenna we're using.

cheers,
Aaron

Title: Re: circuit representation of antenna as a source
Post by RFICDUDE on Apr 12^th, 2010, 6:20pm

How does it depend on what kind of antenna we are using (just curious)?

Also, I don't think it is ok to have both a Vs and a Is, rather it is one form or the other (Vs with a series R or Is with a shunt R).

Title: Re: circuit representation of antenna as a source
Post by aaron_do on Apr 13^th, 2010, 3:06am

Hi,

the EM wave is a power wave so I figure the antenna could theoretically be designed so that it converts the EM power into a current. I don't know exactly what kind of antennas are out there though...

As for the Norton or Thevenin equivalent, I think you can get a mixed model of the source just by assigning part of the voltage as a norton equivalent and part as a thevenin equivalent. From the load point of view they are the same...

cheers,
Aaron