Hi Arka

Take a look into the "balance" folder under 1D results. It shows the energy part that is lost - either by radiation or ordinary losses. There you can find out what's going on with your structure.

Another thing that strikes me on this picture is this large waveguide port on your cylinder. Are you getting the correct modes - I mean the mode(s) you expect - at it? Because other possible problem may be that CST is not getting the correct mode on the port, or the energy is being coupled to other modes different than the one you expect.

good luck

marcelo

Prof. Perotoni,

Thank you for your insight on this. Sadly I am not quite able to interpret what exactly the Balance in 1D results shows. Apparently in my case 3 separate Balance curves are shown (probably given the fact that there are 2 ports and amongst them the 2nd port has 2 modes). Something called Power is also shown in the 1D section but it doesn't show power radiated as such. Could I kindly have some help regarding interpretation of the Balance curve? It's attached alongwith.

As for the port size, I was advised by my supervisor to use a port which is suitably large enough (keeping a width of 4 times the radius on each side) for the dielectric waveguide being used and have used dimensions suggested. I shall be looking into that as well. Thank you for your opinion on the same.

Ask for the far-field monitor.

After it is calculated, if you display the abs value, a box is shown with the radiation efficiency and total efficiency.  The definitions of these from CST help are shown below.

Radiation efficiency: The antenna radiation efficiency is defined as the ratio of gain to directivity or equally the ratio between the radiated to accepted (input) power of the antenna:

Total efficiency: The total efficiency is defined as the ratio of radiated to stimulated power of the antenna:

Compared to the input power, the stimulated power considers any occurring reflections at the feeding location.

Also, if you have a far-field monitor, Power Radiated is included in the 1D results Power folder

Mr. White,

Thank you for your reply.

Ya I did not have a far field monitor earlier and once I included it the Power Radiated is showing up. I did not use the monitor initially as I thought that the target was not to use the structure as an antenna to radiate into free space but to have as good a transmission to port 2 as possible. 

I do now have the Power plot in 1D results and may I know how the power balance could be checked from here? Lets say the stimulated power should be equal to the power radiated + power transmitted to port 2 + power losses + power absorbed at port etc. This is what I would finally like to observe, the different parts of what happens to the input power.

As for the curve I obtained, the different factors adding up to Power stimulated isn't quite clear.

You can add an axis marker that will give you the values at the frequency you want.

From your plot:

At 92.5 GHz only about 40% of the power stays in the problem - 60% is reflected back out of the input port.  About 19% of the power is radiated.  The power absorbed at all ports is an error term - it is power unexpectedly missing from the problem.  It happened in a simulation of mine when I modelled my coax insulator as a hollow cylinder and modelled the inner conductor to fill the gap.  It went away when I made the insulator a solid cylinder and put the centre conductor in it and let the code sort out the overlap automatically.

The power is RMS power so rather than having 100% as total reflection the power folder has 50% as total reflection.

Strangely my the S11 parameter curve at 92.5 shows a value of -12 dB which implies that less than 1/10 th of the power fed in is reflected from the input port which is why the curve seems a bit stange to me. The power accepted is way too low apparently unless I am missing something here.

Hi Arka

As Mr, White pointed out, it seems that part of your energy is being radiated - you can place a field probe close to the structure to see if the frequency of 92.5 has a peak in electric field amplitude.

It also seems that after 92 GHz the balance goes down, that implies a loss, either by radiation or losses.

As for the mode coupling (energy transfer between modes), it can be seen under the S parameters. S1(1)2(2) tells the transmission coupling between both ports, from the first to the second mode. Ideally you want that to be as low as possible.

Hi

I was thinking of this as an antenna problem and forgetting you have two ports.  60% of your power is going out of both the ports, not just being reflected at the input port.  If S11 is 12 dB then 6% of your power is being reflected and 54% is leaving through port 2, and I expect  S21 is just under 3 dB.

Part of your problem is the 19% radiation which is too high for most practical purposes, and probably requires re-design of the transition, but also the "Power Absorbed at All Ports" is a symptom of CST having failed to calculate the model properly, and at 20% is too high to ignore.  You probably need to rebuild your model in a different way - e.g. don't bother making holes in dielectrics for conductors, but let CST override dielectric with conductor as it does by default  - and try again and see if the "Power Absorbed in All Ports" goes away.

Mr. White, Prof. Perotoni, 

Thank you so much for your opinions regarding my query. I shall follow your suggestions given for visualizing the problem first and try to improve my model to lower the radiation and power absorbed at ports and see what happens. 

Back again  with a few updates:

Mr. White,

The analysis given by you at 92.5 GHz holds correctly and S21 is -2.6 dB (as you suggested) and the power flow can be visualized. 

One query that I still couldn't answer back then is that of the Power Accepted which was too low despite a comparitively good S11 and on checking the reason seems to be because the Power Accepted in this case is an algebraic sum of the Power Accepted at Port 1 and Port 2. The Power accepted for Port 1 can be verified by the  S11 parameters.

Strangely though the Power Accepted at Port 2 is negative and the sum of these two therefore turns out to be lower. Might there be any explanation for this?

The rest of the problem though is pretty well explained, the radiation observed by adding the far field monitor and reducing the radiation is my next immediate target.

The power in port 2 is negative because it is going out.  The power in port 1 is positive because it is going in.  Add them together and you get the power that doesn't go out of a port.  93.7% of the incident power (this is for input at port 1) gets in.  6.3% is reflected - this is 12 dB return loss (S11).  57.5% of the incident power goes out of port 2.  This is 2.4 dB insertion loss (S21).  The remaining power is about 36.2%, some of which is radiated, some lost in materials and some as the error term (if it is still there).

Oh okay.

Thanks again. Now it seems perfect. :)

Just a small update, the power absorbed term seems to be representing power going into the higher order modes in the dielectric waveguide. I had simulated 2 initially but on simulating more higher order modes the term decreases somewhat.