# cpw monopole FDTD source defination

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#### john4

##### Newbie level 6 Hi Gurus

Can anybody suggest me how to define source and port for cpw monopole antenna for S11 computation in FDTD.

regards

#### iyami

##### Full Member level 4 Add a resistive voltage source between the antenna and the ground plane,
then measure current flow and voltage drop along the resistor.

#### john4

##### Newbie level 6 Hi Iyami

Since in cpw monopole antenna we have ground plane on both side of the feed.

So we have to define two source or one source.

Because I have seen some paper where authors have define two source.

regards

#### iyami

##### Full Member level 4 Ground plane = voltage 0. So even if you have several pieces they should give identical results (usually you connect different ground plane regions to assure that).

It would help if you have a sketch/image or a pdf file which describes your model.

Also, do you use your own FDTD code or some commercial program (they usually offer some advice on how you should set up your model).

#### iyami

##### Full Member level 4 Hi Iyami

Please find the topology of the cpw which I need to model. regards
I don't know whether you go for a 2d or 3d simulation but I would connect the two grounds at the bottom as a first test.
If your design is symmetrical then you could consider using the symmetry to half the size and use only one ground plane.
(which would correspond to identical sources left and right).

But I don't see anything wrong in principle with just using one of the two ground planes for adding a source.

In any case the real model is important as well. You should know how the real model looks like (e.g how are the ground
planes connected, how will the source be added) and base your decision on that.

#### john4

##### Newbie level 6 Hi Iyami

Thanks,

Its a 3D model.

Yes, the design is symmetric, in that case how to implement that
Suppose I wan't to consider ground at right side of feed then I should cut the structure from the middle of the feed and consider half portion upto right side ground.

What should I do at the left side of the feed , Should I define the boundary like PML

I have never implemented that can you elaborate it little bit.

regards

#### iyami

##### Full Member level 4 Hi Iyami

Thanks,

Its a 3D model.

Yes, the design is symmetric, in that case how to implement that
Suppose I wan't to consider ground at right side of feed then I should cut the structure from the middle of the feed and consider half portion upto right side ground.

What should I do at the left side of the feed , Should I define the boundary like PML

I have never implemented that can you elaborate it little bit.

regards
Implementation of symmetric model is fairly easy. Suppose we cut in the x direction and want to cut at the end of the ith cell.
then the data from the (i+1)th cell you need has to be replaced by the data at the same distance from the other side of the
cutting location (e.g E_x(i+1) <= E_x(i)). If you cut in the middle of the nth cell then you would for example need to do (H_x(i) <= H_x(i+1)).
Essentially this amounts to one copy operation after the update of the other fields (e.g. instead of computing E_x(i+1) you just use the value of E_x(i)).

But I would suggest first to run a normal 3d simulation and actually compare the results you get when you

1) connect the ground planes (you could try to connect below (= down in your picture) or behind the structures)
2) use 1 source
3) use 2 sources (left and right)

There is no perfect answer. Trying out all setups you can think of is in my opinion the best option. It's what I would do
(and did do) when faced with such problems.

#### john4

##### Newbie level 6 Hi Iyami

Thanks, I guess I have to do some implementation to see where I get the best result.

Suppose I have two source as you suggested (option-3), then how I compute S11.

That is how I should use two sampled voltages and two sample currents at resistors of each source.

I have seen in literature some authors have used this, but I am not getting how to use two sampled voltages and current to compute S11 of the structure.

If you can explain me this I guess I can solve my problem

regards

#### iyami

##### Full Member level 4 Hi Iyami

Thanks, I guess I have to do some implementation to see where I get the best result.

Suppose I have two source as you suggested (option-3), then how I compute S11.

That is how I should use two sampled voltages and two sample currents at resistors of each source.

I have seen in literature some authors have used this, but I am not getting how to use two sampled voltages and current to compute S11 of the structure.

If you can explain me this I guess I can solve my problem

regards
let's assume the ground planes are connected then

1) the voltages should be the same
2) you should measure the total current (=> add the currents)
3) you should keep in mind that the resistors you use for inserting the source are in parallel

but just using one source should really give very similar results.

(You could also just average currents and voltages since that halves the total current and doubles the resistance)

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#### john4

##### Newbie level 6 Hi Iyami

Let me try different options you suggested.