Modeling a PCB meander line antenna

Hello to all,

I'm new here and I don't know if this is the correct section where to post (if it's not correct I apologise in advance).

I'm studying a physical meander line antenna printed on a PCB and there is no ground plane beneath the antenna.

Do you have any advice how to set the ground in FEKO? Is it mandatory to have a ground plane for the simulation?

Your opinions are more than welcome,

Thank you

Manuel

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JIF

Hello manuel_lulea

There is no requirement for a ground in FEKO. I assume you are going to excite the antenna with a voltage source (edge port?) and the only thing that you need to do is specify the positive and negative sides of the port / source.

I can't provide more info without more details from you. Why did you think that a ground plane is required? Did you run into a problem when trying to perform the simulation?

Altair Forum User

Hello manuel_lulea

There is no requirement for a ground in FEKO. I assume you are going to excite the antenna with a voltage source (edge port?) and the only thing that you need to do is specify the positive and negative sides of the port / source.

I can't provide more info without more details from you. Why did you think that a ground plane is required? Did you run into a problem when trying to perform the simulation?

Hello JIF,
thank you for your answer. I will attach a file to show you better.
1. I'm using right now a wire port, instead of edge port. Can I use it? (In term of results what's the difference?)
2. If I understood correctly, I can use a wire port 'free floating' that is connected to one side to the antenna, the other in free space, right?
3. As you see from the picture, I need to include a lumped element like an inductor between the two traces of the antenna, is it the correct way?
4. If the thickness of antennas's traces is 35micron should I use cuboid instead of rectangles?

Thank you for your help,

Manuel

<?xml version="1.0" encoding="UTF-8"?>

JIF

Hello manuel_lulea,

Thanks for the image - it makes it much easier to see what you are doing and provide meaningful suggestions.

To get back to your original question of ' Is it mandatory to have a ground plane for the simulation?':

FEKO does not require a ground plane - you can simulate what you want, but that does not mean that the antenna will work the way you think it should. For your meander line model, you need to add a ground plane for the antenna to work. You can think of the meander line antenna as a monopole - a monopole won't work without some sort of ground plane since it is using the ground and its electric symmetry to model one half of a dipole. I hope this makes sense to you. Maybe this helps: Start with a dipole, then turn it into a monopole by halving it and adding a ground plane and then compact the monopole into a meander so that it takes up less space.

There is nothing wrong, from FEKO's perspective with what you are doing. Instead of wires, you can use edge ports, but they have limitations on the boundaries of SEP dielectrics (does not look like you are using SEP though). Have a look at the article below if you do and want to use edge ports with SEP dielectrics since it shows how they should be constructed.

For your antenna to work, you will have to add a ground plane. If this is a microstrip feed, the ground plane should be added at the bottom, but not under the antenna - it should be added to the section of the model that currently has nothing (on the side of the voltage source). I found an image that shows what you should be constructing (see below). What you are missing is the bottom section in the image (note that there is a ground plane on the bottom, but only on a section, not under the antenna).

If you don't understand, send me your model (not just a picture) and I'll modify it for you.

Altair Forum User

Hello manuel_lulea,

Thanks for the image - it makes it much easier to see what you are doing and provide meaningful suggestions.

To get back to your original question of ' Is it mandatory to have a ground plane for the simulation?':

FEKO does not require a ground plane - you can simulate what you want, but that does not mean that the antenna will work the way you think it should. For your meander line model, you need to add a ground plane for the antenna to work. You can think of the meander line antenna as a monopole - a monopole won't work without some sort of ground plane since it is using the ground and its electric symmetry to model one half of a dipole. I hope this makes sense to you. Maybe this helps: Start with a dipole, then turn it into a monopole by halving it and adding a ground plane and then compact the monopole into a meander so that it takes up less space.

There is nothing wrong, from FEKO's perspective with what you are doing. Instead of wires, you can use edge ports, but they have limitations on the boundaries of SEP dielectrics (does not look like you are using SEP though). Have a look at the article below if you do and want to use edge ports with SEP dielectrics since it shows how they should be constructed.

e voltage source). I found an image that shows what you should be constructing (see below). What you are missing is the bottom section in the image (note that there is a ground plane on the bottom, but only on a section, not under the antenna).

<?xml version="1.0" encoding="UTF-8"?>

If you don't understand, send me your model (not just a picture) and I'll modify it for you.

Hello JIF,

thank you so much for your advice. I did the changes you suggested me, but I still have some problems.

What I'm doing is to model a physical antenna printed on a PCB ( I performed a VNA test on it) and from this test I know the S11 parameter of the antenna.

The antenna should operate at 868 MHz (ideally), and the S11 I measure at that frequency is around -10 dB.

With the FEKO model around 1 GHz I can see a resonance peak, but it's -0.1dB

Do you know why there is this big discrepancy?

I attach here the model of the antenna.
(In this model I used Wire port, but the result doesn't change when I use edge port)

Unable to find an attachment - read this blog

JIF

Hello,

I looked at your model and the first thing that I noticed is that the meshing was too coarse. You have selected 'Fine' meshing and usually that would be sufficient for a fine mesh, but in this case geometry (line widths and even lengths) are very small compared to a wavelength. Are you sure about the material parameters, dimensions and inductance value? When I simulate the model (with a refined mesh), the resonance is around 3.5 GHz - thus a factor 4 higher than where you expect it. Could you please verify the parameters and dimensions. Maybe a photo of what you measured so that we can see what you built matches with what you are trying to model?

An example of a reasonable (maybe a bit fine) mesh for this model looks like this:

<?xml version="1.0" encoding="UTF-8"?>

What I have done is to ensure that the triangles representing the line is not wider than the line itself. I have also ensured that the triangles are are not larger than the height of the substrate. These are general rules that I use to guide meshing.

Altair Forum User

Hello,
the dimensions and the parameters are correct. For the sake of clarity I attach the photo of the PCB antenna (the SMA connector has been soldered between the feeding pin and the ground beneath the PCB)
The two ground layers, one on the same level of the antenna, the other beneath the substrate, are connected trough several vias. I also tried to include these layers in the FEKO model but nothing changed.

As you can see from the VNA measure there is a peak around 900 MHz and a small one at 3 GHz (This seems to be the ones obtained from the FEKO simulation).

Anyway, thank you for the suggestion about the mesh.

<?xml version="1.0" encoding="UTF-8"?>

Johan_Huysamen

Hi,

Based on my experience, electrically small antennas such as these are very sensitive to the measurement setup. For example, the presence of the measurement cable could add a resonance as fields couple between the structure and the outer conductor of the cable. Basically the currents induced on the outer cable shield could radiate like part of the antenna.

To illustrate this, I have created two version of your model: one which includes an SMA feed to closely represent what you have (based on the picture you shared) and another with the same setup but including an added section of the cable outer (shield). See the images below:

<?xml version="1.0" encoding="UTF-8"?><?xml version="1.0" encoding="UTF-8"?>

Comparing the results for these two you will see that the added cable section introduces a resonance at around 1GHz (without affecting the higher frequency resonance).

When doing the measurement, I would not be surprised if this lower resonance was very sensitive to the position and orientation of the antenna relative to the feed/measurement cable.

Another consideration is the added inductor. In your FEKO model you have a perfect 22nH inductor that is applied over the whole frequency range. In practice an inductor would have a complex impedance that varies over frequency. It might even become capacitive at certain frequencies. This would significantly affect the behaviour of the antenna. To illustrate this I again used the SMA-fed model above, but changed the inductance value to 10nH. In the chart below you will see that this affects the higher-frequency resonance:

<?xml version="1.0" encoding="UTF-8"?>

To accurately model the frequency dependency of the inductor you might consider measuring its response (S-parameter) and adding this as a general network (imported from a Touchstone file) to your FEKO model.

Altair Forum User

Hi,

Based on my experience, electrically small antennas such as these are very sensitive to the measurement setup. For example, the presence of the measurement cable could add a resonance as fields couple between the structure and the outer conductor of the cable. Basically the currents induced on the outer cable shield could radiate like part of the antenna.

To illustrate this, I have created two version of your model: one which includes an SMA feed to closely represent what you have (based on the picture you shared) and another with the same setup but including an added section of the cable outer (shield). See the images below:

<?xml version="1.0" encoding="UTF-8"?><?xml version="1.0" encoding="UTF-8"?>

Comparing the results for these two you will see that the added cable section introduces a resonance at around 1GHz (without affecting the higher frequency resonance).

<?xml version="1.0" encoding="UTF-8"?>

When doing the measurement, I would not be surprised if this lower resonance was very sensitive to the position and orientation of the antenna relative to the feed/measurement cable.

Another consideration is the added inductor. In your FEKO model you have a perfect 22nH inductor that is applied over the whole frequency range. In practice an inductor would have a complex impedance that varies over frequency. It might even become capacitive at certain frequencies. This would significantly affect the behaviour of the antenna. To illustrate this I again used the SMA-fed model above, but changed the inductance value to 10nH. In the chart below you will see that this affects the higher-frequency resonance:

<?xml version="1.0" encoding="UTF-8"?>

To accurately model the frequency dependency of the inductor you might consider measuring its response (S-parameter) and adding this as a general network (imported from a Touchstone file) to your FEKO model.

Hello Johan,
thank you for your advice. Since I'm quite new with FEKO could I ask you to send me the model you did including the SMA connector and the coaxial cable?

Then I have some comments for you:
1. Where did you calculate the S11? (I did calibrate the VNA including the coaxial cable, instead the last part of the SMA connector - the ones soldered to the antenna- is not included in the calibration.

2. You are right. The measurements were not that stable, but they were influenced to the surrounding.

3. According to the data sheet of the inductor the real behavior is like an RLC circuit with (R = 0.190 Ohm in series with L = 22 nH) with a capacitor C = 0.12 pF in parallel. Is it possible to include a circuit like this in FEKO? (maybe I can neglect the resistor?)

Thank you,

Manuel

Johan_Huysamen

Hi Manuel

I have included the model with the SMA feed and cable outer section here. Please note that I might have broken some of your original parametric geometry definitions while modifying the model. (The geometry of the antenna is unchanged, but it might not update as you change some of your variables.) Also note that this model was created in the latest version of FEKO and it won't be possible to open it in older version.

For both models using the SMA connector (with and without the added cable section), I used a coaxial waveguide port located at the end of the SMA connector. You can see this represented by a red ring in the images above. This would be equivalent to calibrating the measurement to this point.

The parameters reported for the inductor in its datasheet would have been measured at a single frequency. For a really accurate representation, you would have to measure its performance over the required frequency range and then include that in your FEKO model.

Kind regards,

Unable to find an attachment - read this blog

Altair Forum User

Hi Johan,

thank you. As you said, I can't open the file. I have the latest student version (v.2017.2) of FEKO.
Which version did you use? How can I fix this problem?

Could you help me please?
Thank you,

Manuel

Hi Manuel

I have included the model with the SMA feed and cable outer section here. Please note that I might have broken some of your original parametric geometry definitions while modifying the model. (The geometry of the antenna is unchanged, but it might not update as you change some of your variables.) Also note that this model was created in the latest version of FEKO and it won't be possible to open it in older version.

For both models using the SMA connector (with and without the added cable section), I used a coaxial waveguide port located at the end of the SMA connector. You can see this represented by a red ring in the images above. This would be equivalent to calibrating the measurement to this point.

The parameters reported for the inductor in its datasheet would have been measured at a single frequency. For a really accurate representation, you would have to measure its performance over the required frequency range and then include that in your FEKO model.

Kind regards,

Unable to find an attachment - read this blog

Altair Forum User