Helix monopole Antenna

How to use openEMS. Discussion on examples, tutorials etc

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MPCEngineer
Posts: 31
Joined: Thu 02 Sep 2021, 18:29

Helix monopole Antenna

Post by MPCEngineer » Thu 02 Sep 2021, 18:41

Hi,

I'm a newbie and I am trying to simulate a 7 turn helix mono pole above a ground plate.

Using the examples script Helical_Antenna.m as the base I have changed the number of turns and coil size.

The frequency of operation is 868MHz.

I change the frequency range in script but I must be doing something very wrong because the impedance is always zero.

Any help anyone?

Thanks,

MPC
Z.jpg
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Attachments
helix.m
(6.11 KiB) Downloaded 90 times

thorsten
Posts: 1475
Joined: Mon 27 Jun 2011, 12:26

Re: Helix monopole Antenna

Post by thorsten » Thu 02 Sep 2021, 20:14

Hi,

your mesh is completely not okay for the antenna.
The simulation domain is way to large (unnecessary), and your helix only covers exactly two lines per width, thus making it more a brick than a coil.
I did not check in height direction but I think it was similar. Have a closer look at the mesh in relation to the helix from the tutorial and try to scale everything fitting your dimensions and frequency range.

Maybe you try again by first scaling everything from the tutorial helix to your frequency range (keeping turns and such) and then increase turns and so on...

regards
Thorsten

MPCEngineer
Posts: 31
Joined: Thu 02 Sep 2021, 18:29

Re: Helix monopole Antenna

Post by MPCEngineer » Fri 03 Sep 2021, 13:31

Thanks Thorsen,

I have studied and improved the mesh but still the real impedance is zero.

Any further thoughts?

Many thanks,

MPC
Mesh2.jpg
Top view of mesh around helix
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Mesh.jpg
3d view of helix mesh
Mesh.jpg (89.96 KiB) Viewed 1919 times
Z.jpg
Impedance
Z.jpg (25.46 KiB) Viewed 1919 times

MPCEngineer
Posts: 31
Joined: Thu 02 Sep 2021, 18:29

Re: Helix monopole Antenna

Post by MPCEngineer » Fri 03 Sep 2021, 15:04

Forgot to include the script file.

Thanks,

MPC
Attachments
helix.m
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MPCEngineer
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Re: Helix monopole Antenna

Post by MPCEngineer » Fri 03 Sep 2021, 18:41

Hi All,

With a Cartesian mesh over the helix antenna will OpenEMS identify is helix as a continuous structure?

I have attached a PEC_dump.vtp image but I am not sure what I am looking at?

Any thoughts?

MPC.
Attachments
PCE_dump.jpg
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MPCEngineer
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Re: Helix monopole Antenna

Post by MPCEngineer » Mon 06 Sep 2021, 17:30

Help.......

I have tried many different mesh setups but the results from openEMS do not match reality.

Physical experiments show a 8 turn helix monopole is close to resonance at 868.5MHz BUT the results from openEMS are not even close to reality. I guess its something to do with the mesh setup but I just can not find the issue.

Any help gratefully accepted.
helixMono.m
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Helix2.png
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Attachments
Helix.png
Helix.png (68.58 KiB) Viewed 1834 times

thorsten
Posts: 1475
Joined: Mon 27 Jun 2011, 12:26

Re: Helix monopole Antenna

Post by thorsten » Mon 06 Sep 2021, 20:45

Hi,
I see several issues.

First, why is your ground plane this asymmetric? Secondly I see a resonance at ~727MHz (imaginary part going through zero) with a very low resistance.
You surely need a matching network, but I think you port is much too long, it should only be very short and your first turn should start close atop the ground.
That will already act like a (parallel) capacitor and should already improve your matching I believe.
And lastely, who said the resonancy frequency would depend on the number of turns? It does not! It depends on the overall length, width and so on and in this case the length of your port does count as well.

What you can see in the PEC_dump is how the helix is mapped onthe the FDTD grid. Obviously a fine mesh would result in a better resolution of the helix, but from what it looks like this should be okay for a first attempt.

best regards
Thorsten

MPCEngineer
Posts: 31
Joined: Thu 02 Sep 2021, 18:29

Re: Helix monopole Antenna

Post by MPCEngineer » Tue 07 Sep 2021, 09:43

Hi Thorsten,

Thank you for the comments.

I have experimental results from a 7 turn helix on an offset ground plane. The results show a -11dB return loss at 858MHz. I also know the radiation pattern is asymmetrical as well.

I have tried several port lengths to see if that has any effect but it does not appear to affect the results, you still get the doughnut radiation pattern.

I am also very confused buy the radiation pattern. The radiation is a perfect doughnut? the ground plane does not effect the radiation pattern? this I find difficult to understand.

Is it possible the port is not fully connected to the ground plane or helix? Are there any rules regarding the end points of a port?

Any help greatly appreciated.

MPC.

MPCEngineer
Posts: 31
Joined: Thu 02 Sep 2021, 18:29

Re: Helix monopole Antenna

Post by MPCEngineer » Tue 07 Sep 2021, 10:40

Hi.

The radiation pattern remains a perfect doughnut with or without the helix. The pattern changes position but does not change shape.

The ground plane appear to be transparent to the pattern.

MPC.
Helix Radiation.png
Helix Radiation.png (99.37 KiB) Viewed 1791 times
No Helix Radiation.png
No Helix Radiation.png (98.29 KiB) Viewed 1791 times

thorsten
Posts: 1475
Joined: Mon 27 Jun 2011, 12:26

Re: Helix monopole Antenna

Post by thorsten » Tue 07 Sep 2021, 16:45

A doughnut is not the pattern of a helix. The ground plane is not transparent, it seems to be the radiating element with or without the helix elongating it.
Have a look here how it should look like: Tutorial: Helical Antenna

You are clearly looking at the wrong resonance (frequency)... Maybe you try it first with an infinite ground (aka PEC boundary condition)

br
Thorsten

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