Helix monopole Antenna

How to use openEMS. Discussion on examples, tutorials etc

Moderator: thorsten

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

Re: Helix monopole Antenna

Post by MPCEngineer » Tue 07 Sep 2021, 17:39

Hi Thorsten,

I know from VNA results on the PCB in the photo that the antenna resonates at 868MHz.

I did not expect openEMS to have the same results as real life but the doughnut pattern does not make sense, unless I am looking at very large distance from the aerial. I am obviously doing something wrong or confused....

The antenna is a Helical Monopole Antenna. So the pattern should be similar to a 1/4 wave whip aerial.

I will give the PEC idea a go and see where that takes me.

Thanks for the feedback. Grateful for any thoughts......

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

Re: Helix monopole Antenna

Post by MPCEngineer » Wed 08 Sep 2021, 13:02

Hi Thorsten,

I have taken a step back and instead of doubting the model I have increased the mesh around the helix by its radius.

My thinking is the helix is probably acting more like an inductor/tranmission line? Maybe the resolution of the magnetic field is very important and is probably very very important close to the helix.

Using a wider grid around the helix has started to improve the results and I now get a resonance close to 868MHz.

I guess the tilted doughnut pattern is due to the ground plane being small compared to the wavelength. So the antenna is acting like a small dipole source but is tilted due to the small ground plane.

My only thoughts now are why is the feed impedance only 2 ohms when measuring the physical antenna I get 36 ohms.

MPC.
Helix results.png
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Helix Results 2.png
Helix Results 2.png (66.33 KiB) Viewed 1073 times

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

Re: Helix monopole Antenna

Post by thorsten » Wed 08 Sep 2021, 19:42

I had another look at your script because this doughnut just does make no sense for a helix antenna.

You Helix radius is set to 2.8mm that means the diameter is only about half a cm! Is this really correct? Judging from your picture this look much bigger?

And comparing it to my tutorial helix it makes even less sense. Because it has a radius of 20mm for 2.4GHz.
The rule of thumb is to have a diameter of lambda/pi, that is for 2.4GHz ~40mm and for 868MHz that would be 110mm. You are with your 5.6mm a factor of 20 below that...

And considering all this, it makes sense that this acts more like a monopole and not like anything like a helix... and it does not really fit to the picture either???

regards
Thorsten

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

Re: Helix monopole Antenna

Post by thorsten » Wed 08 Sep 2021, 19:49

Additionally for very small antennas like this the actual feeding structure make a huge difference as well and just using this simple lumped port from ground to the helix may not be a good model. You may need to model the (I assume) coaxial feeding as well because it might make a huge difference in terms of antenna matching...

regards
Thorsten

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

Re: Helix monopole Antenna

Post by MPCEngineer » Thu 09 Sep 2021, 14:26

Yes the dimensions are correct. These type of aerials are used a lot in 433MHZ and 868MHz products.

Your example script file is for a axial mode helix antenna whereas I am wanting to simulate a normal mode helix antenna.

The normal mode helical antenna is an electrically small antenna.

Your help has been invaluable, thank you.

MPC.

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