Simple skin effect compensation works but is it valid???

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

Simple skin effect compensation works but is it valid???

Post by MPCEngineer » Wed 22 Sep 2021, 12:13

Hi All,

I have been experimenting and simulating a normal mode helix antenna using OpenEMS and after a lot of work on the mesh and help from the forum I have been able to achieve a really good resonance match with physical results.

The problem has been the antenna impedance at resonance. I was simulating and getting around 2R but the physical antenna was around 36R.

I tried AddCylinder to replace AddCurve to simulate a physical wire with a diameter instead of a line with zero diameter. This had little effect on the antenna impedance.

What I then tried is to simulate the skin effect. At 868MHz using 0.7mm diameter wire the resistance of copper is about 5340 times larger than the DC resistance. By reducing the Kappa copper setting in AddCylinder by 5340 to 11161 instead of 56e6 I now have a very good match between the physical and simulated results. The simulated impedance at resonance is now 45.5R

Can anyone comment on how valid this is to just adjust Kappa to account for skin effect OR was this a fluke???
Attachments
Helix normal mode impedence Kappa 11161.png
Helix normal mode impedence Kappa 11161.png (24.47 KiB) Viewed 936 times

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

Re: Simple skin effect compensation works but is it valid???

Post by thorsten » Thu 23 Sep 2021, 09:10

Hi,

well this does not really sound to good. If your antenna would be working properly you should get much higher R indeed, but not because of (heating) losses, but because of radiation (kind of matching to the 377Ohms of free space).

I have the feeling you just replaced the (missing?) radiation with heating losses in your conductors. So it seems your antenna is not working as it should?
Do you have the correct boundary conditions and not like PEC or PMC? Because for a "working" antenna, switching from PML to PEC should really detune your antenna and thus showing that it radiates and not just heats up.

br
Thorsten

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

Re: Simple skin effect compensation works but is it valid???

Post by MPCEngineer » Thu 23 Sep 2021, 14:19

Hi Thorsten,

The antenna is a normal mode helix. From the research I have done a normal mode helix antenna (NMHA) should have a very low impedance (less than two ohms).

If I use a perfect conductor for the Helix openEMS does suggest a very low impedance at normal mode resonance which agrees with theory. This would suggest the simulation model is working. The model also predicts a high efficiency suggesting all the power is being radiated.

Because of the skin effect the copper wire will have a very high resistance at 868MHz compared to DC. By decreasing the copper material conductivity in the simulation I was trying to model the current reduction due to skin effect. Using this model the antenna efficiency is predicted as 6% by openEMS and the predicted impedance is very close to measured. I believe the antenna is loosing a lot of power as heat.

Just in general is it valid to model skin effect by decreasing the material conductance?
My thoughts: As all the current is flowing in a small 2um layer at the conductor surface and the FDTD simulation is working at a larger scale of 0.4mm. I guess the restricting current effect of skin depth can be averaged over the larger FDTD 0.4mm scale by reducing the material conductance.


Thanks,

MPC.

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

Re: Simple skin effect compensation works but is it valid???

Post by thorsten » Fri 24 Sep 2021, 07:34

Hi again,

okay then I misunderstood or misread ;) I though you were expecting 36Ohms and got only 2...

To be honest, for such low frequencies I would not even bother with ohmic losses, they are usually negligible at least for antennas like this. I would expect a measured efficiency of >90%.
And no modeling such losses in FDTD is very difficult and would require a very good skin effect model which we currently only have for planar conductors (see AddConductingSheet, well if the model really is good is another question).

regards
Thorsten

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