Electrically Small Antenna - S11 > 0

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vath
Posts: 3
Joined: Tue 30 Nov 2021, 14:18

Electrically Small Antenna - S11 > 0

Post by vath » Sun 05 Dec 2021, 21:21

Greetings and thank you very much for the development.

For the past couple of weeks I am trying to simulate an electrically small antenna (dipole like) to extract impedances for a matching circuit. The only issue that I have run into is that for the low frequencies the the S-parameter does not stabilise < 0. I have tried everything, increasing the mesh, increasing the boundaries, increasing the bandwidth of the signal. The simulation time is up to 10h but still no change. :cry:

The bandwidth is 10 MHz - 4 GHz, f0 = (f_start + f_stop)/2, and fc = f0/2. I tried both MUR and PLM_20 as boundaries. Fine mesh of 0.15mm, coarse mesh of 1 mm, and a simulation box of +- lambda. The dipole is excited by a 50 ohms lumped port, which is 2x2x0.035mm in size (gap between the dipoles). I tried to increase fc (bandwidth) but the energy levels remain the same at circa -7dB... I can confirm that from 600 MHz to 4 GHz the result is correct. I did try to simulate only the 10 MHz to 600 MHz part with the same result. I have read the FDTD theory and I don't know if I missed something (https://en.wikipedia.org/wiki/Finite-di ... ain_method).

Also, to reduce the simulation time I was wondering if I could use symmetric planes? I did a quick search on the forums but I didn't find any one explaining how, so please correct me on what I am doing wrong.

For symmetries XZ: Electric; YZ: Magnetic; XY: Open;
BC = {'PEC' 'MUR' 'PMC' 'MUR' 'MUR' 'MUR'}; % boundary

Then the mesh limits should be something like;
mesh.x = [0, ... , SimBox(1)];
mesh.y = [0, ... , SimBox(2)];
mesh.z = [-SimBox(3), ... , SimBox(3)];

HexAndFlex
Posts: 15
Joined: Sat 06 Oct 2018, 09:11

Re: Electrically Small Antenna - S11 > 0

Post by HexAndFlex » Mon 06 Dec 2021, 22:58

While I haven't looked in detail at this. For the small size of the antenna and the huge frequency range you are trying to simulate, I am not surprised that it it taking many hours, perhaps hundreds of hours to simulate.

Hopefully you are already aware of the limitations of electrically small antennas so apologies if I am teaching you to sick eggs(British expression). The rule is pretty much that you can choose one of the following.

1. Small, broadband and very low efficiency
2. Small, narrowband and efficient.

As you go smaller you can make up for this to some extent with an ATU, but this will be a hi-Q tuner and hence very narrowband. Eventually you get to the point that the losses in the ATU dominate that of any radiation. At 10MHz the bandwidth the untuned VSWR will be probably be way into the thousands. To tune to this you will need huge inductor values that will either be physically large, and or very lossy.

vath
Posts: 3
Joined: Tue 30 Nov 2021, 14:18

Re: Electrically Small Antenna - S11 > 0

Post by vath » Wed 08 Dec 2021, 21:58

Thank you very much for your response!

I am aware of the limitations and how to over come them! Here are some links for people that might be interested in the future. Regarding the simulation.... I forgot to mention that the overall size is 130mm by 105mm. Unfortunately I cant share the exact design right now. If I have the permission I will come back and upload it.

Moreover, I have dropped the bandwidth to 10-600 MHz, to get a better estimation of the impedance in order to test the matching on Qucs. The other parameters has been altered as well, f0 = (f_start + f_stop)/2, fc = f0/2, simulation box +- lambda. Unfortunately the result is still the same, as you can see on the attached figure. This simulation took circa 8 hours to complete and it was similar with one employing a smaller mesh, so I am sure that mesh is not the issue.

Any advice on what the issue could be is more than welcome!
Attachments
Lambda_Res_400_200.png
Lambda_Res_400_200.png (16.55 KiB) Viewed 2828 times

HexAndFlex
Posts: 15
Joined: Sat 06 Oct 2018, 09:11

Re: Electrically Small Antenna - S11 > 0

Post by HexAndFlex » Wed 08 Dec 2021, 22:25

I think this is just quantisation noise. The values are so small the limited precision of the digital math may be having an effect.

vath
Posts: 3
Joined: Tue 30 Nov 2021, 14:18

Re: Electrically Small Antenna - S11 > 0

Post by vath » Sat 11 Dec 2021, 17:55

The issue was resolved by increasing the BW, fc = f0, keep a relatively tight mesh and changing the boundaries to PML_20! The increase in BW resulted to a very very very long simulation where the energy was too much for the MUR. I don't know if there are any disadvantages by switching to PML_20.

The impedances are very very close to what I expected, thanks again for the development!
Attachments
LambdaDiv4_Res_200_100_LargeBW.png
LambdaDiv4_Res_200_100_LargeBW.png (15.33 KiB) Viewed 2777 times

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