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I have the following circuit for the SIM900's antenna connection, using the A10393 antenna from digikey:

enter image description here

In closer look:

enter image description here

First of all my knowledge on RF design is not great. My questions now are what should the trace width be in order to match a 50 Ohm line and how I can calculate the value of L2. I read the datasheet and I also have a link below, and it gave different values for L1 and I did not quite understand it. The zero frequency (R0) is what the mid value of the resonance range and should I care about the L1 component?. I couldn't avoid placing a through hole via because I should have placed the antenna on the bottom side.

Frequency is 850 - 900MHz, the GSM range. The board is two layers and its thickness is 1.6mm. The distance from the source pin to the antenna feed is around 15mm.

No GND plane passes underneath, on both layers, the antenna (it is only the board plastic) or near it at a distance of around 5mm (datasheet recommendation 2.5mm).

Datasheet: http://www.antenovam2m.com/documents/download/62712f84669a7974ad90987d256beb9f503605e766135

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  • \$\begingroup\$ What frequency? Is this a two layer board? What's the board thickness? \$\endgroup\$ – The Photon Sep 16 '14 at 17:47
  • \$\begingroup\$ If you google "microstrip calculator" you will find lots of sites that calculate the line width needed for a given substrate material and thickness. Use 2 or 3 and make sure the results agree before trusting any of them. \$\endgroup\$ – The Photon Sep 16 '14 at 17:51
  • \$\begingroup\$ @ThePhoton Regarding the frequency it 850 - 900MHz (GSM). I'll try this with the sites. \$\endgroup\$ – dim23 Sep 16 '14 at 17:54
  • \$\begingroup\$ Also, note the wavelength of 900 MHz is about 200 mm (in microstrip). If you can keep the length (source to antenna) less than 20 mm, matching should not be critical. \$\endgroup\$ – The Photon Sep 16 '14 at 17:57
  • \$\begingroup\$ @ThePhoton The total length from source to antenna is not more than 14.5mm. Does the via cause any kind of problem? \$\endgroup\$ – dim23 Sep 16 '14 at 18:00
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I don't think you are following the rules set out in the document closely enough. For a start here is a link that opens up a document that describes the device. Here are the critical things rolled into one picture: -

enter image description here

I'm not saying that how you've mounted the interface circuit won't work but if it doesn't then you'll kick yourself for having to buy a new PCB. Also they have the groundplane extending to 2.5mm from the edge of the antenna - I think you should do the same for the reasons above. As @ThePhoton points out, it's unlikely that such a short feedline is going to give problems BUT/HOWEVER following the instructions in the data sheet is realistically your only course of action and if it says 50 ohm impedance tracking then go for it.

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  • \$\begingroup\$ I don't have anything passing underneath the antenna, is just the plastic for about double the distance stated in the data sheet. The layer that is shown as red dots is the restriction layer to keep everything out. So as I understand there is not gonna be any kind of problem for such a short feedline, correct? \$\endgroup\$ – dim23 Sep 16 '14 at 20:12
  • \$\begingroup\$ I don't understand what you mean by "plastic". Regarding the feedlines, I can see they are short but, if you build it and are not happy with performance then you'll always be doubting not making the tracks thicker to be compatible with 50 ohms. Been there, bought the T shirt, redid the board and then found out it was something else. Moral of the story - get it as correct to the data sheet as you can then if you have a problem you know it's not the PCB. \$\endgroup\$ – Andy aka Sep 16 '14 at 20:26
  • \$\begingroup\$ So I will need to change the configuration of the components to match the data sheet, in order to be on the safe side. Thanks for helping! \$\endgroup\$ – dim23 Sep 16 '14 at 20:45

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