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I am trying to match a gsm antena, doing it on a pcb, first time in practice. I understand the whole process to adapt it, I've done it in simulators. However, I don't understand the need for a pi matching network, since (theoretically) and LC circuit is enough, or a series and shunt capacitors. So why the extra component?

Plus, I understand even less the suggested pi matching network on the uC datasheet. It shows 2 shunt capacitors with a normally 0-ohm resistor in between. Aren't the capacitors redundant?

When working on a pcb which components should I prefer to do the matching? Are stubs a bad idea? I never see them referenced to do the matching.

I forgot to add: I'm going to do this with a vna, the configuration I have is the antenna pin from the uC directly connected to the matching network and then the antenna. Should I do this with the uC powered or turned off.

DataSheet: https://cdn-shop.adafruit.com/datasheets/sim800h_hardware_design_v1.00.pdf

Suggested pi-network: enter image description here

Hope was clear enough,

Regards,

Pedro Lopes

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  • \$\begingroup\$ What data sheet? What device? What pi matching network? \$\endgroup\$ – Andy aka Jul 14 '17 at 10:44
  • \$\begingroup\$ Just added it to the main question. \$\endgroup\$ – Pedro Lopes Jul 14 '17 at 10:54
  • \$\begingroup\$ And the antenna (this is hard work prompting you)? \$\endgroup\$ – Andy aka Jul 14 '17 at 11:02
  • \$\begingroup\$ I'm having trouble with the second sentence, 'I understand the whole process to adapt it, I've done it in simulators'. If you're asking questions about the difference between LC and pi, then you haven't been doing enough playing around in simulators to understand anything. Think, will the uC be powered on or off when it's using the antenna? In that case, should you measure it with it on or off? \$\endgroup\$ – Neil_UK Jul 14 '17 at 11:19
  • \$\begingroup\$ I know how to adapt with either configuration. I just don't know why we need a pi configuration, when with 2 components you can theoretically match the antenna. Regarding the on/off question, makes sense, thank you. I was just reluctant powering something and connect it to the VNA. I've only done it with passive elements, such as antenna. \$\endgroup\$ – Pedro Lopes Jul 14 '17 at 12:07
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However, I don't understand the need for a pi matching network, since (theoretically) and LC circuit is enough, or a series and shunt capacitors. So why the extra component?

Think about what a simple inductor and capacitor provide regarding the input impedance. At resonance, the impedance looking into the LC network from the chips point of view is resistive because at resonance the phase angle is 0 degrees. However, from the perspective of the antenna it doesn't see a resistive impedance and this will be critical for a lot of antennas.

So, the network is designed to look resistive in both directions and, as a result, it doesn't quite run at resonance. The extra capacitor makes the network symmetrical so therefore, for the sake of impedance phase angles, it only needs to be analysed in one direction.

That extra capacitor (from the perspective of the chip or the antenna) turns the off-tuned LC (that now looks more inductive than resistive) into an impedance that looks purely resistive. This is because that extra capacitor is like power factor correction. In fact it IS power factor correction if you look into it. It looks resistive both ways providing you design the values to suit the operating frequency, the antenna impedance and the chip's output impedance.

Regarding the suggested pi network, those components can be fitted by you (the user) to suit the particular antenna you choose.

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  • \$\begingroup\$ Thank you for the explanation. Could this bypassed by not adding the 0 ohm resistor, with just a shunt capacitor? (assuming that it can be matched solely with a shunt capacitor) \$\endgroup\$ – Pedro Lopes Jul 14 '17 at 12:39
  • \$\begingroup\$ A capacitive voltage divider will not produce input and output impedances that are resistive. The chip may be ok with that but the antenna will be disturbed and possibly disturbed badly. \$\endgroup\$ – Andy aka Jul 14 '17 at 12:42
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Key to understanding PI networks is the circulating current ---- its the same current in both capacitors, but you get to pick the capacitors.

schematic

simulate this circuit – Schematic created using CircuitLab

Pi networks with equal caps are intended to work with equal Zsource and Zload.

Pi networks with 1:5 ratio of caps will produce 5:1 voltage ratios, and thus 25:1 impedance ratios. Some vacuum tube ham transmitter output matching, or general purpose antenna-tuning boxes, use these extreme capacitor ratios.

If you have 1,000 volt plate and 1 amp (nominally 1,000 ohms) in vacuum tube TRIODE, to match to 50 ohm antenna, that 20:1 ratio comes from 4.5:1 cap ratios.

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