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I have designed a setup for 433.92MHz wireless device (a key fob). The source impedance is not equal to 50Ohm. The load impedance may or may not be 50Ohm depending upon the choice of the antenna. For now, I have given a block diagram for a setup with a PCB loop antenna below. Wireless Key Fob Block Diagram

The antenna tuning setup block diagram is show below. Antenna Tuning Setup

Usually, the antenna tuning guides tune the antenna for 50Ohm source impedance. But in this case, the source impedance is not 50Ohm. So, is the tuning procedure same regardless of the source impedance?

Also, I've not done antenna tuning before. Is the antenna tuning setup correct?

Some of the articles I referred:

  1. Nordic Semiconductor - Antenna Tuning - White Paper
  2. A stackoverflow question on antenna tuning
  3. Antenna Tuning for beginners

Suggested matching network design schematic from Atmel Antenna Design Guide. Suggested antenna design

The equivalent representation schematic of the system is given below: Equivalent representation of the system Here, Source Resistance = 465 Ohm, Parasitic Capacitance = 1pF, So, Source Impedance = (166 - j223) Ohm

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This is a setup I found on the Nordic semiconductor dev website:

enter image description here

Impedance matching with source impedance different from 50 Ohms

This is the situation you have. You have an output from a PA, a section of a transmission line (eg. 50 Ohms), and your antenna. You want that the impedance looking from either end of the TL to be 50 Ohms (in a 50 Ohms system). That is why there is another matching network right at the output of the PA, in addition to the one right next to the antenna. Think of this extra matching network as an extension to the output of the PA such that you think of this as its output.

Now the matching network close the PA also serves the purpose of filtering harmonics, which could be an issue when passing regulatory certifications. That is in addition to matching.

Your antenna tuning setup is Ok. Just make sure you calibrate your VNA right at the point where the antenna's matching network starts.

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  • \$\begingroup\$ I followed the antenna design guide for the Printed Small Loop PCB Antenna, for which Tapped Capacitive Network is suggested. I have added the equivalent circuit of the system in the question. I'm not sure how to follow your advice of making the impedance 50Ohm as seen from either side of the TL. \$\endgroup\$ – spockshr Jul 27 '20 at 16:00
  • \$\begingroup\$ @spockshr Using the VNA appropriately, measure the impedance of your antenna right at the feed point. You would make this measurement using the Smith Chart on the VNA. With that impedance, you can go ahead and use an online calculator to find the components values for a pi-network (or a suitable matching network) to make the antenna complex impedance equal to 50Ohms. Place those components right next to the antenna and re-measure the impedance now at the feed point to the matching network, it should be closer to 50Ohms than what you initally had with no matching. \$\endgroup\$ – Big6 Jul 27 '20 at 16:19
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    \$\begingroup\$ @spockshr a better measurement, imo, is the return loss (S11). You can also measure that with the VNA. I think this gives you better insight because if it's better than 10dB, you know that most of the power (90%) is being utilized by the antenna. \$\endgroup\$ – Big6 Jul 27 '20 at 16:22
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But in this case, the source impedance is not 50 ohm. So, is the tuning procedure same regardless of the source impedance?

No, you need to convert the source to purely resistive first. That's a fairly trivial exercise by using a capacitor in series with the output to counteract the inductance (j223): -

enter image description here

The reactance of the capacitor cancels the reactance of the inductance (j223). At 434 MHz a 1.64 pF capacitor has the right reactance to cancel the j223 leaving just the resistive element of the source.

So now you are trying to match an output impedance of 166 ohms resistive to the loop antenna. It's easier if you convert the antenna's impedance to it's parallel equivalent if doing it by hand. The j207 of the loop is 75.9 nH.

Then if you use this calculator you'll find that the impedance is 75.9 nH in parallel with 34.3 kohm. This of course assumes that the loop impedance actually includes radiation resistance in the 1.25 ohm value.

So, you are trying to convert a source resistance of 166 ohms to an antenna load resistance of 34.3 kohm whilst consideration is made for the 75.9 ohm parallel reactance of the coil.

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  • \$\begingroup\$ I followed the antenna design guide for the Printed Small Loop PCB Antenna, for which Tapped Capacitive Network is suggested. I have added the equivalent circuit of the system in the question. I'm not sure if I can follow your advice of converting the source to purely resistive. \$\endgroup\$ – spockshr Jul 27 '20 at 15:59
  • \$\begingroup\$ @spockshr I'm not going to help you. Reason: you've changed the impedance in the question from being series inductive to series capacitive and not cared about the repercussions on any answers given. This has basically screwed-up my answer. So why should I bother to help you? I have had to go back to my answer and edit in the old original picture so that folk don't think I'm a moron. That's wasted enough time for me. \$\endgroup\$ – Andy aka Jul 27 '20 at 17:18
  • \$\begingroup\$ I beg pardon for this. I realized my mistake and edited the question. I was under the impression that when I edited the question, you would be notified by stackoverflow. I appreciate your effort and would bear in mind your suggestion. \$\endgroup\$ – spockshr Jul 28 '20 at 3:29
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    \$\begingroup\$ @spockshr do you think that getting a notification would have saved the day? No, once you have answers, then editing your question can invalidate them as your edit did. Appreciation maybe but that is shown in different ways on this site. \$\endgroup\$ – Andy aka Jul 28 '20 at 7:53

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