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RFarmer v0.2 testing and antenna matching

A project log for NANDfarm => RFarmer

Small, easy to use, cheap ZigBee developement board for the internet of things.

Radu ConstantinRadu Constantin 08/08/2014 at 15:031 Comment

Unofficial rules for PCB trace impedance design:

1. You can never have too many vias.

2. Add more vias

3. Follow the datasheet instructions

4. Every PCB has different impedance so you may have different matching values for different PCB from the same batch.

5. You can never achieve more than 20-30% efficiency without a Network Analyzer that works at more than 2.5GHz.

6. If you are new at this, do yourself a favor and start with a dipole antenna.

7. Before you buy the matching components make sure their tolerances are in specs. A 0.2nH tolerance may not sound like much but it may be the difference between a working device and a pile of nerves.

As you may have guessed by now, I started testing the second version of the RFarmer. Once all the parts arrived I assembled the boards and I was ready to start tackling the matching circuit for the antenna trace. Imagine my frustration when I found out that the university where I can work with the network analyzer was closed for the summer vacation. What to do, what to do…

There must be some other way to match my circuit, right? Surely there is a hack-able way to do this, right? Right? ... No there isn’t! Not a full match at least.

Let’s talk some basics theory that was once learned and then forgotten.

In circuit theory we learned that maximum power is transmitted through a load when the resistance of the load is equal to the resistance of the source. This is DC. When we talk about AC, the same rule applies, but resistance is called impedance. Now, impedance varies with the frequency of the AC signal, and this is where it starts to be interesting. When you start working with high frequencies things start to get a bit voodoo. The main idea is this. The higher the frequency, the more interference you get from the things around the trace (other traces, vias, pcb dielectric, etc.) When you design a design a trace for an antenna, the main (only) impedance you wish to have is from the capacitor formed by the trace itself and the ground plane on the other side of the PCB. But things are not that simple. As you increase the frequency, stray capacitance (read inductance) will start to form between the trace and other traces near it. Even if you have a ground plane around the your trace, the lack of vias between the two ground planes will screw with the total trace impedance, leading to weird stuff like the device working when your hand gets close.

---To be continued---

Discussions

Łukasz Przeniosło wrote 08/21/2014 at 06:39 point
Hey Radu,
Thanks for the very thoughtfull post. I will surelly take your notes under consideration when my board will finally get to be assembled :).

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