Recurrent step when making an IoT device, the antenna matching is a key activity to get the right radio performance for your device. You radio strip and antenna must be tuned to match a 50 Ohm impedance. For doing this I’m using a miniVNA Tiny Vector Network Analyzer (until a switch to my Rigol Spectrum Analyzer) as described on the previously linked post.
The impact of a correct antenna matching has been addressed in an old post on this blog also.
Currently, to tune my antennas, I’m also using the Atyune tool. This tool is free and really good to make the tuning but also to get a better understanding of what you are doing. Let’s see how to proceed.
Get the radio circuit response
I won’t go to detail on how to capture the radio profile of your device as you can find all the details in my post on how to use a miniVNA Tiny to tune your antenna. Basically at the end you have the response of your antenna over the frequency your are addressing:
Basically here we have a good starting point with a SWR @ 868.1MHz for Sigfox and LoRaWAN reaching 2.52:1 This means the 1dBm loss.
Import in Atyune software
To import the data into Atyune, you need to export the S1 data into a file. At first you need to select the target frequency you want to match with in vna/J by clicking in it. You will get a (1) market as above. Then you can export the data : in vna/J tool (the one used to capture the device radio response) you need to go to menu Export >> S Parameters.
This is creating a file with extension s1p. Now in the Atyune software you can select this file and import it with the S11 type.
Now we need to find the right circuit to maker the matching and for this the tool have an internal optimizer. So you can go to Optimization Menu >> Find best Circuit. At this time you can select the type of circuit you want. here i can select a circuit with 1 component. Atyune will automatically propose a solution:
In this example, we have one inductance to add in parallel, the frequency is not centered on 868.1 but a bit after. The result is still really good as on the target we have a result close to one.
You can zoom the graph by going to scales tab and select the right Frequency range after switching Autoscale off.
Visualize the passive component impact
An option I really like in the Atyune software is the ability to modify the passive values to visualize the impact on the antenna response.
In the reality it is really rare to get the exact expected result and you need to manually tune the circuit. The software allows to you simulate the impact of changing the values and identifying the direction of the pic depending on the value you select. This help you to understand if you should increase or reduce the values of your passives in the circuit.
Between the two images above, the inductor is 4.7pF vs 3.9pF. when changing the value dynamically you will see the movement and direction of the signal response and be able to find the right compromise.
Compare theory and reality
Don’t forget to make measure after making the antenna adaptation and verify the response. In the case illustrated above, The optimal value has been 6.8nH instead of 12nH.
You also need to take attention to the surface and position of your antenna when making measure as the environment really impacts the measure. you can see that on the video above taken with a Rigol DSA815. This spectrum analyzer has the advantage of showing the frequency response in real-time. But I don’t know yet if it can exports s1 parameters to be use with atyune.