This morning, some colleagues helped me to test the rf performance of one of my sigfox tracker. I’m really not an RF expert, I try to improve but there still be a long way to go…
In this design I’m using a TD1204 and a Wurth smd 868 antenna ; I have a 100 ohm impedance net between both (sure it is bad) and LC circuit (that sound not to be the good one – btw).
The process to to the test was to use a reference emitter using an antenna with 0db loss / gain and an emission power known ; measure the received power from this to another one with a spectrum analyzer. This gives a power loss reference due to the environment. This as been done in a specific room avoiding wave reflection. Then change the reference emitter by the device to be tested and see the difference in the power reception. Turn the device and test in different directions.
In the best case, the power loss with this antenna is -10db classing my device in the U2 device class. (not really good, but sound acceptable regarding the size of the object) The results vary a lot depending on the axes of the device regarding the antenna. This means with a such antenna you have to expect to have multiple antenna at different location to receive the message to ensure you get it !
So the antenna worked as expected even if the impedance of the circuit is not really the good one and if the ground plan was really short due to the sizing of my prototype. Interesting to see.
On the side it was interesting to see the sigfox channel hoping and the different power pic coming from this channel hoping and multiple transmission. Making sigfox protocol concrete on a scanner screen ! As much as we have seen the used frequency is around 868.1xx.
Now, I need to understand how to create a good matching circuit … seems to be a really more complicated question … any post around this could be interesting to read… please post link.