IoT design a usually a matter of antenna as already seen in different previous blog post. Antenna performance is the assurance of your capacity to deploy your object in larger zone and a way to save energy by reducing transmission power.
As we will see, if you get a hardware component and simply put it in a box its radio behavior will be totally different as the box is impacting the transmission.
This post will practically show you the impact of a box on a device radio quality.
Let’s start by introducing SWR concept : you are transmitting a signal from a component (a emitter) to an antenna. If they are correctly match, in theory, all the power is transmitted to the antenna. We have a SWR equal to 1.0:1
In the reality the matching is not perfect and de transmitter impedance is not exactly wire impedance and not exactly antenna impedance. As a consequence every time a part of the power is not transmitted to the antenna but returned to the source.
As this part is not transmitted to the antenna, this energy is lost for the transmission.
The SWR is not the only factor impacting, once the power is transmitted to the antenna, the antenna itself has its own radiation capacity : the received power is not transmitted equally in all the directions and we can have antenna with or without gain. An antenna with gain will potentially radiate more power than received (but usually not in all directions). An antenna with no gain will usually radiate less power than received.
At the end your device transmission power will be something like :
minus Power Lost in the connectivity to the antenna
minus Power Lost in the antenna itself.
Basically many things are impacting your antenna matching, first of all the wire between your emitter and the antenna, you can see this post on antenna wire design for more details. The next important thing is the box where you put your device and it’s direct environment.
It means as soon as you change the environment around the antenna you get a different impedance and a different performance. As we will see this performance is really different and this is the reason why the operators in LPWA request you to certify your device even if you just change the box or just put a certified device in a box.
So let’s take the example of a device communicating on Sigfox with this basic response over the air without any box:
We see we have a pic indicating where the loss is minimal. The tiny white arrow is corresponding to 868,2Mhz ; the central Sigfox frequency. This indicates the antenna matching is quite good. The RL is -12dB.
The following table is indicating the corresponding Power Loss … less than a 1dB. This is really great and indicate the impact of the wiring between transmitter and antenna is really low.
As a consequence the radio performance is really good as we can see this report from sigfox backend ( You need to trust me as this has no comparison reference other than my experience for this location )
Now we are going to put the device (same one) in a standard ABS plastic box and close the box :
As you can see, just by changing the environment (not the location, not the power) we are really impacting the transmission. Basically we have a loss of 10dB by adding the box. As the transmission power is 14dB originally it’s a lot and in percentage of the power it means going from 25mW to 5mW equivalent. So 90% of the provided energy has been lost in the transmission.
Now let’s see what we get from the VNA analysis once in the box :
As you can see the signal is different and not centered anymore. The white markers on the right are indicating 868,2Mhz and now the antenna is centered on 810Mhz. The SWR value given by the VNA is about 11.0:1 corresponding to a theoretical loss of 80% about 6-7dB.
To solve this problem we can add a matching circuit between the emitter and the antenna. This is a couple of inductor and capacity to filter the signal and modify the inductance. My post on the use of a VNA will help you to understand how to make it and the link to the website I use to calculate the values. I did not had the right inductor/capacity but use some of the more appropriate. The result is quite impressive:
The horizontal scale on this capture is not the same as previously, here the pic is at 840Mhz, It’s not yet as perfect as the beginning but better. The SWR is now corresponding to a loss of 2dB equivalent to a loss of 36%.
This have an impact on the radio performance as in this transmission result :
For sure this is really less than the initial transmission, free in the air. But we can see the saving of the 4-5 dB (from 6-7dB of loss to 2dB of loss as given by the VNA). In fact you have to trust me a little bit as the rssi is not a stable value and you need to get more transfer for having a strong opinion.
If you have follow all what I’ve written until know you may ask me why we have about 10dB lost in the box when VNA says 6-7dB and why once matching corrected we still have something like about 6dB lost when we are expecting 2dB.
So ! this is a good question ! why this box is impacting the transmission about 4dB whatever the antenna matching is.
If any of you can give me some input on this point I’ll be happy to know !! let’s comment 😉
Putting the antenna in a box is not the only thing impacting the transmission : the place the box will be installed also impact the performance. In the following example you will see the difference for exactly the same object, in the same box, in the same place. The difference is : in the first communication the object are nothing around the 20cm, in the second configuration a metal plate is magnetized on it back (normal use case for this device). Check the results
|Antenna||RSSI First (free)||Rssi Snd (magnetized)|
In the concerned box it already have a large metal, magnetic area in the same place so it is really related to the environmental change more that the presence of a metallic plate blocking the signal (even if this is also playing a role) . The environmental change is important to consider, particularly when the object is transported by a human moving and putting it in its hand or a pocket. The technology of the antenna and its choice can positively or negatively impact these environmental interactions.