I calibrated my VNA for open, short, and load and took a S11 (reflection coefficient) measurement using a pigtail connector soldered on my BGM121 based PCB. The lowest point on my graph was around -10 db which was centered at around 2.41 gHz. I also noticed that my PCB was extremely sensitive to noise. Attaching cables to my board that weren't in the antenna loop area affected the VNA measurement. In addition when I placed both my fingers on the sides of the board, the lowest point dropped to around -20 db and the frequency shifted a bit to around 2.44 gHz. (the drop in power loss made little sense to me because I expected the reflection coefficient to get worse because of human body contact -- was it possibly the placement of my fingers on the side of the board that made the reflection coefficient better?)
After taking these measurements, I removed the pigtail connector and replaced it with an 0402 package 1/16W resistor that connected the RF_OUT pin with the ANT_IN pin (the pin that was previously connected to the pigtail connector). I connected my PCB to the development kit through the mini simplicity connector and uploaded a bluetooth advertising program on it. I used the Blue Gecko phone application to see if there was a signal and did not receive anything.
I would like you to verify my logic for this below and answer the following questions:
1) Although at -10 db the board is only reflecting back 1/10 of the signal and 9/10 is being radiated by the antenna the 1/10 reflected signal interferes with the 9/10 being radiated. So although, there is a strong enough signal being radiated by the antenna, there is a signal integrity issue with it which causes the radiated packages to contain corrupted data so my phone cannot detect a signal. Is this logic correct?
2) According to the datasheet, adjusting the antenna loop width affects the frequency of the signal. My problem was not that my antenna was off frequency. The reflection coefficient of the antenna was at its lowest at 2.41 gHz which is within the bluetooth band. Will adjusting the antenna loop help me with my problem? Will adjusting the antenna loop help with reducing power loss?
3) My PCB was extremely, extremely sensitive to noise. Anything small that came near the PCB, such as metal that wasn't even covering the antenna loop area significantly affected the performance. What is the reason for this? Is it because my PCB requires me to tune the antenna loop? If I tune the antenna loop to the optimal size for my application, will it not be that sensitive to noise? I have taken similar measurements with the BRD4302A test board for the BGM121 module and have noticed that the BRD4302A is much more robust to noise. I can put my keys on top of the BRD4302A without significantly affecting its performance. However, as I mentioned above my PCB is extremely sensitive to noise. What could be the possible reasons for this and how would I go about fixing it?
4) If I need to adjust the antenna loop width to tune my antenna, how would I use my S11 measurement to find out how much I should adjust the loop. My PCB has 4 layers so I can only be able to adjust the top and bottom layer (be either scraping off copper or adding solder) and see how that affects my S11 measurement. Is there a better way to go about this? Is there a size that most people increment their antenna loop width by because I have absolutely no idea if I should increase it by 0.01 mm or 5 mm. If you could give me a decent size to increment it based on past experiences, that would greatly help me when implementing this in my newest design. When I do adjust the antenna loop width, should I recenter the position of the BGM121 module on the board?
5) If my issue with not getting a signal was not due to a signal integrity issue but rather due to losing too much power that the signal transmitted wasn't strong enough to be received (although I doubt this because my phone was right next to the PCB), how would I increase the power transmitted in the code so that I maximize the TX power of my module and can see if I receive a signal when I do that.
Please thoroughly read through all my questions as they have multiple parts. I am currently in the process of designing and fabricating a new PCB and would appreciate it if you could give me quick and thorough responses to my questions.
1. -10dB of reflection loss would mean that the antenna is fairly well tuned at that frequency, so it should be OK. However, you may want to have this S11 level across the entire BLE bandwidth. You range issue can be related to many factors as listed in the KBA link below. It is also recommended to make sure the code you are using is correct.
2. See above 1. Also, it may happen that the measurement setup is wrong. Please share some photos from your measurement setup.
3. It tells that your custom HW might not be optimal. Please share your design files for our review. It is recommended to open a SalesForce case for this and for point 2 above too.
4. Using 4 layers, it would mean several design variants.
5. Use max power settings available.
In general on this issue and topic, we'd recommend to you to open a SF case and share your design files & meas setup photos for our review in order to be able to give you more exact suggestions.