Matching antenna on circular board

[Johannes]

Hi 

I'm working on a IoT solution with a small circular board, 26mm in diameter. I was suggested by Johanson to use the 2450AT18D0100 antenna and a lowpass filter designed for nRF52840, the 2450FM07B0037. There are however a few things I don't really understand.

1) Is there a reference design for these kind of antennas? 

2) Does the lowpass filter replace the entire matching circuit or should I use the lowpass filter and then a matching filter? I.e. a matching filter between the FL1 and the ANT1 in the image below.

3) The antenna itself confuse me. The datasheet shows the ground pins on both sides to be directly connected to ground but some files sent to me from Johanson show both sides decoupled via capacitors. I was trying to find information about these on johanson webpage but I couldn't and I don't understand how these affect the tuning of the radio or what values to start with. Any suggestions? Below is my initial attempt, any comments?

/KafP

[Andreas]

Hi,

 

1. Usually there is a reference design for the antenna in the antenna datasheet. This specific part has 3, depending on the feed direction and clearance size.
2. Yes, the Johanson filter replaces the matching network in our reference design. The performance is very dependent on the layout though, you need to replicate the layout in the filter datasheet as closely as possible with regards to positioning and grounding. You seem to be missing a trace between the VSS_PA pin and the center pad: 
   
3. We do not know anything but what is in the datasheet unfortunately, but this is a quite common way to tune the center frequency of some antennas. A few pF is usually a good starting point, you will have to measure the VSWR using a VNA and test various combinations and see how the center frequency of the antenna is affected. If you do not have a VNA both Johanson and we can do this for you.

 

Best regards,

Andreas

[Johannes]

Thanks, it makes more sense now.