- I'm planning on allotting a 40mm x 40mm ground plane underneath the patch antenna
Note that the ANT1818B00DT1516A datasheet specifies a 50mm x 50mm ground plane underneath the patch antenna.
If the ground plane is larger or smaller then there will be effects on the overall performance of the antenna. You can read more in this Maxtenna application note. It states:
When the ground plane size changes the resonance frequency of the patch, where the matching is optimal, changes as well.
So if your ground plane is too small (or too large) the resonance frequency will change, essentially detuning it from 1575.42MHz and making it less sensitive.
In the graph below you can see the centre frequency varying by as much as 40Mhz when the ground plane is too small, the total bandwidth is also reduced. The gain is reduced by ~20dBic which can be seen in the application note.
- Can I have part of the antenna ground plane located underneath another small pcb that is mounted on top of the PCB with the ground plane on it?
Yes, this will work, but will also reduce the sensitivity. Ideally you want an unobstructed area around the antenna.
However, many commercial devices exist that have very densely packed enclosures, often with a screen on top. Consider the Garmin Nuvi below:
You will notice that there's not space for a large ground plane above the antenna and the screen is on the other side of the PCB. However, it's likely that a number of different patch antennas were tested before the devices were mass produced, so it's not directly comparable to your scenario.
If you want a small PCB then consider mounting the GPS antenna on the opposite side of the board to the screen. Allowing the correct size ground plane and test it. You might be surprised just how good the GPS receiver is.
- Do I connect the antenna ground plane to the ground of the entire PCB?
No. The ground plane is for the GPS patch antenna only and is separate from the rest of your PCB. You should not connect it to any other part of your PCB.
As a separate note you should consider the antenna feed. It should be run on the opposite side of the PCB and ideally impedance matched to 50 ohms. This likely won't matter for a small run of hobby boards but might be important for a high performance system, because GPS runs at ~1.5Ghz which will likely be impacted on standard FR4 PCB.
From the chip antenna datasheet:
https://www.johansontechnology.com/datasheets/antennas/2450AT18B100.pdf
It looks like you wire up a 50 ohm transmission line to the chip, then void the copper ground and power planes underneath the chip per their instructions. This isn't too bad of a layout, and using the chip antenna prevents you from having to do a more precise antenna on the board. These are usually a bad idea, because the constraints for your board get tougher, and would probably require a few design iterations to get right in addition to expensive test equipment to characterize. So you're on a good path.
The thing to remember when designing controlled impedance is to get the specs on trace width and layer stackup from the board vendor you're going to use. They will help you define those things on your layer stackup diagram, and also the trace widths for different controlled impedances. Don't trust your own software; always go with the board manufacturer.
In my experience, the shorter you can keep your transmission lines, the better. In your image above, why not decrease the distance from C23 to L1? Minimize this; try to have as short a launch into your chip antenna as possible. Keep the matching network components - it seemed like it had better performance in the datasheet.
Best Answer
you can use the Johanson 2450AT43A100 without the 50ohm feed line to reduce space and mount is as close as possible ti the output pin of the BT SoC. Because you are using a ground plane which is longer than a quarter wave length the only difference if you use a less wide ground plane is that the radiation polar diagram will be more of a donut shape this means that the antenna will have significantly higher gain on the sides than on the top/bottom. Please note that by referring to "the the Antenna" i mean the whole set including the ground plane. The antenna will behave as half wave dipole.