I am designing a PCB including a GPS module. I am using the Locosys LS20031 which has a antenna mounted on it. The module is mounted on the board with 1" plastic studs and a connector connects it to the PCB . The question is if it's good/wrong to lay a layer of copper on the PCB area underneath the module.
Will it reduce the antenna gain/sensitivity? It's better to put a keep-out area instead of copper?
Electronic – GPS module antenna shielding
antennagpslayoutpcb
Related Solutions
Generally, "support for active antenna" means that the receiver can supply DC power (typically 3.3V or 5.0V, current-limited to about 30 mA) up the antenna cable. With a highly-integrated module like this, that support is generally done on the PCB that it's mounted on, rather than inside the module itself.
While this module may work perfectly well with passive antennas (especially if the connection is short), active antennas will pretty much always give you a slightly better noise figure.
- 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.
Best Answer
It is typical to avoid placing any copper underneath the GPS antenna for the reasons you stated. You should be careful routing anything below the GPS module because it may leak and cause interference. Route them as short as possible. The module itself would have a ground plane if it was needed.
Looking at the datasheet of the product I can see a hint that in their reference design they removed all copper layers below the antenna, as they should have.