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I am designing a four layer board in which a patch antenna is used for a GPS receiver.The antenna's feed is a through hole and reaches the bottom of board, where it meets a 50 ohm transmission line. The bottom of patch is filled with ground but the layer adjacent to the ground filled layer is a power plane.
Should the layer adjacent also be ground or will this influence the performance of patch?
I've had a similar issue while changing the power plane, a quick fix is just to delete the outline and try again. If that doesn't work maybe this article at Labcenter about power planes under ARES will be useful.
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.
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.
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
This should be answered in the datasheets. Without knowing anything more about your particular situation than you presented here, I would assume that there should be no copper at all below the patch antenna. The keepout rectangle probably extends a bit all around the patch antenna too.
Again, details should be in the datasheets. If this patch antenna is for a specific frequency or specifically for GPS receivers, then its datasheet should give you all the relevant layout rules. There should also be rules on how the connections are routed. Metal nearby can significantly change the antenna characteristics to the point where performance is degraded. The GPS module datasheet may also have information on how it can be connected to particular antennas.