antenna
I am designing a GPS board with an SMT patch antenna (SGP.25cfrom Taoglas). I understand that the rf feed is 50Ohms impedance controlled track. But the datasheet shows 2 feeds – inner feed and outer feed. What are the 2 feeds. Does the inner feed mean that I can place the GPS on one layer and the antenna on the other layer and connect the RF track via the inner feed. In that case how is the impedance controlled through the via.
Thank you.
A full size patch antenna (normally about 25 × 25 × 4 mm) will typically have a gain of around 5dB and for an SMA connector pair insertion loss for even fairly cheap connectors is likely to be under 0.5dB at 1.5GHz. For coax RG-174 is a pretty popular (and cheap) cable often used on consumer grade GPS antennae. Running the numbers through this Attenuation and power handling calculator over that short distance the loss at GPS frequencies is only 0.3db. So you should still have a net gain of over 4dB.
Most modern receivers will work well with that and for example the Fastrax IT520 documentation recommends a loss of less than 1dB when using a passive antenna which the above combination achieves. In a vehicle application sometimes the effect of too much gain can be mixed, you can for example start picking up weak reflected signals that can cause multipathing effects. Other than extended testing with a particular module there's not really any way to predict that reliably.
For ESD protection I don't really see an issue if you're using an encapsulated antenna and the SMA connectors are normally always mated. But a little extra ESD protection isn't a bad thing and while the low capacitance protection diodes made for that purpose are small they do normally meet all the usual ESD standards.
So in general I'd say give the external passive antenna a shot and compare against an active in some real-world tests. Some of the cheaper antennae are actually quite good, so grab a few of each from a variety of manufacturers and unlike reworking a PCB design they can easily be compared. It sounds like you've talked yourself out of an on-board antenna for other valid reasons, but I don't see signal loss to be significant enough over that short distance to worry about that side of things.
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
No, you need to leave a hole through the solder mask under the inner feed and provide an isolated pad to solder it to. Anyway, if you feed from the inner feed, then the outer feed is going to become a stub that could cause some rather major reflection issues.