antennaembeddedgpsgsmpcb
I need to put two antennas for GSM and GPS communication on my PCB. My final product will be embedded in a user-friendly plastic box with buttons, leds and switches.
I have not choosed yet which type of antenna I will put on my design (PCB antenna, ceramic antenna or SMA antenna), so I am wondering if all those types of antennas work if they are completely inside a plastic box or if some parts have to be outside.
Amtenna signals shouldn't interfere since the GSM and GPS are on different bands. However, the electronics might interfere with each other. Put GPS and GSM modules on their own ground planes such that all the parts are covered, then connect ground and supply in one place, plus bypass it with capacitors right there. Use at least three - some high quality 22pF in small package (0402), 1nF, and 100nF. At least the first two should be with NP0 dielectric. It is also good idea to bring the power from the other layer and to connect the two grounds (for GSM and GPS planes respectively) as close as possible to the input power connector (or battery).
Naturally, you want both modules as far as possible from each other, it doesnt matter whether it is on the same layer or not.
Connect to RF inputs through short microstrip. Make it thin, cover it from all sides with ground (including the other layer). Also use small high freq. parts for input filters (if its not in the module already).
Antenna performance depends on whether it is active or passive. Active ones are usualy external as they are bigger. Passive ones have low gain and it might take while for acquisition to identify enough satellites in case of GPS. The receiver correlates background noise with known pseudorandom transmission pattern of the GPS satellites since the signal is that weak. Its a wonder it works.
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
Generally, plastic does not interfere with an RF signal. Metal is another story.
However, any plastic has a dielectric constant and can detune anantenna that is in close proximity. Detuning from proximity to a dielectric will happen mostly for plastic located close to a full size resonant antenna, and relatively little for a miniature (e.g. chip or ceramic) antenna.
Whether the effect is significant will depend on many factors, not least of which is how much performance you need from the antenna system. In other words, if you have enough margin in your application, even if you reduce your antenna efficiency by a few dBs you may not notice the difference.
Detuned antennas can generally be re-tuned by adding components in the antenna feed circuit, or adjusting their values. But avoiding trial-and-error by knowing exactly what measures to take requires expertise and use of specialized and expensive test equipment (particularly if, as in your case, you are talking about UHF or microwave frequencies).
If you are thinking of using a PCB antenna, you can try using a reference design, paying close attention to dimensions and PCB material properties. However, to be sure you've got it right, it may be best to enlist the services of an RF engineer to fine-tune it. This would be the case whether or not the antenna were boxed inside a plastic case.
As suggested earlier, ceramic or chip antennas are more self-contained and less likely to be detuned by nearby plastic, particularly if there is a bit of space between them and the plastic case. However, they cost more than a PCB trace and generally have less efficiency.
Avoid using plastic that is loaded with conductive material (metal, carbon to make it conductive) will reflect or absorb RF, or metallic paint.