Proper PCB Layout For GPS Antenna

I am currently working on a project that involves creating a custom PCB. The board has a UBLOX MAX-M8 GPS Module, as well as a Rainsun GPS1003 SMD GPS Antenna.

I’ve created versions of this PCB in the past with the antenna right next to the GPS module as shown in the old_pcb photo below. This system would get GPS time after a few minutes, but never was able to get a position fix. I had to solder a LED leg to the pad and antenna to separate it from the PCB to get it to work.
old_pcb_not_working

I noticed on other boards that use this GPS and similar antennas that the antenna is separated from the main PCB on a section that usually sticks out and is separated from other components. In my new PCB, I did something similar (as you can see in the new_pcb photo), but I wanted to see if anyone had any insight on this.
new_pcb

Thanks!
Noah!

You MAY get a better result without the ground plain around the antenna & trace, These WILL attenuate the signal.

With the so called ‘antenna’ so close to the GPS antenna input the PCB layout is not going to make a lot of difference.

Those tiny ceramic sticks are pretty useless as antennas, just accept it.

If you want the GPS to get a fix properly then fit a better (and bigger) antenna.

Not a radio engineer, eh? :laughing:

You clearly do not understand what an “antenna” is.

An antenna couples electromagnetic energy into free space, where “free space” means just that. Space unencumbered by things that block electromagnetic radiation such as copper foil and PCB traces. And an antenna must for anywhere near maximum efficiency, be tuned by its dimensions and shape, to the frequency involved. And part of that tuning involves the transmission line between the transmitter/ receiver and the antenna itself. At the frequencies involved here, a random PCB trace does not “cut it”, it has to be carefully designed in dimensions as well as the parameters of the PCB material and the ground plane on the opposite side.

The most practical antenna in this case is actually a quarter wavelength of stiff wire calculated for the frequency, projecting perpendicular to the PCB. Given that you are mounting this in a protective non-conductive case so that the antenna does not get distorted, this would be quite a good solution.

There are only very limited circumstances when one of those tiny ceramic sticks is acceptable as a GPS antenna.

They are used in light high altitude balloon GPS trackers and whilst once the balloon is high in the air they perform OK since they have a very good unobstructed view of many GPS satellites, it can take an age for the initial ground based fix.

The ceramic sticks are also used where the controller can use GPS assist (as in a mobile phone for instance) wherby the GPS satellite empheris information is downloaded via the mobile phone.

For a comparision of how the type of antenna affects GPS performance take a read here;

Note how badly the ceramic sticks do and how much better a plain bit of wire does.

The datasheet for the antenna will show you the correct layout for the PCB around the antenna - follow this exactly.

For the trace between the module and the antenna you need to use 50 ohm microstrip.
Calculators for PCB microstrip are available online, you need to know what your PCB is
made from (usually FR4), and how thick it is (or more precisely the thickness between
the trace layer and the groundplane layer.
https://en.wikipedia.org/wiki/Microstrip

Note that for receiving only the antenna match is typically less critical than for transmitting,
so you may get away with poor layout, but for GPS signal strength is crucial you need
every 0.1dB of antenna efficiency you can glean.

Thank you for the link to the website!

This project happens to actually be a HAB Tracker that I’m working on. Although the ceramic antenna will likely work well with my tracker when it is thousands of feet up in the air, I’d rather put a more effective quarter wave wire to increase performance early on, and when it lands.

Thanks!
Noah

This was incredibly helpful. I am an Amateur Radio Operator, so I am aware of the importance of using 50 ohm cable, but I didn’t know it applied to PCB traces!

My PCB actually also uses the DRA818V VHF transmitter module which passes through a low pass filter before going to an SMA connector. My traces are extremely short (a few mm at most), but I’ll definitely check out using a 50 ohm microstrip for that.

Noah

Ah, the bit that most people forget about, it might be OK in the air, but when it lands …

I have tested one of the ‘commercial’ and well known GPS tracker boards that are intended for HAB use and uses the ceramic stick, that also performed badly, no good if you need to do a ground level search.

Its not difficult to arange the PCB so that you can add bits of guitar wire as antennas. Add some small holes nearby and you can fit 1/4 wave radials as well, which does help. A 3/4 wave vertical improves reception further.

There are further comments on the use of wire antennas on GPSs here;

This is microwave stuff, mate!

Definitely going to be reading into that a bit more, never knew the topic was so complex! Very interesting though, thanks for the help.

That’s absolutely perfect. Thanks a bunch.

That makes sense. I’m definitely interested in using this tracker for ground purposes as well, as it uses the Ham Radio APRS network. Been working on this project for over a year, glad it’s finally coming together.

Not just the traces! It also applies to the material used for the circuit board the trace rests on. Remember that the impedance of coax is based on the material used as insulation, as well as the physical dimensions of the conductors.
Paul

If you look at the second PCB image in post #1, the ‘track’ between device and antenna is around 1/100th of a Wavelength.

Does a track that short need to be closely impedance matched ?

If so what about the SMT pad for the antenna pin for the GPS module, does that pad need to be impedance matched also ?

I dont know the answer.

However those ceramic sticks have loss of around -5dBm over a simple wire and in marginal GPS situations that 5dBm loss can be a major issue.

Did I not mention that?

Is it? What is the wavelength?

Not so much impedance; it becomes part of the length of the antenna itself.

Especially on such a high frequency, just a few mm added to the length of the antenna can change the ideal receiving frequency for the antenna a significant amount

Try looking at external antennas also.

https://www.adafruit.com/?q=gps+antenna&sort=BestMatch