Active GPS Antenna Modification

Receiving in the mid-L Band can be a bit of a challenge, Many fun things are in this band like Inmarsat, Iridium, Glonass, GPS, GoES, NOAA exc
Generally speaking you need to make a tuned high gain antenna such as a patch, bi-quad, helical or 1/4th wave snub on a dish.

I have made a few of these as shown below.


These require external amplification, filtering, calculation, precision measurements, lots of copper and tuning.

But there is an easier way people have floated for quite some time, Taking an active GPS patch antenna, replacing the filter and the antenna with the band required.

But if we actually have a look at some GPS ceramic patch antenna datasheets they actually have a wider band than you would expect … ranging from 30 to 100mhz of reasonable VSWR
it appears as if we can (at least try to) keep the patch included, what might have worked out better is if my patch is rated for not just GPS but also GLONASS.

The antenna i have is the ANT-555 magnetic GPS antenna from Sparkfun. This antenna appears to be a cheap chinese alibaba find as if you search for its datasheet each link appears to show a different one. I used the datasheet as supplied by sparkfun that states the antenna works with 2 LNAs that are sandwiching a SAW filter and obtaining 27dBm of gain in 1575.42MHz +/-3MHz

Well let’s start by cracking into one and see what’s going on, after opening it up and removing the shield i was shocked to find that all the circuitry was very small as shown below next to a US quarter.

Right off the bat you can see that there is a power extracting network, one small chip, another small chip and some passives.

The first chip does not appear to be an LNA and after some investigation it appears to be an absurdly small SAW filter, after some investigation it appears to be an SF1186K-3 filter in an insane 1.45 x 1.15 mm package by RFM.

So 3 things were just found out, for one the datasheet does not match up with the actual product so we can safely believe the rest of it might be total garbage, Two any idea of me replacing this filter have just went out the window, and any idea i have had on what SAW filters look like and how they are manufactured have also gone out the window.

So we are left with 2 options, 1 i could try and deadbug a filter in its place, 2 i could just short it out.
Not able to find a wide enough band SAW filter of a size i could easily deadbug without causing a feedback oscillation  i went with option 2.

after removing the filter in the least destructive way possible (yanking it out with pliers after all else failed and accidentally ripping up a passive i had to replace) i just a jumper in place.

Now i resoldered the can and ran some tests, i was less than impressed by both its performance on iridium and GPS.

I rectified this by extending the rather lacking ground plane with some FR4 soldered on to the grounded can and some vias as shown below taped to a tripod for testing

 

It looks fairly nice and on top of that it works rather well. I did testing with an AirSpy for its reasonable noise floor, high gain, proformance at the desired frequency and bandwidth

 

As you can see it does more than a respectable job at capturing iridium packets and these packets can be rather easily decoded!

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