Tuning GPS patch antenna - Page 1
Forum section: electronics / rf-microwave

author: jmw
author_rank: Frequent Contributor
timestamp: September 19, 2020, 01:34:40 am
content: I noticed some GPS modules have cuts and scratches on the top side metal and discovered a note in the datasheet that this is part of antenna tuning during production.Given the volume production of these modules, is this work done by hand or machine? The marks don't look like they followed any pattern, so maybe it's a technician who nicks the silver at random until the return loss meets a spec.

author: Orange
author_rank: Frequent Contributor
timestamp: September 26, 2020, 07:28:16 pm
content: Interesting question.Looking at the 'mutilations' they are 90 degrees offset (aprox). Probably to make a correct symmetric circular field in addition to the SWR.

author: ogden
author_rank: Super Contributor
timestamp: September 26, 2020, 09:00:13 pm
content: Depends on the volume and labor cost. Particular antennas definitely are hand-tuned. Cuts of the machine would be much more straight.

author: cdev
author_rank: Super Contributor
timestamp: September 29, 2020, 02:23:06 am
content: Its done so they resonate at the GPS frequency. The environment, ground plane of any antenna but especially a high-Q ceramic antenna varies quite a bit depending on what the ground plae (especially) is but also it influences the characteristics of the polarization versus merely gain.A simple calibration could probably be automated with a test jig in a very basic simple anechoic chamber or possibly even without it, if the space is big enough. Of couse now with Ubox amd most other manufacturers the target is a significantly wider range of frequencies than it used to be. the patch antenna will work but its not ideal. Just like you can get away with using a GPS antenna for any sigals in the 1550-1600 MHz range. Kind of.GPS patch antennas are actually non optimal as far as circular polarization too because as the angle becomes closer to the horizon they become mostly horizontal polarization. this makes them particularly suceptible to low angle reflections because its the polarization that allows them to reject even numbered reflections.Depending on your use case, it may matter and then it may not, but the one thing that always matters is if its resonating in the right band.. Patch antenjas are not my favorite GPS broadband antennas, thats for sure.They are very finicky and even identical models from the same batch and same manufacturers on the exact same size ground plane vary noiticably. Luckily with GPS if you have the patience, you could probably work out a great way to test them and maybe even tune them just using the gps satellites themselves as your signal sources.the best broadband multiconstellation antennas would be archimedian spiral antennas, you can flip it over and use the other side for the opposite polariztion as a dish feed.I would suspect or some variant. A piinwheel antenna using a 90 degree hybrid and four arms works well. Going from two to four arms brings a big improvement but additional expense for the broadband hybrid.Quote from: jmw on September 19, 2020, 01:34:40 amYou can bet its done by machine - a network analyzer likely combined with some proprietary antennas and signals to check the polarization? also a stepper motor and controller perhaps? - Without something to rotate it and measure the polarization is just going to be able to tell you where its resonating though, nothing about symmetry or the stuff you would want know for a precision antenna, but, good enough to get it working okay.You could build a robot with an arduino or raspberry pi to do anutomated testing, the arduino controlling stepper motors to move the antenna around while the GPS collects data. It would need to be outdoors in the middle of a field.patch antennas are, good enough for most GPS use. Just use a ground plane for them. (Doh!) they dont work without one. Up to a point the bigger the better. A ferrite bead to decouple the feedline helps a bit too.