Homemade Helix GPS antenna
Introduction
The GPS comes generally with a small one inch patch antenna.
While this antenna has a good sensitivity and a small size, you probably noticed some positioning errors due to indirect reception path, especially in urban areas or close to a wall.
Improve the GPS reliability
Build a helix antenna
The backfire helix antenna is quite easy to build, cost near nothing and definitively solve this position error problem.
Various helix antennas can be found, i used this one as a guide : http://lea.hamradio.si/~s53mv/navsats/analog.html
Most GPS provides power through the antenna connector, in this case i had to add a 18pF capacitor for DC blocking purposes and avoid a short circuit with the new antenna.
A classic ceramic capacitor can be mounted on top antenna. Value is not critical, just select one from 10pF to 47pF.
I also did the same modification with a M8N module that has a built in magnetometer.
Another example using a SMA connector, the DC blocking capacitor is soldered between the antenna pin and SMA connector, close to the GPS module on other pcb side.
Results
Neo6: Patch - Helix antenna
On the images bellow you can see two GPS traces, first image is obtained running the helix antenna with +/- 3m error.
The image on the right show the original patch result with more than 20 meters errors/jumps.
If you are really looking for a very high reliability i would recommend to use a NEO-6 GPS module instead of the now popular M8N, this old NEO-6 device is less subject to position glitches. (see next example)
If you want to use a magnetometer you can use a separate HMC5883l module.
Neo8m: Patch - Helix antenna
In this test two GPS modules are used at same time, the left GPS trace show the original GPS Neo8m module and to the right trace another same module running helix antenna with SMA connector.
You can notice the North-East spike, those spikes or quick jumps can occurs as well using a helix antenna.
The position change jump quickly 10m or more and return slowly to original position.
This jumps issue looks related to neo7/neo8 family and can occur using only GPS(USA) as well.
My flying test platform
I did my tests with this light tricopter, the magnetometer module is mounted on a 20cm vertical mast while the GPS module and helix antenna is mounted on the tail boom.
When i switch the PositionHold flight mode it looks like my tricopter is nailed in the sky, no more fly away or erratic moving.
I highly recommend this antenna when flying in a urban zone where multiple path reflections are a problem for the GPS.
