So I read an interesting article about GPS antennas called "Adding a GPS Chipset To Your Next Design Is Easy".
A few points to bring up that I have concerns with dealing with my M8N antenna.
1. Active vs Passive Antennas. Two paragraphs within the article describes the difference between Active and Passive antennas. According to CSG Shop's specification for the NEO-M8N it comes with a low-noise regulator and RF filter built-in. So I'm assuming that it is a active antenna.
2. Antenna's requiring adequate plane. If I read that document correctly, these GPS modules may require a GPS plane as they are installed on a PCB that does NOT have 40mm of side to them.
Quote: "Generally, patch antennas in the 15- to 25-mm size range with a least a 40-mm (on a side) ground plane will give the best performance in portable equipment, but this may be too large for your application. This could force you to look at smaller antenna topologies such as linear chip antennas."
3. The next concern is to mitigate the noise interference from FC, ESCs, and PDB. Since my Y6B is set up with a clam shell cover and my M8N is attached under and close to the all the electronics, I may need to develop a shield "ring" connected to the shield can and then connect that ring to RF ground through an inductor at a single point.
Quote: It's common in VHF and UHF RF shielding to connect all points of the shield can to the PCB's ground plane. This can be a mistake at GPS frequencies, since the open-air wavelength of a GPS signal is so much shorter than UHF. Depending on the size of the shield can, if there is current flow across the can, the shield can will be able to resonate near GPS frequencies resulting in interference or de-tuning of the GPS RF.
By developing a shield "ring" connected the shield can and the inductor, the inductor will filter any EMI-induced current flow. The ring connected to the shield can will prevent any current flows or resonation issues.
I'm not an electrical engineer and need guidance from those out there who are. Did I interrupted this correctly? and if so I could use some help with developing the "ring".
Doug, I've found your house...
Pls, mask GPS coords.
No worries, I have plenty of fire power and two dogs that like to chew on things. LOL
Yesterday I decided to perform a bench test outdoors in open skies and I used two of my copters for the test. The test ran for 1 hour with the first 45 minutes running on 5v USB power from the laptops. At 1451 battery power was applied for the remainder to the test (15 minutes).
The Y6B is setup with
LEA-6H as secondary GPS mounted on a pole approximately 10 cm
NEO-M8N as primary GPS mounted under clam shell cover just 15 mm above the Pixhawk
The OctaQuad is setup with
LEA-6H as primary GPS mounted on a pole approximately 9.5 cm
NEO-M8T as secondary mounted external to clam shell cover just 25 mm above the Pixhawk
There are some interesting results to share.
1. LEA-6H never performed as well and the NEO units but one has to take into consideration that these units are single band therefore limited to what satellites it is able to see and lock onto.
2. NEO-M8N I tested was a dual band unit and it performed quite well for the most part. I was a bit surprised to see that it performed almost as well as the NEO-M8T which was a triple band unit.
3. NEO-M8T showed the best results and improved when battery power was applied around time 1451. I noted that the HW power level on the Pixhawk was registering at 4.52 volts but when I powered up the X-8, I saw the power jump up to 5.2 volts. So it's very possible that the GPS needed the extra power to improve signal detection and quality.
4. My initial ground test conclusion is that the LEA-6H units are limited in which satellites they can see (GPS) but according to UBLOX it can be flashed to pickup Galileo satellites. They seem to not be affected by EMI but that hasn't been extensively test as they are mounted up and away from most of the electronics.
The NEO-M8N performed quite well in close proximity of Pixhawk, LiDAR, and FPV transmitter among other electronic components. Little EMI was observed during this test.
The NEO-M8T was by far the winner of this test as no EMI was detected and it performed very well even in close proximity of ESCs, Pixhawk, LiDAR, and FPV transmitter. Only downside I saw of this unit is the lack of an on-board compass but if you plan to use two GPS units, I would recommend the M8N as secondary and mount it on a mast using it's compass and then mount the M8T as primary either on a mast or anywhere close to frame.
Tomorrow I plan to run test two which will include hovering both air vehicle at same location and altitude to gather more data points to share.
All NEO units I tested came from CSGShop.com.
M8T - Triple Band with Ground Plane and EMI Shield, no compass.
M8N - Dual Band w/45mm Antenna
Interesting that I just came across an M8N with Ground Plane and EMI shield and a compass. Must be a new product.
All units I have are passive.
As for price point, you get what pay for. No guarantees when you buy of Ebay that it isn't a Chinese knock off.
Look at CSGShop for their 25mm M8N antenna. I have one installed on my Antenna tracker. It's still it bit pricey for the market pricing your interested in.
Flight Test Today.
See results below.
Flew both copters this afternoon and saw some very stable results. Unless someone can see a noticeable difference between the ground test and this afternoon flight test, I'd have to say that noise wasn't as big a concern to my GPS units as I had thought. The M8T performed better than any of the GPS units with M8N pulling a close second.
Interesting results. Not what I expected but thats the reason for tests.
OBTW, anyone notice that 3DR no longer sells the LEA-6H? Writing on the wall. They did switch to the NEO-7M. Not sure why. Does anyone have information as to why they prefer that over the NEO-M8 series?
Here's the way I have my CGS M8N setup and I've had no issues running 3.3 rc5. It's mounted on a copper clad board that is grounded and all the wiring runs through the CF pole. 12 sats inside and 18 outside with HDOP consistently 1.2. I'll do a short flight the next time I'm out and check the logs to see if I get any errors.
Would you be able to confirm if you still have problems with INAVErr using this new gps(neo-m8t) from CSGshop? what your logs shows? There is quite a lot of people thinking that the inaverr was related to interference from other electronics... the NEO-M8T has EMI protection as also a larger ground plane so if interference was the problem in first place this new gps should show some improvement or maybe not even show any problem in INAVErr...
I (and I believe many other as well) would appreciate your reply.
Hi, thanks for sharing this information! I am looking into replacing my 3DR GPS receiver with something that has a ground plane – the CSG M8N EMI seems to be a good fit, except it doesn’t include the compass. So my first question is if I might be able to use the CSG receiver for GPS and the 3DR receiver for compass capabilities (connecting both to my PixHawk flight controller)? Do you know if PixHawk would support this? Also, I’ve been trying to get in touch with CSG to see if it’s possible to easily configure the CSG M8N + compass with a ground plane. Basically putting the following products together:http://www.csgshop.com/product.php?id_product=173 and http://www.csgshop.com/product.php?id_product=202
Any insight or recommendations? Thanks!
Pixhawks supports up to 2x GPS units and 2x compasses (however, one is internal to Pixhawk) so it's possible to do as you stated.
Both my Y6 and X8 have dual GPS antennas on them.
X8 has the M8T as primary GPS and my LEA-6H GPS as secondary with the compass connected via I2C.
Since you plan to purchase both the M8N+Compass and the ground plane M8N, I would recommend you keep the M8N+compass on a mast far enough away so as to not cause EMI to the compass. The ground plane antenna can be mounted in close proximity of electronics as it's shield and ground plane are quite good at mitigating EMI issues.