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".



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the content of my post just above

And Randy's reply have been suppressed


It was stated that this post was the one to also discuss about INAVERR errors with 3DR and M8N GPS

Please tell us is this changed, for everyone to be aware


I did not see either post. 

There is a lot of information coming out, but as far as I'm concerned, nothing yet conclusive or even much help getting to the bottom of this "problem". As I found and others have reported, a high INAVErr is not exclusive to the M8N but also affects the LEA-6H. It may not be as widespread, but is happening. Therefore I think before talking about shielding and such for the M8N, the high INAVErr issue (whether innocuous or not) needs to be nailed down as to what is the exact cause.

I'm confident it will be, just maybe not on the timescale many of us would like. It would be great if there was nothing to worry about, but based on comments from a few devs, it doesn't leave me with warm fuzzy feelings.  

I now believe Craig Elder's explanation for high INAVErr as being caused by use of the M8N is not 100% correct. That's just my opinion but I think there is enough evidence to support that opinion.  If Doug wishes to not use this group to discuss the unanswered questions, and only focus on shielding and EMI rings, that's fine with me.


It's fine that folks are using this to discuss the error rate issues.  I'm starting to see that the error rate is possibly from a timing issue between the GPS unit and Pixhawk processor.  So a message drops and is registered as a nav error but the airframe is stable and isn't flying erratic, then I believe the firmware the DEVs have developed is catching and ignoring it in favor of other sensor information.  This is good as no one sensor should have total control but a fusion of data helps the FC make the right decisions for flight.

My concern with bad nav errors needs to be addressed to whether its the firmware on the M8N, interference, or something else.  Seeing that much discussion out there is on GPS plane interference, I thought this discussion was a good idea.  After all, who's to say that the design of the GPS unit isn't causing the nav errors due to inadequate GPS plane or EMI ring?   I feel that either of these may help reduce the problem many are reporting as errors.


I'm still planning to setup a few tests in the days ahead as soon as I get a few more parts, so I can test my theory and report on it.  But at this time weather is holding me up.

Good day Fellows,

Antenna are my 'passion' as it were, and I have been following this discourse with some interest. As I do not use either APM or PixHawk in any of my planes I cannot comments on the effects being experienced by the various commenters. However, there does seem to be some confusion or lack of definitive info regarding the antenna, ground planes and shielding, etc.

If you will allow me to add my pennies worth, perhaps I can clear up some of the smaller issues. I suppose my comments will not really help find the solution to whatever your actual problems may be, but it may help some of you to not waste time down avenues that need not be explored. 

If my post is too long, shoot me down...

This should perhaps be a blog, but it really is aimed at this group so...

Firstly, the Article "Adding a GPS Chipset To Your Next Design Is Easy".referred to by Doug has a number of truths, but on the whole is quite misleading, untrue in places, and untrue by inference in others. 

As I need to extract sections from the article to comment on, I wish to give the author credit from the onset:

All extracts from the article at the above URL are credited to  Jeff Wilson, of Electronic Design, as published August 10th, 2011.

In these discussions the typical antenna referred to is a patch style antenna.- It is necessary to know how it works. In essence , it is a 'square' ( not always square) of conducting material, spaced a design distance away from a conducting ground plane. A conventional patch antenna  is approx 1/2 wavelength on a side, spaced a few mm above the plane, and the ground plane is typically a full wavelength on a side. This will give the highest directivity achievable from the design, with the lowest possible back-lobes , resulting in little pickup of RF noise from the rear. The radiation pattern is a half hemisphere upwards from the ground plane, with a reduction in directivity towards the sides. 

The patch antenna is in fact two coplanar dipoles, spaced apart a certain distance, radiating in phase. This gives the 'gain' or directivity of the patch antenna. The ground plane is an essential part of its design. The size of the ground plane will affect two elements - The Feedpoint matching, and hence the SWR on the antenna, and the directivity of the antenna. 

A patch can be designed to work with a small ground plane, from an impedance matching point of view, by reducing the ground plane and re-determining the patch feedpoint. So smaller ground planes are possible, as will be seen in my measurements. However, a smaller ground plane will reduce the directivity of the complete antenna. If the ground plane is reduced to the bare minimum ( and the feedpoint match re-determined), the resulting antenna will almost be omnidirectional, ie, it will pick up the 'noise' from the A/C electronics behind it just as well as the GPS Sat signal.

A further factor is that the GPS signal is Circularly Polarised, so the GPS receiver antenna must likewise be. One of the methods of achieving this is by 'clipping' two opposite corners on the patch - there are other ways. This clipping is not arbitrary - the size thereof is by design.

There are other types of GPS antenna developed for low volume applications, the 'chip' antenna for example. This is a linear polarised antenna, and so right away has a 3db loss in signal due to cross polarisation. It is also much smaller and much more inefficient, and will have a great loos compared to the larger patch types.

Now, if you did the calculations of my patch above for a GPS antenna, you would have found that the antenna is huge. At 1575MHz one wavelength is 190mm, so the patch should be approx 95mm on a side.....The size is reduced by placing the patch element on a material that has a high dielectric constant - Air, in the computation above has Er=1. You have seen the GPS patch antenna are on a ceramic substrate which has a high permittivity , often with Er around 40 to 100.

Wavelength = Speed of Light Co)/ (Frequency X (sqr(Er))

So with an Er of 60, our patch element reduces to 25 x25 mm.

The patch is most efficient with a substrate Er of 1, and reduces with increasing Er.

This loss can be regained by adding a low noise amplifier ( LNA)

However, the ground plane cannot be reduced  without losing directivity.

Since the GPS patch antenna manufacturers go with the flow in the ever increasing need for 'smaller', the antenna are reduced in size to suit the application, rather than the performance. I mentioned the impedance mismatch when reducing the ground plane. This requires correction if the optimum transfer of received energy by the patch is to be transferred to the GPS receiver.  It is the same for all RF interfaces - the impedances must match as best as possible. An SWR of more than 2.5:1 is not really acceptable.

So some manufacturers will make the patch on a small, or almost non-existant ground plane, correct the feedpoint placement, and ship them out, normally with a recommended size ground plane to be used. Others will bias the ground plane size more toward obtaining best directivity. Somewhere in between is a balance, and the recommended ground plane should be used. Changing this of your own bat will worsen either the matching or the directivity, if not both.

Probably none of you will actually design your own antenna, and maybe not even your own GPS receiver breakout board. But at least the above verbage should help you to understand if the design you have purchased stands a chance of working...

In the quoted URL, this is said -

“Passive antenna” designs are more complex and can be susceptible to noise coupling into the antenna ground plane if not correctly isolated from other noise-producing components on the PCB.

This is untrue - The antenna is simply that - the actual antenna ( the patch element, the ceramic substrate, the ground plane) is the same for a passive or active antenna. To make the antenna an active one, an amplifier is added - that is all. The antenna is no more or less susceptible to noise pickup - in fact , the active antenna is more susceptible, since the amplifier will now actively amplify any noise within its passband.

A further comment:

There’s one thing to note when deciding between chip and patch antennas, though. Patch antennas will provide the best signal performance for their size, as they receive signals on all sides of the patch. Linear GPS antennas (chip or dipole) will generally only receive signals along one of their axes. This results in linear antenna designs being at least half as sensitive (i.e., around –3 dB) compared to patch antennas, and most will probably be around 25% as sensitive as a patch (or about –6 dB).

Again, not true. The radiation pattern for the patch and the chip antenna can be designed to be very similar. The patch does not 'receive on all sides' and the chip not - they 'receive' according to the designed radiation pattern, and a chip antenna with a pattern in only one specific direction would not work for GPS and would never sell...And this is NOT the reason for the -3dB of the chip antenna directivity - the reason is that the Patch is designed for circular polarisation, while it is very difficult to do so for the chip antenna. So the GPS signal suffers the 3dB loss because of cross polarisation.  What is important with ALL antenna, regardless of type, is to understand what sort of radiation pattern you need, and what the antenna can provide. For GPS we would like a pattern that is a half sphere upwards, with maybe a -3 to -6dB loss on the horizon ( to eliminate pickup of man-made RF noise from the horizon). Most GPS patch antenna deliver this, if used with a suitable ground plane...

All GPS receiver manufacturers ( of the chip level device) , such as the various Ublox devices mentioned - Neo, Max, etc, have taken care of ALL emc issues related to signal integrity and noise effects to the device itself. External devices, the LNA, etc, are the responsibility of the board level integrator. 

And this is the most confusing of all-

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 wavelengths 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.

Absolutely untrue. The first sentence is correct, but the concept of a shield can is used on every GPS Chip and many other RF devices - Video TX, RCS TX, etc. The can must connect to the ground plane correctly, ie, the spacing between connections to ground, if not solid, must be a very small portion of the wavelength we are trying to shield against. These small GPS module's cans are soldered every 3 or 4 mm, so the frequencies that may enter' these gaps are in the region of 20GHz.....And if the can could ever resonate ( not possible - it is connected to ground..) the resulting current flow in the can material would be on the outside, and penetrate very little into the material ( skin effect) and would NEVER enter the interior of the can. Imagine a copper sphere, hollow inside. Make a small hole, and stick some coax cable into this hole. Solder the screen of the coax to the sphere. Connect the other coax end to your transmitter and transmit. Now try to measure the rf exiting the sphere...If we could measure any useful signal out of the sphere  we could place antenna inside metal fuselages...

The simple way to avoid this is to create a shield “ring” that connects to the shield can and then connect that ring to RF ground through an inductor at a single point. The inductor filters any EMI-induced (electromagnetic interference) current flow while the single-point connection prevents current flow across the shield can (and any resulting resonation).

This has no meaning at all and no practical application at the frequencies of interest.  This concept is certainly used a lot in the analogue world, especially on high input impedance amplifiers, as a signal guard ring, but never in the application stated! The shielding can is already a shield 'ring' - a mere ring would do nothing - it implies being open on two sides. 

The shielding can is solidly connected to the chipset PCB ground plane, and this ground plane is brought out to many 'pads', on the side of the chipset PCB ( just check any Ublox GPS chipset datasheet for example..)- these pads are then soldered the  main PCB ground plane, which forms part of the Patch antenna ground plane in an integrated GPS. If this grounding is maintained, then all is well. And a connection to any part of the ground palne is as good a ground as you will get.

And then the discussion got lost with fast edges, and clocks, etc - nothing to do with fitting a GPS chip-set to a PCB and antenna...


I did some measurements with a Taoglass ceramic 25mm square Patch fitted to a copper sheet ground plane of varying size. This particular patch antenna was designed by the manufacturer to be fitted to a very small ground plane , 30mm x 30mm

Here are some pics of the antenna in various stages of clipping the ground plane:

Plan here is 140mm X 140mm

here 85 X 85mm

                     here 27 x 27mm

The SWR measurements at GPS L1 - 1575MHz are

190x190 - 3.9:1

140x140 - 3.8:1

85x85    - 3:1

60x60    - 2.4:1

50x50    - 3.2:1

40x40    - 3:1

35x35    - 2.3:1

27x27   - 1.5:1

25x25   - 1.4:1

25x25 is basically no ground plane - just the silvered plane on the underside of the ceramic antenna. This antenna does not have directivity suitable for our applications...

Network Analyzer measurements of 190mm x 190mm ground plane version:

Marker 1 is the GPS L1 freq. Marker 2 is Glonass - L2

This is with the 27x27mm ground plane - marker 1 the GPS L1 freq.

The GPS ceramic antenna -TOP                                                    and the underside.

                         A linear polarised ceramic chip antenna - normally has to be placed at the edge of the PCB


n Active antenna from the top - note the small ground plane, also not the greatest directivity;

                                                                                         The antenna from the bottom - with a shield can ( NOT a ring..)


To conclude - 

All the thinking as to what size the ground plane should be, if the GPS module should be active or passive, etc, as far as the technical implementation, should have been taken care of by the GPS module designer/manufacturer. However, there are modules done with 'cookbook' recipes, in home shops, that are sold, and these can only lead you astray. Go for something with credibility, from a reputable supplier, WITH specs and data on the antenna directivity. Adding an LNA to increase gain does not increase directivity, and can worsen signal to noise ratio if not done well (shielded, etc).  In a fixed wing plane you can easily get away with a very small ground plane antenna - in those Inverted Lawnmower things, there is so much RFI from the ESC's, and everything is in such close proximity, go for the largest ground plane based antenna possible.

None of this may have anything to do with the problem you chaps are experiencing, but as I said, it may prevent useless journeys...

The Nampilot -

BTW, I have posted many blogs on antenna issues, design etc - some may be of interest to you - search for nampilot, SurVoyeur and so on...



And by the way those flying Inverted Lawnmower things are AWESOME!!

peace :)

So this basically means we are stuck with the CSGshop's design or whatever other vendor's design of the M8N and if we try to add a ground plane we may increase the directivity but the impedance matching and hence the SWR will go to hell !!

I have a question though, does having a smaller ground plane necessarily mean better impedance matching as suggested by your tests above ?

Nampilot, thanks a lot for sharing your seemingly infinite wisdom...

Always a joy to read from you.


If I implied that a smaller ground plane is better matching, I apologize - 

The size of the ground plane is driven by Patch antenna theory, and as I indicated, it should be 1 wavelength long on a side, or around a 1 wavelength diameter disc if round. This is close to optimum for back lobe suppression, and with the correctly designed associated patch radiating element, optimum for directivity.

The impedance matching of such a patch element on such a large plane can easily be achieved.

Reducing the ground plane size will reduce the antenna performance in terms of back lobe suppression, and directivity to some degree, but the patch element can still be properly matched .The antenna in my pictures shows the connection pin - the impedance match is made by moving this pin closer to or further away from the element center - center is 0 ohms, edge is high impedance, and 50 ohms somewhere in between.

So the manufacture may choose to use a smaller than optimum ground plane, for size reasons, and then match the patch element for that size plane. The patch in my pictures was designed for a 30mm plane , from the mnfr specs.

So the bottom line is, the plane should be the size recommended by the patch designer/manufacturer, since the impedance was matched for that set of conditions. The smaller the plane the poorer the back lobes suppression, and potentially the poorer the directivity.

Changing an existing design without recomputing the affected elements merely messes up every design aspect...

My tests show that the plane had to be small for optimum impedance match, as was intended by the manufacturer...

Hope that clears it up.

The Nampilot...

OK so building a custom ground-plane is definitely out... coz' I don't have the equipment or the know-how to pull this off without making things worse...for now it would be best to focus on the INAVerr and get some dev help on the issue of missing GPS samples  

Thanks for the insightful data and test results.  So at this point I see no need to alter my existing boards from CSG Shop. 


So at this point I suspect EMI caused by ESCs, Motors, and Flight Controller my affect the signal  strength if antenna is located too close to them. 


So my questions are 

1. What reasonable distance is optimal for mitigating the noise from the electronics?  Is there a formula that allows us to determine an optimum displacement for the GPS antenna from the electronics?

2. Are there other options to help reduce EMI?

3. Is there a correlation between the increasing NAVErrors folks have seen lately with respects to the types of GPS units on the market and the Pixhawk/firmware?


After reading you write up, I feel I do not have the right knowledge or equipment to build and test a larger plane for the GPS antenna and have to trust that my manufacture performed all computations and settings needed for the ground plane I have on my 25mm and 40mm antennas.

Lastly,  You mentioned that us who fly "..inverted lawnmower things" (funny) should use as large a ground plane as possible due to EMI from ESCs, What size would you recommend assuming size doesn't matter?  

Thanks again for shedding light on this topic.

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