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".
Thoughts?
Replies
Sorry to ask this but english is not my native language and I'm some how lost in translation...
The GPS signal is extremely weak. Things that jam or severely degrade gps in my experience include some cameras, Wii IR camera, Beaglebone black hdmi module, some SSD motherboards, and the internal oscillator on the stm32f4. Part of this is due to the sensitivity of ublox receivers, I didn't see some of this interference on the mediatek. I'm sure various EMI mitigation techniques have been already discussed, including a larger ground plane.
DOP is certainly an indicator of EMI issues but only when it's significant enough to lose satellites. A much better metric is the horizontal and vertical accuracy that ublox receivers output, but last I checked, the apm firmware didn't log or display it. It's a better indicator because it seems to be related to the signal to noise ratio so can fluctuate a lot while the number of sats doesn't change.
As a future point of reference for anyone who accesses this information, the best horizontal accuracy I have received is 0.6-0.8m inside out lab with an amplified GPS repeater. In the field, a good value is ~1.5m and anything greater than 3-4m is getting noticeably interfered with. Of course it doesn't necessarily mean you're getting 0.8m accuracy but it's an indication. For reference, I observed these results with both a 3dr ublox 6 and csgshop m8n. I'm using a stm32f4 based autopilot with Beaglebone and odroid u3 also onboard. There's also 2 xbee pro 900s, one of which is right next to the GPS antenna. Hope this helps.
Hi Dan,
I'm sorry for my question, what do you use the software on Beaglebone autopilot? Sorry for the question off topic.
Cheers,
Sorry for the slow reply. I use the beaglebone as a co-processor for my own stm32f4 based autopilot. There are some more details in the videos here and here if you're interested.
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...
Enough-
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
A
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...
How are the locations and sizes of the small cuts on the perimeter of the patch for tuning are determined? I understand the patch is tuned in real time while viewing a network analyzer but how is it know where to place the cuts and how many are required?
Nampilot, thanks a lot for sharing your seemingly infinite wisdom...
Always a joy to read from you.
AkCopter,
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...
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.
Hi Doug,
Firstly, what GPS module do you have? Is it one of these two?
Is the Taoglass patch antenna parts number one of these :
CGGBP-356A 02
CGGBP-184A 02. ]
If so, the the ground plane should be 70mm x 70mm. This is quite clear in the Taoglass antenna specs. It certainly does not look like that in the images... I think your second last sentence must be modified - I think the mnfr did NOT design as per the antenna requirement Specs...
Remember, that does not mean the GPS will not work - it means that the impedance match is wrong, so power is lost, which they make up for with the LNA, but the bad side is the loss of directivity and subsequent pickup of radiated noise from below.
I strongly suspect that radiated noise is the biggest culprit in this situation. The best way to discover this would be to attach a similar antenna with ground plane to a spectrum analyzer and measure the detected emissions with the craft at full power.. not easy without the equipment.
Regarding your questions -
1. No there is no easy way to estimate this without measurement. The radiation decreases with the inverse square of distance ( a simplification, but good enough for this), but you have no idea how high the levels are to start with.
2. Yes - by shielding the noise generators, which is always the best approach anyway. There are some metallic sprays which can be used to create a 'shielded can' from your AP enclosure, etc - not totally effective as the cables still exit unfiltered, so the cables conduct the energy out from the enclosure. Cables can be fitted inside braided shields. ECS should be as close to the motors as possible - the 3 phase wires to the motors are the big culprit - hi current hi frequency square wave PWM signals with harmonics into the many hundreds of MHz. In essence, you want the electronics inside a shielded can, the same as that little can over the GPS chipset. That is very difficult, if not impossible, to achieve, but all the hi current, hi frequency cables should be shielded and grounded at a common point - place a disc of copper foil, maybe 60mm diameter, at the base or some convenient position in the craft, and connect all shield, braids and grounds to it. Braid shield all signal leads, servo leads, power cables. A lot of work, can add weight, but it does work.
WRT your last sentence - I would recommend a ground plane ( with appropriate antenna of course) of at least 70mm x 70mm, placed maybe 200 to 300mm above the avionics. This is almost guaranteed to work in even the most stubborn cases. In less stubborn cases, both dimension can be reduced, eg, in a light craft with lower power to the motors in flight. I would not go less than 40mmx40mm antenna designs, maybe 100mm min above the craft. Any smaller and the antenna sees as much noise from below as GPS signal from above.
As an aside, you may be able to extend the ground plane on this module ( assuming it is one of the two in the pictures?)
Scrape the ground plane copper clean around the PCB periphery, and cut strips of double sided PCB , same thickness, and butt them up against the GPS PCB plane. Solder all along the seam, making sure to cover the joint. A thin copper foil over the joint would be great. The strip width should be such that the 'new' dimension ends up 70mm x 70mm. Don't leave big bumps of solder along the joint. Don't short out other signal tracks to the ground plane in this process..
Then raise the PCB to say 150mm above the avionics and see what improved..
Your question 3 I cannot answer - we have our own autopilot and software so it all works differently..However, with respect to the 'types of GPS on the market' - they are all pretty much the same - all have similar sensitivities, all have similar numbers of tracking channels, all use GPS engines from similar manufacturers, so there is not a lot of difference among them. Where the BIG difference lies is in their integration into a complete GPS unit, ie, when mated with an antenna, the 'correct' ground plane, etc.
A simple check is to obtain the antenna mnfr and part number, get the data sheet, and see what the mnfr recommends as a ground plane. If the GPS unit you are looking at complies within say 10% of this size, go for it. Else avoid like the plague...Unless you are flying normal craft, or your lawnmower is not noisy...
I will be back next week to chat some more - on the way to the Namib Desert tomorrow - we will be darting 5 Desert Lions and fitting GPS SAT tracking collars for a conservation project.
Have a look here just for fun:
http://desertlion.info/news.html
Cheers
The Nampilot..
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