GPS Antenna selection - Page 1
Forum section: electronics / rf-microwave

author: snx
author_rank: Contributor
timestamp: October 28, 2018, 09:52:46 am
content: Hi,I want to build a project that includes a gps module with an antenna. I'm not sure how to select the antenna.The casing is made of aluminium (1.8 to 2.5mm thick). As i want to keep the device dust and moisture free, i would like to use a antenna inside the case, if that is possible.Sadly i don't have much knowledge about gps so i hope that someone here can help me with my questions:1.) Is it possible (at all) to use a gps antenna inside the closed aluminium enclosure. The front-panel is 1.8mm thick. If yes, what antenna should i choose?2.) If it cant be used inside the enclosure, would it be possible to cut out a square in the front panel and put the antenna right in that square? Over the antenna there would be only the front panel foil (.9mm).3.) If i need a antenna thats outside the device, are there any very small Antennas with SMA? (like 5-10mm long, not the 5cm long ones you know from wifi routers).Thanks for your help.

author: hagster
author_rank: Frequent Contributor
timestamp: October 28, 2018, 10:04:21 am
content: The GPS antenna will not work behind any metal structure. No matter how thin.You can get GPS antenna with o ring seals that mount nicely on these cases with the SMA connector on the inside. Antcom sell them for the aircraft market, but there may be cheaper alternatives.

author: LapTop006
author_rank: Unknown
timestamp: October 28, 2018, 01:04:48 pm
content: Quote from: hagster on October 28, 2018, 10:04:21 amThat's not *quite* true, but even metalised window coatings are enough to kill almost all reception. A proper metal diecast case is certainly enough thatYou can get nice plastic outdoor enclosures, that's what I'm planning on using for a similar project.Quote from: snx on October 28, 2018, 09:52:46 amDigikey have a surprisingly decent selection of antennas, it's worth noodling around and finding a bunch of potentials and ordering one of each, they're mostly pretty cheap.

author: snx
author_rank: Contributor
timestamp: October 28, 2018, 08:18:06 pm
content: HiI'm using one of those aluminium frame cases, see attached picture.I guess it would be possible to laser-cut a piece of plastic as a replacment for one of the metal endcaps, then i would place a gps antenna next to it. The available space on the plastic sheet would be 15x65mm so i guess i would be possible to mount the following antenna to the plastic sheet:http://www.taoglas.com/wp-content/uploads/2015/04/AP.10E.07.0039B-1.pdfWould that work out?One thing i noticed about gps, that you only need a "good condition" to get a signal, not maintain it. For example, with a gps module that has a chip antenna, i can't get a signal in my house inside, but if i go out and "pick up" a signal, and then go back in, the signal remains and works.My other plan would be to use this antenna:https://docs-emea.rs-online.com/webdocs/0dbd/0900766b80dbde7f.pdfWhich has a o-ring seal as described, but this would add add around 25mm in lenght to the case, which i would like to avoid.

author: cdev
author_rank: Super Contributor
timestamp: October 28, 2018, 10:22:54 pm
content: Quote from: snx on October 28, 2018, 09:52:46 am1.) No, it isnt possible to put your antenna inside a metal box. On the other hand, it would make a lot of sense for you if your budget permits to use a plastic radome over your antenna to protect it from damage and your receiver from ESD.The Taoglas antenna you linked to is a dual band antenna with an integrated LNA, designed for both GPS and GLONASS. Is that what you are looking for?The square antenna needs to be outside any metal enclosed area, ideally on top, but its really designed for PCB placement. The Taoglas - and all patch antennas need to be facing upward or close to it.Their most popular style is a square patch of (ideally silver plated) metal fed at a point slightly offset in a diagonal direction from its center, in such a way that it acquires right hand circular polarization.The silver plated or plain metal patch is attached to a ceramic substrate picked for its dielectric properties, which make the patch much smaller than it would need to be with air but this also makes it much more narrow-banded (working over a narrower range of frequencies) and makes it more finicky about having a ground plane.The Taoglas antenna you linked to is a compact one and its profile is higher (lower S/N ratio at the antenna, and IMHO, slightly higher chance of damage if its not protected inside of a plastic radome.)It has an integrated low noise amplifier which means it requires a voltage to drive it.One thing, though. Most modern GPS receivers are sensitive enough to not need an LNA unless the antenna is on the far end of a long piece of coax. Since your device is a one piece unit you dont need an LNA. Is your unit a multi-band receiver? If you don't know can you share what GPS module you plan to use? Multi-constellation GPS performs significantly better under some conditions. It all depends on what your application is, whether you need it.The usual patch antenna is designed to work with a flat ground plane of at least 50-70 mm square and wont function properly without one.A smaller patch antenna is likely to need one too, even though its smaller. A decent ground plane may be even more important if the actual patch is smaller.The typical size of a ceramic patch antenna is around 25 mm square (not 10 mm square) and if you have the space for a larger square it will likely deliver significantly better performace. 10mm square is very very small for this kind of antenna. Thats why they need an LNA. Although taoglas is very much a reputable manufacturer and I am sure that antenna will work, the fact that its size has been so aggressively reduced implies tradeoffs. If you are able to go with the Sarantel antenna my gut feeling is I would consider that before getting an amplified 10 mm ceramic patch.Unless your design both requires the antenna to be small and provides a significantly larger ground plane I would worry about the ground plane being inadequately sized. Ask them.Note that the patch antenna absolutely requires both that a decent ground plane exist, the bigger the better, a smaller antenna may even be more dependent on a decent ground plane, and also all patch antennas start to lose their circular polarization at lower angles so they absolutely require that the antenna be pointed up within a range of at the most 15 or 25 degrees of vertical to work well. Also they should be in the clear without other antennas, or any other metal around them.These kinds of situations all are the Sarantel antenna's strong points- major strong points- a QFH frees you from the orientation issue. Their implementation of it is unique in that its a fraction of the size of one without the high dielectric design they use. However its bandwidth is likely lower than the Taoglas.Frankly, the bandwidth specified for that tiny Taoglas patch seems likely to either be overoptimistic or they manipulate it with filtration in the LNA. The LNA contains an SAW filter which is standard in many better GPS filters now. I just don't know. You should take what I am saying with the caveat that I am not an engineer, just a hobbyist who likes to fool around with GPS, especially antennas.No, foil won't do, it would likely cause problems for the signal.Most modern GPS receivers are a PCB which provides some ground plane area plus mounting for the antenna and they use a plastic case. Is there any reason you cant do that?

author: deBug
author_rank: Contributor
timestamp: October 28, 2018, 10:58:11 pm
content: >I guess it would be possible to laser-cut a piece of plastic as a replacment for one of the metal endcaps, then i would place a gps antenna next to it. The available >space on the plastic sheet would be 15x65mm so i guess i would be possible to mount the following antenna to the plastic sheet:That would work or do it out of PCB material with the copper removed, You get drilling, cutting and screen print for "free" and can pick from a few colors.I saw a product with built in GPS in a alu. box the other day were they have done this. It was in a black Alu box and the PCB at one of the ends was made of PCB, very hard to spot and the screen printed text looked professional.I believe Mikes Electric has an old video on how to do frontpanels with PCB material.//Harry

author: cdev
author_rank: Super Contributor
timestamp: October 29, 2018, 12:41:37 am
content: Unless somebody can live with very patchy fixes becoming available, assuming that this panel was on one of thesides, if this worked any better than erratically, I would be surprised.Without more info about the application, its hard to say what would work and what wouldn't.I think the popular picture of what GPS's can and can't do is not very well-informed when it comes to the constraints imposed by the need for a decent antenna placement. With a good antenna and an optimal placement on top they can be very accurate.Quote from: deBug on October 28, 2018, 10:58:11 pm

author: snx
author_rank: Contributor
timestamp: October 29, 2018, 09:02:07 am
content: Quote from: cdev on October 28, 2018, 10:22:54 pmMy GPS module is the NEO-M8Q or NEO-M8N, it has 3 conncurent GNSS and you can select from BeiDou, Galileo, GLONASS, GPS/QZSS, so i guess an antenna who can do GPS and GLONASS would be fine, then i could relate on both systemsThe Taoglas is saw has an UFL Cable on it.So if the Antenna needs to be out of metal, i could 3-D print a new front panel what hase a Dome (8mm high) where the Antenna is housed.) My main Problem is that i don't like to use a stick antenna because it adds alot of lenght to the case and has the danger of mechanical damage because it looks out of the device. A Dome would be a much better geometry.Would it help if i send a hand-drawing of my idea ?

author: cdev
author_rank: Super Contributor
timestamp: October 29, 2018, 03:59:51 pm
content: Sure, send a drawing, or just tell us, what is it, what is the desired use?Okay, here is a rule of thumb. A patch antenna will perform best with a ground plane thats whatever size the manufacturer suggests and the smaller it is (I don't care what they say in their literature, this is physics) the worse performance you'll be able to get. You will get noticeable improvements with better ground planes with even small increases in size up to a size of a CD, about, and smaller improvements beyond that. With almost no ground plane (say, a hockey puck antenna thats supposed to be stuck to a metal object without one) you'll get quite crappy performance. But it will likely be able to get a fix, if your receiver is semi decent, but the quality of that fix will be quite poor. Stick that magnetic antenna to a small piece of steel (ideal) or even just place it on an old CD and zap, the fix will improve. You will see it in U-center. I strongly suggest you utilize its lesser known functionality to log the performance of different antennas along with the SN ratio using different prototype settings, also call your manufacturers and ask them how to get optimal performance. In my experience good manufacturers love to give tech support. Thats worth more than gold because they will help a lot. Ublox also has a BBS where you can ask questions.For a hand held unit, I would go with the Sarantel antenna as thats its strong point. You can put it in a dome, sure. Unless you have a flat area to put the patch antenna thats bigger than a tiny area on the end of your box, I would not use the tiny patch antenna, but what do I know, I might be wrong. Get two sample units, one of each and try it out.*How accurate does your setup need to be?* Most GPS applications don't need to be super precise, they just need to 'work'. Does it matter to your application to know where something is within say 2 or 3 meters or less (outdoors) or is say 5-10 meters off and jumping around a lot okay? Will your moving vehicle or person be located in cities with many tall buildings or in forests a lot?

author: hamster_nz
author_rank: Super Contributor
timestamp: October 29, 2018, 05:51:25 pm
content: ...Just throwing this in here (mostly so I can follow the thread)...The attached image is fromhttps://www.novatel.com/an-introduction-to-gnss/chapter-5-resolving-errors/gnss-data-post-processing/- it is the expected performance of GPS system and various correction systems that can be used e.g. SBAS (a.k.a. WAAS in North America). An uncorrected standalone single receiver is expected to get 'only' to 10m , no matter how good the antenna is.Most of the errors are outside of the receiver - which is how PPP can get 0.10m with post-processing to remove ionosphere and orbit errors. No matter how good your antenna and receiver is, you will still hit these limitations.I think you quite quickly get no better performance for your efforts to improve signal strength.

author: hagster
author_rank: Frequent Contributor
timestamp: October 29, 2018, 06:44:29 pm
content: CDEV is correct in everything he says.Modern GPS engines can cope with much weaker signals than the original GPS spec defined. HOWEVER... They do this at the expense of accuaracy and more importantly and dynamic performance. Basically modetn engines pull the signal from the noise by averaging the signals over longer and longer times(actually by a combination of coherent and noncoherent integration). If the signal timing or doppler changes within this averaging period the results get smeared and peaks get reduced. Hence the accuracy drops and the reaction speed to changes of direction and speed is reduced.Add to all this, the receiver has to work harder(hence consumes more energy) and struggles to get updated ephemires. Hence unless you are especially constrained with antenna size and placement you are still best to attempt an optimum config if at all possible. This also gives headroom for users to do dumb things.Here is an example of a panel mount GPS antenna that will probably do the job.http://www.panorama-antennas.com/site/index.php?route=product/product&product_id=117Also unless you have a long feed cable there is little advantage(or disadvantage) to using an active antenna over passive.GPS and GLONASS and Galileo all use the same frequency for most civilian signals so any GPS antenna is ok for these. Currently there are very few receivers than make use of civilian signals on L2 or the various E freqs for Galileo so I wouldnt worry about multiband gps antennas currently. This will probably change in the next couple of years and there will be real advantages to multi-frequency systems in the future. Especially for accuracy.

author: cdev
author_rank: Super Contributor
timestamp: October 29, 2018, 07:12:11 pm
content: GLONASS L1 signals are roughly 1590 to 1615 MHz. Which is high enough above the 1575.42 MHz center of most of the rest of the L1 constellations that some antennas don't work very well. Most COTS GPS antennas work somewhat, but they vary a lot.In the future, we're going to see more and more GNSS receivers that also support other frequencies and GNSS systems, especially another frequency band centering around 1176.45 MHz. Using both at the same time, along with short baseline correction data collected at cell sites near the user will lead to off the shelf receivers in the near future having a lot more accuracy than many setups officially do today.But even now the accuracy can be surprisingly good. better than manufacturers usually quote.

author: hagster
author_rank: Frequent Contributor
timestamp: October 29, 2018, 07:22:03 pm
content: I thought GLONAS had changed to a CDMA from signal like GPS. Havent followed where they were in updating the constellation recently though. But I thought that was one of the enabling changes that has spurred on combined GPS/GLONAS receivers.

author: cdev
author_rank: Super Contributor
timestamp: October 29, 2018, 07:36:24 pm
content: This PDF has a lot of data on the various GNSS systems basic characteristics:http://www.unoosa.org/documents/pdf/icg/2018/ait-gnss/03_gnss.pdf

author: snx
author_rank: Contributor
timestamp: October 29, 2018, 08:18:33 pm
content: Find attached the image of the case i wanna use, and a sketchy 3D drawing how i would replace one front panel by a 3D-printed one to allow a dome for the ceramic patch antenna.

author: cdev
author_rank: Super Contributor
timestamp: October 29, 2018, 08:31:56 pm
content: Ask the manufacturer. It might work okay.I have a "PND" GPS, a (Mio 310x circa around 2006) car GPS used normally on the dashboard that has an end mounted chip antenna. That works but the very same GPS with an external antenna with a CD sized ground plane works markedly better. Tested in Manhattan which is a very difficult challenge for a GPS.

author: hamster_nz
author_rank: Super Contributor
timestamp: October 29, 2018, 08:44:06 pm
content: Quote from: cdev on October 29, 2018, 07:36:24 pmAnd this one too:https://www.u-blox.com/sites/default/files/products/documents/GPS-Compendium_Book_%28GPS-X-02007%29.pdf

author: snx
author_rank: Contributor
timestamp: October 30, 2018, 05:49:46 pm
content: Quote from: cdev on October 29, 2018, 03:59:51 pmDo you mean ground plane on the patch antenna, or on the Ground of the UFL Cable? If the cable is meant, the UFL Connector of ground would connect to the pcb GND poor which is ~100x60mm

author: hagster
author_rank: Frequent Contributor
timestamp: October 30, 2018, 07:31:15 pm
content: I think I was imagining a die cast box. In all honesty, the difference between having that bulge for the GPS antenna and just having it behind a flat RF transparent sheet wont be huge and not be worth the extra effort. Still use a ground plane though.

author: cdev
author_rank: Super Contributor
timestamp: October 30, 2018, 07:37:02 pm
content: The patch antenna (sans LNA and SAW) is just a precisely shaped square of foil on top of a flat piece of ceramic material with the feedpoint offset slightly at a diagonal from its center. Think of how a sandwich made with a very thick piece of ceramic cheese would look if it only had two pieces of very thin metal "bread". Near the middle but slightly offset there is a dab of solder, that connects to a post which in your case goes to the input of an LNA. That spot is connected to the center conductor of the coax otherwise. Beneath that is another flat piece of metal which is larger which is connected to the shield of the coax. That is the 'ground plane" and its electrically ideal if its more than a few wavelengths in size (large) but for the purpose we're proposing for the antenna, it can be smaller, lets say 50 mm square might be a reasonable size. Does it give you a minimum size in the documentation? The optimal size for the top part of the antenna likely varies if the ground plane is smaller, or the antenna will shift in frequency such that its not where it is supposed to be.The manufacturer should be able to supply specifics as that's what they do for a living.I mean the ground at the feed point of the antenna. Not the rest. BTW, ufl connectors don't stand up well to repeated connection/disconnection.