Affordable GNSS Antenna - Page 1
Forum section: electronics / general

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: May 02, 2020, 02:21:54 pm
content: I built a small PCB to carry the UBLOX Max-M8W-0-10 andthe project is here(I still have a few PCBs if anyone wants one).Anyway, it's working fine but, with the fancy building-mounted antenna I have for my NTBW50A GPSDO, the signals for the GPS satellites are great (45 - 50 dB) but the GLONASS satellites seem not to be picked up but they are when using acheap antenna like this onein my basement window (at lower signal strength). The SBAS and WAAS satellites are still picked up. The GNSS configuration screen from U-Center is below and that would seem to indicate that the GPS, SBAS, and GLONASS satellites are all in the L1 band.It occurs to me that the issue is likely the frequency band of the antenna but maybe it's because the Max-M8W prefers GPS over GLONASS satellites?A friend is looking at the ZED-F9P board from Sparkfun for his boat and asked me about antennae. I went down a rabbit hole this week looking at various GNSS antennae and saw that they can cost $200 or $300. So, my question is this. What's a reasonably priced ($50-$60ish) GNSS antenna and where can I get it from?

author: cdev
author_rank: Super Contributor
timestamp: May 05, 2020, 03:14:40 am
content: For a boat you dont need extreme accuracy, you need the bands you want to receive and you want it to be extremely well waterproofed. And I would get at least one backup antenna too, "just in case" Most boat antennas are made to fit onto a threaded pole.

author: Berni
author_rank: Super Contributor
timestamp: May 05, 2020, 05:40:51 am
content: GPS L1 and GLONAS L1 are not the same frequency. So the antenna needs to be designed to have a wide enough response to cover both.As for the ZED-F9P you will only get the accuracy out of it when provided with live correction data from a near by stationary receiver. Otherwise you get the same ~2m precision that you will get out of any other GPS/GLONASS receiver. And for a boat 2m is plenty anyway.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: May 05, 2020, 09:28:06 am
content: Quote from: Berni on May 05, 2020, 05:40:51 amThanks,I know GPS and GLONASS are on different frequencies, the question is which ones? If you type "GNSS antenna" into the Digikey search, one of the parameters is this weird mish mash of different band collections.As for the accuracy relying on a nearby stationary receiver; are you sure? That would negate the whole point of it being >$200. Even my cheap UBlox Max-M8 can receive the SBAS and WAAS signals via satellite (which was news to me after I bought one and powered it up!)

author: Berni
author_rank: Super Contributor
timestamp: May 05, 2020, 02:54:47 pm
content: Its easy enough to google. GLONASS L1 is 1602.0–1615.5 MHz and GPS L1 is 1575.42 MHzAlso the Ublox ZED-F9P is actually cheap for its functionality. It is the lowest cost L2 RTK GNSS receiver on the market and is driving the market to a new low cost price point. In volume the modules cost less than 100 USD. But you are not the target audience for it. The real audience for it are selfdriving systems for tractors/agroculture, surveying, drone autopilots. The reason being that normal GPS is not accurate enough for that, but this new RTK technology can provide positional 2mm accuracy. The big deal is that RTK is hard to do with only L1 so a receiver with L1+L2 capability works way better.The reason why it needs a near by reference station is that L2 pseudorange data is encrypted and only the US military has the decryption keys for it. But the L2 receiver can still measure the phase information of the L2 frequency carrier by ignoring the pseudorange data. So they use a 2nd reciever to find out the carrier offsets of all the L1 and L2 satellites in a place near by (This includes all of the atmosphere distortions) so that it can compare them to the phase information received by the moving receiver, resulting in a milimeter accurate distance between the two receivers. So the absolute result in reference to earth is only accurate to the milimeter if you also know where the reference receiver is down to the milimeter. (Technically the GPS presudorange data is also referenced to a location on earth, but that is in reference to a certain US military base where the equipment is located for monitoring the GPS constellation and uploading the satellite orbit corrections and atomic clock corrections)

author: cdev
author_rank: Super Contributor
timestamp: May 05, 2020, 05:07:07 pm
content: A good broadband GPS antenna with a good pattern can be made by etching a two arm archimedian or logarithmic spiral on a single sided PCB (or removing the entire back side of copper. ) You should use a balun You can either make an infinite balun by using two matching pieces of semirigid coax one as your feed, (see Dyson) the other one as your dummy arm, or you can put an actual balun at the feedpoint and then put an SMA jack on on the back. You could try winding your own balun on the smallest type 61 balun core you can find with bifilar winding, just a very small length of it wound evenly twisted together. Very small core, very short, thin enameled wire. Or -Mini Circuits.1600 MHz is nearing the limits of traditional balun construction but it should still work okay with a bit of loss.Archimedian is a better GPS antenna as far as uniformity of the pattern but you will likely need to etch it. Equiangular spiral is easier to fabricate by hand. Depending on the size you can get coverage over a very wide range of frequencies. 10 to 1 is not impossible for both types. A shallow cone is tempting for GPS because it has gain upward but for accuracy esp. multiconstellation dont do it because the great thing about a spiral is that the phase center of the antenna for two diverse frequencies will be in the same plane and the center will be located at the same spot and thats important. the same goes for dish feeds. the tip of a cone will (usually) be where the feed is and phase center locates for the highest frequency and the butt end for its lowest.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: May 05, 2020, 10:37:10 pm
content: Berni,Thanks for the detailed explanation, I love that eevBlog gets such excellent answers as yours.cdev,Your PCB antenna ideas are very interesting, are there any pictures, links, or design files that you can give us?

author: cdev
author_rank: Super Contributor
timestamp: May 05, 2020, 11:15:04 pm
content: I have a lot of stuff on them. I've only built a few but they do work. They would NOT be even remotely good for a boat however because I have constructed them on foam core, and used materials like copper tape which likely comes off if it gets wet.Because it uses glue to adhere itself. if we got together we could make much more durable PCB versions.The spiral antennas are very useful and I have received a ton of interesting signals with mine. there are a lot of different perfectly valid ways to make them, each with its advantages and disavantages.A number of them were described by Dyson in the 1950s and 1960s. and are easy to reproduce exactly as he made them.The antenna can be inside of a conductive exterior square, which shorts the outside or not. Both work great. You could cut one out of a square and then use the two pieces you cut out to make another one out of that.You could actually make one very easily out of just copper tape and graph paper in the same size as the graph paper. Basically, make a two armed square spiral meeting at its center the two arms meeting in a four sqares where they just touch, did you understand what I mean? Ideally your copper tape is fairly thin as is your graph paper size. You make a square archimedian spiral, implement it as a square, To the RF, the square is seen as a meander line, in other words, in a circular projection, a squre is a sine wave, a meander line, does that make sense? A meander line works well to shrink the size of your antenna a bit. You can see what this looks like with some graph paper and a pencil, just make the closest thing to an archimedian spiral you can using straight lines instead of a circle on the graph paper. The important thing being that the space between the arms should be the same as the width of the arms. You could also I suppose make a hybrid design and incrementally increase the width of the tape you used as you went outward. This might produce an antenna much like a lof spiral. But that antenna would likely have irregular discontinuities and wouldnt perform as well for GPS.You can make archimedian spirals in Inkscape, I am pretty sure. Just make sure it has two arms, not one. You could make a four armed antenna but then you will need a broadband 90 degree hybrid coupler. If depends on how far apart the highest and lowest frequencies you need to receive are.All GNSS satellites I am aware of have RHCP. Make sure you make an RHCP antenna, not LHCP, otherwise you may have to use it backwards.. I will upload some graphics that could be turned into a pcb or cut out of some conductive material with an xacto knife, and taped in a very halfassed way to some cardboard or something. Using a 4:1 VHF/UHF balun. That works too!Please read this for an explanation and the math.(Archimedian spiral)http://erewhon.superkuh.com/library/Electromagnetics/Spiral%20Antenna/Analysis%20of%20Archimedean%20Spiral%20Antenna_%20Chapter%202_%20Caswell.pdfthe equiangular or logarithmic spiral is constructed differently. The angle remains constand and so the width of both arms increases as one moves outwards. The arms of both antennas in the classic design is exactly the same width and the spaces, so a circle should be 50% covered.At one point, the city of Seattle was going to build a park based on a spiral antenna but it seems, not any more.. One of the tales the late musician, Jimi Hendix used to tell involved some story involving communications from the stars that was to be received with such an antenna, telling the human race where to go?But only if it was the right polarization. Kind of like what you want to do!