GPS antenna problem - Page 1
Forum section: electronics / beginners

author: Kemme
author_rank: Contributor
timestamp: February 24, 2015, 03:33:30 pm
content: Hello allI consider myself a beginner in the electronics so I decided to post my problem in this section.So I build a simple GPS receiver board which is based on GPS-GXM-U3 module.The board looks like that:When I connected it to my microcontroller without an antenna I could read the data but position was not very accurate. The difference was about 50km from when I actually was and usually it used 0 satellites.Well, I thought that 'the antenna' was just a stripe of copper ended with an unconnected SMA connector so the results were pretty good.So I connected an active car GPS 5m cable length antenna to the SMA connector, I applied 3,3V to the Vant pin on the GPS module and it doesn't help at all. It seems like the GPS still sees that 6cm copper trace between the antenna and GPS as a real antenna. When I move my antenna around the room and outside the building nothing changes. I need to move the board with the GPS to 'catch' the signal...As a total newbie in this subject I started reading about signals, high frequencies, wave impedances and I came to the conclusion that the antenna should be located as near the GPS as possible. However the amount of information overwhelmed me a little bit.Is it possible that the copper trace between the module and the antenna is just too long? Are there any more tricks that prohibit me from proper using of the active antenna? I just don't know where to start

author: ccayser
author_rank: Contributor
timestamp: February 24, 2015, 04:05:55 pm
content: There are probably more experienced RF users in this forum than I am but I try to share what I know.Usually the antenna should be in the correct distance from the chip or module itself.The trace to antenna socket must have a correct impedance, which is depending of the length, width and clearance of the trace. It is also important what board material you have as different board materials have different dielectric coefficient. It is also important what on the opposite side of the board and that it is not crossing any data/power lines.You can look more deeply into it by searching "PCB transmission line design" and from there you will be able to find the terminology for more detailed search (e.g. microstrip)

author: jpb
author_rank: Super Contributor
timestamp: February 24, 2015, 04:52:57 pm
content: Is the coplanar waveguide (strip between two ground planes) 50 ohms? I don't know the dielectric of your board and its been a long time since I did such calculations anyway.When you connect the active antenna does it draw current (a few 10s of mA)?Try putting it all in a metal box (with a hole for the sma connector).Why have you got stub lines on the two ground connections (2 and 4) (I'm just curious on this).Perhaps your ground plane needs better grounding.

author: ConKbot
author_rank: Super Contributor
timestamp: February 24, 2015, 05:05:20 pm
content: How long did you wait? If it's new or been off for an extended period of time, ephemeris and almanac data will need to update. 15-20 minutes for a full update.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 24, 2015, 05:10:19 pm
content: If that's just a generic 2-layer FR4, that trace to the antenna is WAY too narrow for a proper 50-ohm impedance. It should be closer to 0.11" (110 mil).

author: Kemme
author_rank: Contributor
timestamp: February 24, 2015, 05:22:43 pm
content: Thank you for all the answers.It is a single layer FR4. I used some online calcs and yeah, my impedance is about 100ohm.I had no idea that this trace would make a trouble, what a bummer.Is there any solution for that case? Maybe some additional capacitance between the signal trace and GND? Or maybe the best idea is just to glue the 50 ohm coax cable instead of the trace?

author: albert22
author_rank: Regular Contributor
timestamp: February 24, 2015, 06:38:48 pm
content: If the trace impedance were 50ohm it would not mater very much what length is (other that losses in the pcb material). Assuming the antenna and the input of the gps are also 50ohms.Try connecting a 50ohm cable almost directly to the pin. But first get rid of the trace.You may also check if the input to the gps needs DC decoupling to avoid damaging the module by feeding it with the active antenna supply.

author: SL4P
author_rank: Super Contributor
timestamp: February 24, 2015, 08:52:37 pm
content: Quote from: Kemme on February 24, 2015, 05:22:43 pmThis is often the solution used in production equipment... which avoids many of the physical & mechanical pitfalls of the PCB approach - unless you are committed to the PCB design effort needed.Panel mount connector, with crimped coax leading back to a solder point immediately adjacent to the module pin. Appropriate means of securing the coax that doesn't pinch or affect the capacitance of the cable.

author: edpalmer42
author_rank: Super Contributor
timestamp: February 24, 2015, 09:34:54 pm
content: Would a mistake in the trace impedance really make that big a difference? I'm not an RF guy, but it seems unlikely that such an error would cause the unit to ignore the antenna completely.I know that Trimble actually recommends using RG-6 cable (75 ohms) to connect their Thunderbolt GPS Disciplined Oscillator to the antenna even though both the antenna and the Thunderbolt are 50 ohm devices. Their explanation is that the difference is trivial and the cable is cheap, high quality, and readily available.Ed

author: ccayser
author_rank: Contributor
timestamp: February 24, 2015, 10:07:40 pm
content: Quote from: edpalmer42 on February 24, 2015, 09:34:54 pmYes, the mismatch would make a big difference. Depending of the system and it's specification, it might not be so important if it is 50+-1 ohms or 50+-10 ohms or something else and it might work. There is a small tutorial that will explain antenna impedance and it's importance.

author: lincoln
author_rank: Regular Contributor
timestamp: February 24, 2015, 10:52:39 pm
content: I will touch on one thing, the active antenna needs a bias voltage (which you know) but of the very few modules I have worked with require an inductor/rf choke between the antenna and the power supply. Values I have seen recommended for L1 (GPS band) is in the 33 nH range.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 24, 2015, 11:10:07 pm
content: Quote from: lincoln on February 24, 2015, 10:52:39 pmThe device the OP is using already has an inductor between the Vant pin and the antenna pin to function as a lowpass filter:https://www.maritex.com.pl/media/uploads/products/wi/GPS-GXM-U3.pdfHe didn't mention what antenna he's using though, some of them don't work well off of 3.3v.

author: Kemme
author_rank: Contributor
timestamp: February 24, 2015, 11:50:06 pm
content: I attached a car GPS antenna. Here is the link, I could not find any datasheet.http://www.tme.eu/en/details/gps-mc-a/gps-and-gsm-car-antennas/gps-mc-aIt was just the cheapest antenna I could find.Anyway I scratched the copper stripe and soldered a 50ohm coax cable. Now it is getting really strange.My GPS now sees 8 satellites, HDOP is 0.9! The funny thing is it doesn't matter if the antenna is connected or not.But still, GPS shows wrong same location 50km from where I live and still I cannot reach Differential GPS fix status.Is it even possible that a functional GPS module sees 8 satellites and shows the same, wrong location?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 24, 2015, 11:57:34 pm
content: How do you know it's 50km off, how are you comparing? I'm only asking because, depending on your location, 50km could be the difference between regular spherical and WGS84 coordinates.

author: Kemme
author_rank: Contributor
timestamp: February 25, 2015, 12:13:54 am
content: Quote from: suicidaleggroll on February 24, 2015, 11:57:34 pmI read it from the GGA sentence, copy the values to herehttp://www.latlong.net/Show-Latitude-Longitude.htmland Bob's your uncle. It always shows the same location about 40 km to the south and 15 km to the west from where I live.

author: SL4P
author_rank: Super Contributor
timestamp: February 25, 2015, 02:33:14 am
content: Quote from: edpalmer42 on February 24, 2015, 09:34:54 pmCheck in on the term SWR (Standing Wave Ratio) - which will give you an idea of what you're up against in mismatched impedances.

author: SL4P
author_rank: Super Contributor
timestamp: February 25, 2015, 02:35:23 am
content: Disable DGPS for now, while you're troubleshooting.That will only add to your confusion if the DGPS data is not being merged correctly.Simple one antenna, one set of data will help isolate what is working and what's not.

author: edpalmer42
author_rank: Super Contributor
timestamp: February 25, 2015, 05:21:22 am
content: Quote from: SL4P on February 25, 2015, 02:33:14 amEven though I'm not an RF guy, I am familiar with SWR and it's evil twin Return Loss. Of course, SWR is critical in transmitters because a mismatch can damage the transmitter or, on a high powered system, even melt the cables!I decided to test the question. I fired up a Jackson Labs GPSTCXO ( a GPS Disciplined Oscillator). I connected it to my GPS splitter which is currently feeding two other GPSDOs. The GPSTCXO reported signal strengths up to 50 or 51 dbC, depending on the satellite. Then I added in a 30 cm. piece of 93 ohm coax. There was no change in signal strength. Then I put in about 10 meters of 50 ohm coax between the splitter and the GPSDO. Still no change in signal strength. Why? Because these receivers are more than capable of dealing with line attenuation and multipath. A little cable glitch isn't going to bother them at all. Actually, Jackson Labs also says that 75 ohm coax works fine. So that's two companies that agree that mismatch isn't an issue with GPS receivers.So Kemme, I wouldn't worry about whether the impedance of your stripline is optimum or not. Check for continuity to your antenna by confirming that there's a reasonable current flow through the Vant terminal. If that's good, look elsewhere for the solution to your problem.Ed

author: Kemme
author_rank: Contributor
timestamp: February 25, 2015, 07:29:14 am
content: Quote from: suicidaleggroll on February 24, 2015, 11:57:34 pmLet me quote you once again.It was 1 am here or so when I replied to your post. This morning after reading the NMEA docs I found out that you were absolutely right. GPGGA sentence consists of location data in degrees and minutes while I was searching the location in decimal degrees. Bummer. I just converted the measurement and it showed my home on the map. Great.However it still seems that the antenna doesn't make any difference. I'll make some measurements today.EDIT.Ok guys, I think we found a guilty one. I measured the power consumption with and without the antenna (19mA and 9mA), everything within the specs.Then I started moving the board from one place to another in my room and it turned out that when the antenna was outside 'moving the board' action was actually causing the GPS to 'catch' the signal. Then I could disconnect the antenna and the GPS would still receive the data. When I reset the power of the GPS I could not receive any data until the board was moved again.Now let me show you how my boards look in realityBreadboard, 8 leds and that old computer IDE tape is my prehistoric debugger which allows me to read the data.Now it seems that this is causing the problem! I did another test. I moved the board to the place far away from window where there is a little chance to receive the GPS data, the antenna was outside. I turned the boards on - no data received. I disconnected the IDE tape for 5 seconds and voilà, it started to receive the data. I repeated it 5 times and the result was always the same.I also did 2 testes with antenna disconnected from the board - I was not able to receive any data.Well, it is really interesting.

author: edpalmer42
author_rank: Super Contributor
timestamp: February 25, 2015, 06:17:14 pm
content: I guess it could be RFI, but it seems unlikely. Could something on your debugger be loading down something?Are you reading the data via a serial (or USB-emulated serial) port? If so, stop trying to decode the data manually and get one of the freeware GPS programs that can read NMEA. VisualGPS is one that comes to mind. This will let you see signal strengths, coordinates, etc. easily and quickly.Ed

author: cdev
author_rank: Super Contributor
timestamp: February 26, 2015, 02:39:28 am
content: ConKbot is right, even with a perfect antenna, it will take a minimum of 15 minutes or so after everything is connected, with a sky view, to download ephemeris data. (thats called a cold start) it will require a battery or a supercapacitor to store that data, it should be good for around a week, if you leave it off (or on inside a metal building or car park) for longer than that it will need to go through the 15 minute cold start procedure again before it will give you a 3D fix.A modern GPS should be able to do fine with a passive antenna connected by a short wire. You could- just to see if its working, use a short piece of wire coiled in the same direction as the fingers on your right hand when your thumb is pointing up, with a piece of metal behind it. You could use a taoglas passive antenna, they cost around $5. if it supplies power just use a cheap active antenna - they contain an LNA and cost under $10