Hy guys. I know this issue is not related to Arduino, but my project does imply such a board. Ok, going off topic.
I have implemented a Nexus 7 as a multimedia unit, but being enclosed in the dashboard, the GPS signal is weak almost all the time. So I've considered attaching an external antenna to the device. Now, my car does have an antenna with the receiver placed in a perfect position. But, most likely (since the informations are scarce), those components work on 12v, and the android gps I believe works on 5v.
My question is, it is possible to adapt the antenna to the android? Would I need a separate voltage source? And a level shifter? I should mention that the car GPS antenna is not connected to anything. Also, I've tried to grasp the "how a antenna is powered and works" but it seems I have mashed all that info in a big pile of nothing. So if someone would be kind enough to explain me as you would too a low IQ Neanderthal, I would be grateful.
P.S. I know I could use a bought 5v gps antenna and stick it to the windshield, but my project is sort of a plug & play multimedia unit, so using the existent antenna on the vehicle is preferred.
Where is the car GPS antenna? Usually I think they are on top of the "shelf" above the dashboard, just under the windshield so they get the best possible view of the sky. Even if the car doesn't have GPS there may be a plastic "patch" there so they can have just one dash moulding rather than two, one with a cutout for the antenna. Since the distance between that antenna position and the infotainment unit is very short there probably isn't any electronics in the antenna to power, whether from 12v or 5v.
So, where is the antenna you think you have? Does it have a cable connected to it and if so is that a coaxial cable?
The antenna is on the roof of the car, and from the documentation of the car it goes into the receiver module, and from there, to the OE infotainment with a Fakra connector (coaxial cable).
Unfortunately I couldn't find anything about that module, because I would have to remove the interior roof lining to find a Serial number or something, and let's be serious, I'm not doing that.
What I don't understand, having a coaxial cable, how do you get 12 v to the module and receive signals on 2 wires?
Car type? Where is the receiver module? But I think you will have a hard job using this unless you can find documentation.
You only need a supply to the module if it has a built-in preamp, to overcome cable loss. Straightforward to use the centre conductor of the coax as the +ve and screen as -ve, with coils and caps at each end to separate power and RF signal. If you have a coax plug on the end of the cable I suggest you just try it.
So I've looked at the schematics, and it seems that the antenna goes into a aerial connection unit, that is more accesible. I,ll try to reach it in a couple of days, and hope to find some info with it.
If you unit is similar to the marine GPS boards we used to build, all the receiver electronics were in a tiny box under the patch antenna. Only power and data signals came out to the circuit board we built.
I'm thinking if I have the coaxial cable going all the way to the multimedia unit, most likely the receiver is in the unit (having no logic to transport the data through a coaxial, instead of other simpler and cheaper way). Also, I have the possibility to open the multimedia unit and reverse engineer it starting from the Fakra connector, searching for components that work as a receiver. From what I have searched, that unit I was taking about might just be a female to female connector, placed for easier removal of the GPS antenna.
This would be the logical approach I agree and I assumed it would be used in roof-mounted car antenna units. But different models of the same car may have different infotainment fits, some with and some without nav, so they don't want to build in extra cost. Also the roof position in a car is very unfriendly for electronics - parked in the sun in Bahrain the temperature can go up to nearly 100C, chips generally specified up to +85C.
So I've checked the car head unit voltage for the GPS antenna, and the good news is that it outputs 5V. The receiver is in the head unit. On the other hand, the voltage on the android device pins for the GPS antenna is 0.06 volts. Am I missing something? I've checked while the GPS is active, and searching for satellites. The antenna for the android is a sticker, like a NFC emitter.
Is it a possible the power a antenna with that voltage, or the android devices uses a different technology for lower power consumption?
Also, I've searched the web a little more, and it seems there is another antenna, classified as wifi and GPS antena, behind the motherboard, with 3 pins. I'll check for the voltage on those pins tomorrow, but why would they use 2 antennas for the GPS?
So the head unit can supply 5V for a preamp in the GPS antenna. The GPS input in the tablet doesn't need to do this because there is no lossy feeder between the antenna and the receiver. Antennas as such have no need for dc power. I would be slightly surprised if they share an antenna between Wi-Fi and GPS as Wi-Fi will radiate quite a lot of noise out of band and GPS is essentially having to decode a signal well below noise - anyway even if they do it won't be powered.
The 0.06V you measured is probably just leakage - essentially it's zero. No it won't power the preamp in the GPS antenna (if there is one).
It may well be that the 5V from the head unit is provided just in case it is used with an antenna with preamp - the one you have fitted may not. So worth just connecting it to the tablet and trying it out. If it does turn out to need 5V then you would need to make (or buy) a DC splitter. Couple of coils and capacitors, but not trivial to design or make.
Could you help me with a link to a DC splitter, or a diagram? I found splitters that need the DC voltages to be provided from one receiver, and I would need a splitter that has separate voltage input and signal output. Not sure if that exist.
I found this, I believe something like this would be helpful, but a schematic would be more helpful to adapt it to 5v, and build it myself if possible. If not, I can buy it.
Hope the link is not considered as product promovation, if so, I will remove it.
Actually you don't want a splitter, I chose the wrong term. That unit divides the GPS signal into 2 parts as well as feeding the DC. What you need is a bias tee such as shown here:
$48 seems a lot for a couple of connectors and two or three passives. There are probably DIY designs around, I haven't found anything from a quick Google. But I suggest just trying the antenna you have without the DC feed first, as I said it may not have a preamp.
Would this bias tee be ok?
Seems a little sketchy.
That's the sort of thing, looks much as I would expect. What's the price in EU, £ or $, just out of interest? You will need SMA connectors or adapters.
It's 1£ without a case, and 10£ with the metal case. Shipping is about 2£.
Giving the fact that I will need to connect it to the Fakra connector, I will solder a coaxial cable straight to the board that will have a Fakra make connector to the other end, and for the other connection(RF+DC) I'm still thinking. I need a flexible cable to bend it in tight spaces.
For the moment, I will try to connect the Android to the antenna with a short (15 cm) twisted pair of normal wires, since I can't get any coaxial wire. That shouldn't be an issue, I believe?
Remember that GPS is working at ~1.5 GHz so you need a good cable and take care with terminations. 15cm of twisted pair will probably NOT work very well!
Would a shorter length of cable work? Let's say 5 cm? This type of cable would be just for testing.
Twisted pair will probably not be impedance matched, and at ~1.5 GHz the wavelength is 20cm, so you 15 cm is 3/4 wavelength. Standing waves on the line could cause quite a large signal loss so if it doesn't work how will you know what the the problem might be? Shorter would be better, ideally solder a coax connector matching the one on the cable.
I have managed to solder a coax cable with the proper connector, and I am glad to report that there is a small increase in number of satellites acquired. The values fluctuate a little but they remain in those margins most of the time.
The first picture is with the antenna connected and the second one is without. Now, a biased tee would surely help me,right? The length of the cable to the antenna is about 5-6 meters.
Also, would it be possible for someone to help me with a diagram for diy biased tee, because I would like to test it this month (giving the fact that an aliexpress package takes about 3 months to arrive in my country)?
I have found this. It says it's tested up to 2 GHz, so it would work in teory.