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GPS active antenna spec — Parallax Forums

GPS active antenna spec

john_sjohn_s Posts: 369
edited 2014-11-06 14:21 in General Discussion
I came across this http://parallax.com/product/28502 antenna that I plan to use with my GPS unit.
So far the only requirement for the antenna is that it has to be "active".
Now, the word "active" means that it requires an external power supply circuitry. But how many volts and at what current?
Does it have a built in biasing circuit and the rf coupling capacitor or should I provide it externally?

Since a specification for this particular antenna is non existent I am asking for some help from whoever uses this particular antenna (or similar) to spare any info on how to properly interface it to a GPS module.

Thanks

Comments

  • Heater.Heater. Posts: 21,230
    edited 2014-11-05 12:09
    I don't believe it is active at all.
  • jonesjones Posts: 281
    edited 2014-11-05 13:20
    The one you linked claims to be active, which just means there's an internal preamp. Here's an example of an active GPS antenna that I know is active: http://www.adafruit.com/products/960

    Typically they are powered by DC that is run through the feedline, so no extra wiring is needed to the antenna. The GPS module I used it with (Adafruit's Ultimate GPS breakout) needed nothing else, the power was derived internally in the GPS module, and the receiver would auto-detect the antenna. There's even a proprietary NMEA sentence that gives you external antenna status. As to why you need an "active" external antenna, I think it's mostly to compensate for feedline losses, which are high at >1 GHz.
  • SRLMSRLM Posts: 5,045
    edited 2014-11-05 13:20
    It's an active antenna. The "active" part is all contained in the GPS module and antenna. You don't need to plug in additional power, etc.

    To use you just connect it to your OEM GPS module, and it will automatically be used. One thing to be careful of is that it has a MMX connector, which is pretty rare. Most GPS units use SMA connector or a u.FL connector.

    What GPS are you using?
  • john_sjohn_s Posts: 369
    edited 2014-11-05 17:15
    MMCX connector is not an issue for me as I need to change it anyway..

    Can somebody who uses this antenna post a picture of the other side where I believe might be at least some sticker with "additional" info?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2014-11-05 18:49
    If a 75-ohm source impedance is adequate to power the antenna's built-in preamp, it's quite easy to run power and signal through the same cable. I did this myself once with a PIC-based windgauge. The PIC and sensors were installed in the windgauge housing. It was powered through the same RG59U coax that the NTSC video signal came down through from the roof. Here's a link to the project (#14 at the top):

    -Phil
  • PublisonPublison Posts: 12,366
    edited 2014-11-06 08:03
    john_s wrote: »
    MMCX connector is not an issue for me as I need to change it anyway..

    Sparkfun has an adapter for $8.95. Beats soldering. :)


    https://www.sparkfun.com/products/285
  • john_sjohn_s Posts: 369
    edited 2014-11-06 10:22
    Perhaps I should modify my rambling a bit - as there is NO datasheet and nobody so far knows the exact power spec for this particular antenna I assume its LNA can be safely powered from 3.3V external PS via some 33nH and a small decoupling 22pF cap to GND. This should satisfy just about any GPS module even the one that does not expect an active antenna to be connected to its ANT input.

    Is the LNA inside this particular antenna 5V tolerant is yet another question ...
    GPS ant.PNG
    468 x 295 - 11K
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2014-11-06 11:39
    Likely, but it's probably designed for 3.3v. Every GPS module I've seen has 3.3v bias on the antenna connector.
  • john_sjohn_s Posts: 369
    edited 2014-11-06 14:21
    Circuitsoft wrote: »
    Likely, but it's probably designed for 3.3v. Every GPS module I've seen has 3.3v bias on the antenna connector.

    Thanks, that's exactly what I was hoping for and very reassuring .
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