GPS no longer finding signals [bad GPS antenna]

rrobinetrrobinet
edited March 2018 in Problems Now Fixed
My Kiwi GPS lost the ability to track satellites.   The status page shows sporadic signals for one or two satellites, but it no longer locks to even one.  I have changed the antenna with no improvement and the antenna remains outdoors mounted on a pie pan.  

Comments

  • sv1btlsv1btl
    The same with my Kiwi, some months ago. I've sent it to the seller, but no luck. They've told me that the GPS and the BeagleBone were ok and they sent it back, but GPS never worked again, although I've changed 3-4 antennas.
  • rrobinetrrobinet
    Thanks for the warning.  I've logged on the Kiwi but I'm not sure where to look for GPS activity logging.
  • jksjks
    @SV1BTL Who was the seller? Are you using a feed line extension or anything else unusual?
  • jksjks
    @rob GPS activity is not logged. Maybe there should be an option.
  • G8JNJG8JNJ
    Hi,

    This may sound a bit obvious - but have you checked that the antenna still getting power (DC supply volts) from the KiWi GPS SMA connector ?

    Sometimes when RF adaptors are being used the DC continuity can fail.

    Regards,

    Martin - G8JNJ
  • rrobinetrrobinet
    Unfortunately the Kiwi is in Maui and I am in California, so it will be a few weeks before I can examine the unit.  I'll post the results of my examination here.  In the meantime the Kiwi is now published on sdr.hu and the logs show many users from around the world. 
  • G8JNJG8JNJ
    "Unfortunately the Kiwi is in Maui and I am in California"

    Ah OK............

    Good luck with your faultfinding,

    Martin - G8JNJ
  • sv1btlsv1btl
    edited January 2018
    @jks ;
    GPS does not work to my Kiwi in both cases - with the antenna came from the seller and with an extension as well. 5V are for sure comming out from the GPS antenna's socket.
    It's for good that I can manually calibrate the frequency.
    I've sent you a private message.
  • rrobinetrrobinet
    My GPS problem was corrected by purchasing a third GPS antenna.  That $15 antenna advertised 25dB gain and with it I track 6 or more satellites.
  • rrobinetrrobinet
    I just built a 100' GPS extension cable from a Satellite TV RG-6 coax cable + F to SMA adapters. It is amazing that the GPS signals will travel so far on a $20 coax cable
  • jksjks
    edited June 2018
    I'm a little surprised you got away with that. But RG-6 is okay at GPS L1 (~8 dB loss @ 1.5 GHz per 100 feet) compared to the 4 dB loss of the short 3m of RG-174 that is attached to the Kiwi puck antenna.
  • rrobinetrrobinet
    I too was surprised. I left the rg-174 in place and just extended it with the 100' RG-6. So total loss is perhaps 4 + 8 = 12 dB. Perhaps the improved signals from the better antenna location outdoors compensates for the additional RG-6 loss. We are going to measure the RG-6 loss with a vector analyzer next Monday to get real world data and I'll report back.
  • G8JNJG8JNJ
    Hi,

    You should be able to get away with it if the antenna has a high enough gain amplifier to overcome the cable losses without degrading the Signal to Noise ratio.

    The S/N ratio is generally set by the device used as the amplifier in the GPS antenna, which should be as low noise as possible. Above a minimum value the gain figure doesn't really matter, it's only there to make up for cable losses. Once you overcome the cable loss, adding more gain will not improve the S/N ratio.

    It's exactly the same with satellite TV reception. The Ku band 11GHz is converted down to L-Band 1-2GHz (the same frequency range as GPS signals) by means of the LNB (Low Noise Block) down convertor that is mounted at the dish focus. The LNB typically has 50dB gain so that you can run quite long lengths of coax before the attenuation becomes excessive.

    For example, I use a professional outdoor 'bullet' GPS antenna which feeds four GPS receivers after being split (with a passive satellite TV L-Band splitter with 6dB loss) via 30m (approx 100ft) of RG6 foam filled foil and braid screened coax, and it all works perfectly OK.

    A lot of the magnetic mount GPS antennas with RG174 cable generally only have enough gain to overcome the loss associated with the attached very thin RG 174 coax. Some have more gain than others and as a result will work with longer coax cable extensions, but as they all look the same it can be a bit of a gamble.

    Regards,

    Martin - G8JNJ
  • rrobinetrrobinet
    When the GPS antenna supplied with the Kiwi failed I ordered this $15 GPS antenna from Amazon which claims to have 28 dB gain.

    https://smile.amazon.com/gp/product/B00LXRQY9A/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

    I don't know what the gain was of the original antenna, but this antenna works through the approximate 8 dB of additional loss of the 100' of CAT-6. Your splitter suggestion is intriguing as I plan to install 6 or more Kiwis at the KPH receive site later this month, and I would rather not run extension cords to antennas for each Kiwi. I expect you would use a splitter which blocks DC from all but one of the splitter outputs.
  • jksjks
    You may need an active splitter if you want to use one antenna to feed 6 Kiwis. A passive 1:8 splitter would have a lot of loss. Many of the active splitters, especially the ones you find on Ebay, have a 5V minimum DC input. Your antenna will likely accept 3.3-5V but the Kiwi will only output 3.3V so you'd have to feed 5V from a DC source into the splitter DC port.
  • G8JNJG8JNJ
    Hi,

    Yes, you would need to use an active splitter an 8 way passive would have about 12dB through loss which is likely to be excessive.

    General rule of thumb (approximate) attenuation when using cheap satellite TV passive splitters:-

    2 ways 3.5dB
    4way 6.5dB
    6 way 8.5dB
    8way 12.5dB
    12way 16dB

    Regards,

    Martin G8JNJ
  • WA2ZKDWA2ZKD
    I bought a +40 dB gain antenna on eBay for <$50 and split that
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