Author Topic: Shared GPS  (Read 3154 times)

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Steve Arndt

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Shared GPS
« on: December 09, 2012, 03:51:59 PM »
I'm currently plugged into 3 GPS "pucks".  Any chance the CN Vario, ClearNav and Flarm could share a common signal?

Evan Ludeman / T8

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Re: Shared GPS
« Reply #1 on: December 09, 2012, 03:58:35 PM »
Can't do this with devices that are secure flight recorders (which all three are).  FWIW, I've had good luck so far locating my three gps pucks very close together (almost touching) vs. the normal recommendation of 12" separation.
Evan Ludeman / T8

dg300

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Re: Shared GPS
« Reply #2 on: November 04, 2013, 03:48:20 AM »
Hi All
Since mid this year we started to have interference between CN and Flarm GPS antennas.
GPS antennas although they are receiver are also transmitting because of signal bleeding of  the internal oscillator of the receiving circuitry.
I moved the pucks as far as possible apart but the problem never disappeared completely.
Antennas needs to stay in the instrument panel hood so it stays a completely solution when removed from the glider.
I started looking on the internet for possible solutions. I found something on Wikipedia but don’t think you can connect something running on 1.5Ghz as normal power lines.
I succeeded to use only 1 GPS antenna to power 3 devices being CN,CNv and Flarm. I still have  a fourth connection available if I need an additional antenna in the future. This is accomplished by using a Wilkinson power divider. I found a 1 to 4 port splitter on eBay. I had to modify the outputs because only 1 output should feed the DC to the antenna. In the picture you will see that the 2nd port from the bottom doesn’t have a capacitor in the line (47pf). This is the DC pass-through.  2 of the other ports have also a 220 ohm resistor to simulate a load of an antenna. The 3th port doesn’t have this. I tested and found out that none of the devices has a problem if it doesn’t see a DC load of an antenna. In the final setup all the 22O ohm resistors have been removed.
With this setup I have 13 sats on the CNv and 12 sats on the CN.  Before when the Flarm was powered on the CN dropped to 5 or 8 sats.
From readings I found that a 4 port splitter would have a 6db loss . A 3 port would have 4db loss.
6db means only the half remains.
RG174 cable has  about  1 to 1.5 db loss per meter. So if you shorten the antenna cable you would gain some from that.  The splitter should be centred around 1.5Ghz. Mine is between 800 and 2000Mhz with the center at 1400Mhz.
Currently the power is coming from my CNv.
Please be aware that a setup like this makes it less redundant as having 3 GPS antennas . I don’t think it violates any rules.

« Last Edit: November 05, 2013, 03:27:11 AM by dg300 »