Neo M8P-0 no fix, not enough sattelites

0 votes

Hello everyone,

I am using a board with an Neo m8p-0 with an passiv antenna. 

However, I am not able to get any fix outdoors. 

In the Antenna settings I enabled everything except open circuit detection. Can that be the problem?
I am able to see that there are one or two sattelites detected most of the time, but not more... I also got a C94-M8P running with the same configuration, that is able to get an fix instantly with loats of sattelite detected. And the C94 is using an active antenna as provided.

The Antenna Status looks like:

And the board is sending these nmeas:


Does anyone have an idea what I am doing wrong?

by mstranne asked Dec 6
0 votes
Either your passive antenna is terrible or there is a strong noise source nearby that is jamming the receiver.
Describe your passive antenna and its implementation: patch, chip, PIFA, helix, length and type of coaxial cable and connector, size of ground plane, mounting of antenna, etc.

The antenna configuration settings mean nothing UNLESS the product board you have is designed to use an active antenna but the passive antenna you connected is shorting out the applied DC bias voltage, affecting the receiver's VCC.
by grampy answered Dec 6
by grampy edited Dec 7
Sorry I wasnt able to add images here, so I created antother Answer.
0 votes

I am using this antenna:

thats the schemantics of antenna to m8p:


and here you can see that it is able to find a lot of sattelites, but cannot use them for a fix:


However I am not sure what I am doing wrong now, since the setup was working some time ago ...

by mstranne answered Dec 6
That's not a passive antenna. It takes 3-5V and has a 28dB gain
Measure the voltage on the antenna side of the L2 inductor.
The sort of gain specified and cable length you'd expect signals in the 40-50 dBHz range, you need >= 32 dBHz to decode navigation data
Why it used to work but doesn't anymore?? --- no idea except to suspect a failed antenna or damaged cable, or new noise from other nearby subsystems.

Observations on your schematic:
Remove R22 and R23, they are not required since NEO has internal.pull-ups on those.
Check R21 --it may have been fitted as 10k rather than 10 Ohms or may have been damaged by a short circuit on the antenna cable.

I believe that VCC_RF has a direct DC connection from VCC with no switches or monitors so no antenna configuration commands are required.

Place a 4.7uF to 10uF low-ESR capacitor from VCC to ground to minimize noise on VCC.

The antenna you selected alludes to having an internal filter but it may be of questionable quality, allowing external interference to creep in.  If you are serious about using M8P for RTK you should invest in a good antenna from a reputable manufacturer, and not scrape the bottom of the barrel for the most important element of good RTK. The forum has several recommendations for GNSS  RTK antennas.

Keep all digital signals far away from the RF_IN pin and the RF components.  

Make sure that inductor L2 has a self-resonant frequency of 2GHz or higher ... again, use components from a reputable manufacturer.
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