Hello,

Now you can clearly see that the GSM antenna is connected directly to the coax.
The GPS coax goes to the box.

Bertus

Yes, my question is:
the capacitance i measure at the end of the GSM coaxial cable (2,5m long) is the capacitance of the cable itself or also the one of the PCB traces ?
I measure around 400 500 pF with a multimeter in the 400nF range (not a true capacitor, i know).

 

Hello,

The coax cable can be seen as a infinite combination of L/C circuits.
See this page for some general info:
https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/characteristic-impedance/

For the real information of the coax, you must need to know the type of coax.
It is likely printed on the side of the coax.

Bertus

 

I opened one unit, this is a picture of the inside.View attachment 172183

As @bertus said, the GSM antenna is, as I suggested in my first post, just a fancy PCB while the GPS antenna is that box.

Measure each, resistance should be enough. There should be an open circuit on the GSM antenna and some measurable resistance on the GPS antenna/LNA.

 

Hello,

The coax cable can be seen as a infinite combination of L/C circuits.
See this page for some general info:
https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/characteristic-impedance/

For the real information of the coax, you must need to know the type of coax.
It is likely printed on the side of the coax.

Bertus

Yes i see RG174.

 

Hello,

The capacitance is about 100 pF/m.
The rest you are measuring is likely the antenna itself.

Bertus

 

Many Thanks Bertus and Yaakov.
My next measures are:
the 2,5m of GSM coax cable alone (desoldered from the PCB)
the GSM PCB alone

 

Sorry for the delay.
I measured the cable alone (2,5m of RG174): 266pF
the PCB alone: more or less 2pF
I measured also a 4m cable of RG174 (the antenna is available in two versions: with 2,5 and 4m of cable): 402pF
The GPS module is not easy to open.
Anyway, the measure of capacitance is out of range.
Thank you for your help.