[Tinkerphones] GTA04: about the MMCX socket for external GPS antenna...

H. Nikolaus Schaller hns at goldelico.com
Fri Aug 2 10:44:27 CEST 2019


Hi,
I think I finally understand what the problem is...

Recently I found out that my external GPS antenna with long cable and MMCX plug is broken.
I ordered a new one and then I tried to plug it into the GTA04A4.

It didn't go in as smoothly as I was used to and therefore I applied just a little more pressure
and ...

... snap, the MMCX socket did disappear inside the GTA04 case.

Now what is the reason?

The MMCX plug has a tiny golden slotted ring which should be pressed together when inserting
the plug into the outer ring of the MMCX socket. This ring should then move inside the socket
until it snaps in a notch and keeps the plug from falling off by gravity applied to the cable.

This is the "lock-snap mechanism" mentioned here: https://en.wikipedia.org/wiki/MMCX_connector

Well, making the ring be pressed together and move inwards needs a smaller or bigger force
in the insertion direction, i.e. into the GTA04 plastics case.

My old MMCX antenna had a plug where the force apparently was much lower. And new one needs
a higher force.

Since the MMCX socket inside the GTA04 is not mounted upwards on the PCB but with right angle
(while the one shown on the wikipedia photo is straight), the force is not pressing the socket
onto the PCB but parallel to it. With the result of shearing it off.

I have checked with some Amphenol data sheet and they say:

Engagement Force	< 30 Newtons
Disengagement Force	> 8 Newtons @ 1 - 5 Mating Cycles
			> 4 Newtons @ 100 - 500 Mating Cycles

Well, 30 Newtons is like placing a 3kg weight on it... That is quite a lot, compared to the
ca. 140g of the GTA04. And explains why the solder joints can be sheared off.

Interesting is that it is expected to wear out after more than 5 cycles which is what could
have happened several years ago to my old cable.

A solution would be available in the GTA04A5 where the MMCX socket is a through-hole type
(like the Wikipedia photo but with a 90 degrees bend). This withstands much higher forces
(maybe > 300 N) than the simple surface mounted soldering of the GTA04A4 SMD socket.

So I now have to repair it, which is fortunately trained several times.

Then we have to find out how to make the plug insertion need lower forces...

BR,
Nikolaus



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