loosing square when connecting 1.9GHz Dect Radio test hardware

My test system consist of:

1) 50 ohm modified coax cable. On one side cable is spliced exposing center conductor and ground shield. The other side of the cable there is an integrated audio plug - tip, ring1, ring2, sleeve.

2) TX radio board that has a 3.5mm audio connector and RF connector. Input signal must be inserted into audio connector on radio board and it will be modulated onto a RF carrier at 1.9GHz out of the RF connector on the same board.

3) Silabs ucontroller. Pin 3 of the controller outputs a square wave of 3 V peak to peak.

4) A "3 pin" potentiometer. Used to vary the square wave voltage level.
Pin 1 of the pot is connected to pin 3 of the ucontroller.
Pin 2 of the pot is connected to the center conductor of the spliced side of the 50 ohm RF coax cable.
Pin 3 of the pot is connected to the ucontroller ground node.

5) RX Radio board that has a 3.5mm audio connector and RF connector.

With an oscilloscope I can see the square wave perfectly on ring1 of the 50 ohm modified coax cable.

The problem is soon as I insert the audio plug side of the modified coax cable into the TX radio board 3.5mm audio connecter I notice the square wave goes away on the oscillator.

does anyone have clues to why I loose the square wave when I insert the plug side of the modified coax cable into the Radio board 3.5mm connector? and how to fix it?