What may cause the unlocking of RF transmitter?

Hi,
I use PLL + VCO for my ISM band (2.4~2.5GHz) transmitter design. It works well when I connect a 50 Ohm antenna in front of the PA. But my transmitter will be unlock and still shock when I remove this antenna. This phenomenon only happened at the lower ISM band. Could anyone knows what's the possibly factor will cause this phenomenon ? It seems impedance mismatch will affect RF unlock. What can I do to fix this problem ? Please give me some suggestion about this problem.

Thanks in advance

Tsung Han

You probably need a buffer between the VCO and antenna. Do a load pull on the circuit (using some attenuators and cable) and monitor VTUNE to see if it hits a supply rail. See if there is an impedance that unlocks it. If not, RF could also be feeding back into the VCO from the PA, but more thena likely it's a pulling problem.

When you remove the antenna, very strong reflections occur and then these may affect the VCO frequency stability due to limited S12..