Problem with antenna of a tetra handset in DMO

i have designed a tetra handset, now when DMO(direct mode operation ), i must hold the antenna by hand, and another phone can receive signals and voice is clear. as soon as i undo my hand , the other phone can receive signal but no voice can be heard.
can anyone tell me why?

thanks

The impedance is very sensitive. Any ground plane change
the impedance improving it.
Try to match the antenna the best you can.

I agree, the antenna impedance matching work have to be done.
Use some inductor or capacitor to tune the antenna impedance to be 50Ohm. Ensure the handset battery, cover are in the original position. A vector network analyzer would be used to do the work. When the job is finished, you should test whether the impedance change when you hold it or not. If it does change a lot, then it's the antenna designer's work now. Impedance matching can not do everything for you.
It's preferrable to refer to the antenna engineer.

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