You will loose a bit of the receive sensitivity and some TX power, but nothing you will notice in the real world.
Depending on the antenna and base you have, you are best replacing the complete cable! If this is not possible, just make up and extension coax cable using good quality coax and connectors.
What type of connector is on the end of the current antenna cable? If its a PL259 connector your best cutting that off and crimping on a BNC or N-Type.
Where abouts are you located? If you need help let me know and i'll make up a cable for you.