If you keep the same mounting location but use a shorter antenna, your transmission efficiency will decrease. To compromise on the antenna height, you can only slightly reduce the antenna height, at least making it about 5 centimeters higher than the roof of the car, or you can buy a flexible antenna, which is available. Another option is the shark fin antenna, which attaches to the roof and is similar to those found on sports cars. In either case, as long as the antenna is higher than the roof, the transmission efficiency will still be acceptable.