What exactly is the antenna doing?
If your antenna is defective and need replacement, try eBay for a good price.
If you have no intention of replacing it and interested in keeping it extended at all times, that I'm not sure if at all possible. Normally that happens when the antenna motor itself is broken. The motor when given the juice will extend the antenna, and when the juice is cut off, it retracts. If you want, you can try breaking it by stopping it during the retraction motion. Then again, it will be in a way when you need to go through car washes.
Nothing in life is simple! :silly2: