After some research on this forum and youtube, I decided to replace the bad mast on my infiniti I30.
I was following crazy4maxima 's video. But later on, I found it's very hard to push the sawtooth back into the coil box. But I remember a video clip on youtube about replacing mast for Honda and I think all the mechanics should be same. I put the motor back to the trunk and tried the youtube way. Here is the video:
http://www.youtube.com/watch?v=nEJtooskd9Y

If you don't have anything broken inside, you don't even need to take off the motor, just follow the youtube video above. Remember, the sawtooth should face to the back of the car( I guess maximas are the same).

Another tip, I bought the mast in pepboy, it's 20 bucks, it's just a stick and plastic, priced too much. crazy4maxima claimed he got his on ebay for only 10-12 bucks. If you are not in a hurry, just go ebay to get a cheap one.

In the end of this post, I got a question, I couldn't push the mask all the way down, no matter how many times I turned on and off the radio. there is still 2 inches sticking out. Is this normal?

 

are you using a universal antenne?

 

No, I use the one for Maxima 95-99 model. There is a booklet in pepboy that you can check the model number.