TV antenna

[MacGyver]

Is it possible to use an old TV antenna like the big kind you but on your roof for CB use?

[linx]

No. TV antennas have small elements used to pick up analog tv signals. Also, I think the feedline coming off a TV antenna is 75 or 100ohm whereas CB coax is 50ohms. It could probably be possible to use a higher frequency radio like a ham radio with an old TV antenna, but it's not going to be possible to use the CB with it. 1/4 wave elements in CB freqency is around 9 ft long, so you would need at least that, to be effective. 1/2 wave would be best at 18 ft (give or take).

[Circuit Breaker]

Is it possible? Sure...but you would have to basically rebuild it with longer elements. If the impedance on it is 75 ohms, that isn't too bad and wouldn't present much of an issue. But, like I and Linx have stated, you would need to do a lot of work to it and it just wouldn't be worth the effort. Adjusting it to work on 2 meters or 70 cm would be much easier.

['Doc]

Nothing new here, just a re-hash of what's already been said.

Can you use a TV antenna for CB? Sure. Will it be any good? No. Not without a lot of rebuilding.
One other method of making it into a CB antenna is by using a tuner. That tuner can make it sort of 'look good' to your radio. It'll still be a terrible CB antenna but your radio will be happy with it (but you won't be). About as good/useful as a rubber duck, sort of, which ought'a 'turn you off' real quick .
You might be surprised at what you can 'make' work with a tuner. And I'd be surprised if whatever that 'what' is did anything worth mentioning sometimes...
- 'Doc

[Night Crawler]

One other method of making it into a CB antenna is by using a tuner.

That's the way to do it.
Short the feedline together and run the tuner against a ground. It will work like a random wire antenna.