Well I'm not sure witch way would be better, I like my scanner antenna on top in middle of cab, but not sure where to put the cb antenna now after what you told me about the location of the antenna for the cb, is a stick antenna a better way to go ?
By "stick" antennas, are you referring to the fiberglass antennas?
I think you need to ask yourself which radio is the most important to your own personal needs. That antenna should get the best location.
Mounting the scanner antenna on the cab roof should work just fine, doesn't just need to be in the center. Try to avoid mounting it all the way out at the edge, give it some ground plane to work with if you can, but don't get too hung up on it. Getting the antenna up high and in the clear will give you the best performance. The higher it is the better it can "see" the signals. When you hide it down behind the cab, you are blocking what it can "see". Same thing goes for mounting it off the fender, there is going to be some blockage from the cab.
The CB antenna will work in most places and it could be that you wouldn't necessarily notice the differences. Putting any CB antenna directly behind the cab is going to give you issues. The proximity to the metal of the cab is going to detune the antenna. Probably the worst place you can put an antenna on a pickup is directly behind (and shielded by) the cab.
I always had bad luck with the fiberglass stick antennas. They'd work fine for a while, but then it would seem like they were having issues. Checking them with an SWR meter usually indicated that they were way too short. This was likely caused by the fiberglass bending enough to damage the thin wire wound around the fiberglass core. I would usually end up replacing those fiberglass antennas every year for one reason or another.
When I got my amateur license, I started reading more about antennas. I learned about having them high and in the center of the ground plane. I invested in the tools to do permanent NMO mounts. After that point, all my antennas when on the cab roof. I started installing antennas for others, including a lot of CB antennas. I quickly noticed the improvement of putting a CB antenna on the truck roof, up high and centered on the ground plane. The stainless whip stood up to tree branch strikes a whole lot better. Aesthetically it looks better. They lasted longer, too. In fact, once I started mounting the antennas up there with a decent NMO mount, I never had to replace one.
I'll almost always recommend a Larsen NMO-27 CB antenna with a permanent NMO mount. I've installed enough of them over the years that I'm completely sold. never had an issue with them, and never had an issue getting them down to the 1.1:1 SWR.
I've looked at and tried many of the "consumer" grade CB antennas and I won't recommend them. Doesn't matter what others say, when you work with commercial gear you'll notice the difference.
I'd say use a good CB whip on the center of the cab roof. Stick the scanner antenna off to one side. That will give you the best overall performance.
Also, if you go with a standard NMO mount, you can swap antennas around all day long and never have an issue. Nice thing about standardized mounts like this is that you have a huge selection of antennas to pick from. If your interests change down the road and you decide to get into amateur radio, you can easily unscrew the CB or scanner antenna and screw on one that will do the amateur frequencies. Putting an NMO mount or two on the roof will give you a lot of possibilities. Also, you'll know you did it right the first time and won't ever second guess yourself. Sort of built in peace of mind.
