Batboard

Difference?

I currently have a mag mount on the roof with a 1/4 wave and I see alot of people talk about the Fender Mount 1/4 Wave. Wouldn't it hurt performance?

Is there any advantages?

The roof gives you a better omni pattern for transmitting,
according to everything I've ever read on the subject.
The fender has some lobes that are not a strong as other
lobes (directions), due mainly to presence of the roof.

And, you can put a longer antenna on the fender than
you can on the roof if garage clearance is a problem.

So why does everyone decide to mount them on the fender if it results in not as strong signal going out from the whip?

It's easier to patch up the holes from a fender-mount angle bracket, than from a big NMO hole in the roof.

and probably the clearance problems that Tom In D.C. mentioned.

Aesthetics more than anything. The roof is overall a more superior mounting position.

Overhead clearance shouldn't be a big issue with passenger cars but on trucks/SUVs, yeah.

3 little screw holes or one 3/4 inchhole

Overhead clearance certainly can be an issue with cars.
My Outback is not that tall and I can only safely run
a 16-inch dualbander safely on the roof. A "standard"
dualbander is out unless you like to hear the antenna
hitting the sprinkler pipes in the parking garage.

(The above is based on recent personal experience,
because when you get old you spend a lot of time going
to doctors which requires parking in building garages.)

The 3 foot long VHF midband whips on the roofs of police cars here often hit things and get damaged.

The poster mentioned a quarter wave but not a band. Assuming it's either VHF-hi or UHF the size of it shouldn't be a problem on the roof long as it's not a tall vehicle. Far as fender mounting you would be altering the radiation pattern considerably not to mention the decreased elevation as to having it mounted on the roof. Center of the roof is always the #1 location unless you have clearance factors at play. Not living out in the boonies I use 1/4 waves exclusively, on my truck my VHF whip is a of a whippy small diameter so if I do hit it the whup will bend and not damage the NMO mount. My car has a stout 1/4 wave on it as clearance isn't a problem. The car does most of it's mileage on the highway at 70+ and the heavy whip doesn't de-tune at these speeds as a whippy type does.
My .02

Well I was just saying that everyone says that they've installed it on the fender and I thought well if everyone else is doing it and it does good maybe I would do the same but I guess I'll just keep the mag mount on the center of my truck.

I am referring to VHF 19" 1/4 Wave

If the hole is the issue go with a 3/8" hole mount instead of the big 3/4" style. Easier to install, and a lot easier to plug later. Though they do make rubber body plugs for both sizes smaller is always better when making holes in my opinion.

I've seen several radiation pattern charts that indicate the center of the roof is the best place to mount an antenna. I constantly get my hind parts chewed any time I mount one here though. Apparentyly the State Police Investigators in my home state are slaves to fashion rather than folks who might need a little more umph in their transmit signal on some long and lonely highway some night. This is one reason I gave up the profession and went back to installing electrical systems on construction sites. The politics and BS was killing me a day at a time.

KG4LHQ wrote:if everyone else is doing it and it does good

Most land mobile systems are designed as one with specific goals in the design.

A local agency or business knows where their mobiles and handhelds will be. They can design the repeater or base to give xx% coverage into specific areas with antennas of whatever type specified by the contract bidded on.

These systems are not used like ham repeaters where the users can be in many random locations, with random transmitter powers and antennas. And the other thing about hams is the natural curiosityabout how far it will work." To one service it's a toy, to the other, it's a tool.