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Roof or fender mount... everything else is a compromise depending on how important performance is to you. A 27MHz whip I would probably fender mount due to its height.

If doing fender mount with a NMO, use a Larsen KHFUD cable assembly because they are a sealed unit. The solder-on type will get your coax braid and center conductor wet after the first rain or carwash which will cause corrosion and can change its impedance. Those are designed for roof mounting since the bottom side is inside the vehicle cab and shielded from the elements.

I do prefer the NMO mounts not only because I do way more VHF/400MHz/800MHz installs but also because the RF components are completely enclosed up until the antenna's radiating element itself. The threaded peg style can be affected by any adjacent metallic objects which affects your match... reflected power back into the radio. Especially with a 4 watt mobile (that's theoretical on a good day, terminate a real radio into a test set and it may be only 3.6W stock) you want every bit of power going into the air. With a 30W mobile then 0.5W coming back isn't a huge deal for power loss.

On my Tundra I had removed the center stack cupholder tray entirely and mounted the radio equipment there. If you have a center console try the side of it, inside the compartment itself or if it is a compact like a PRO520 maybe it will fit that slot in between, but would need an external speaker because it's a down-firing speaker.
 

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You want a bit of flex. An extremely rigid mounting would not give and you'd be breaking stuff. There are also opposite cut mounts available for the other fender side available.

NMO27 and NMOKHFUD coax assembly is definitely the way to go for fender and through-roof installs and it's the setup that I recommend time and time again with successful results and good feedback from the end users. In addition it's very easy to change to another radio setup if need be in the future.

Make sure the gasket is in place between the antenna thread mount (well in this case loading coil base) and the fender bracket. They make a thick one and a thinner one, I generally use the thick one on fender brackets but just make sure there is something present. Occasionally they fall off during handling or installation.

Wrapping some rubber splicing tape or similar material would be prudent around the coax to prevent it from wearing the paint from vibration and also to prevent wearing the sheathing from the same vibration. I've also used a small strip of VHB mounting tape which sticks to itself and to the place you want it to be, then it loses its stickyness after being exposed to free air for a bit.
 
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