This is the recommendation as posted in the owner's manual for MicroMobile radios regarding antenna location. A UHF antenna needs a metal surface to 'radiate' off of. The larger, the better. Your bumper choice may not provide enough surface area.
" Installing the Antenna
To get the best range from the MXT400 an external antenna (50Ω) should
be used. The antenna is intended to be attached to the vehicle’s roof,
trunk or similar location. Specific installation requirements vary between
vehicles. Use the following guidelines to install the antenna.
* Where you locate your antenna does make a difference. *
1 Metal surfaces covered by fiberglass or vinyl may affect radio range.
Avoid these locations.
2. Mount the antenna as high on the vehicle as possible. The higher the
3 . If possible, mount the antenna in the center of whatever surface you
4 . The antenna cable is 19.7 feet (6 meters) long. Be sure the mounting
location will allow for connection of the cable to the radio.
5. Be sure the mounting location is clean and dry before installing the
6. Route the antenna cable through an accessible entry point, such as a
rear door or trunk opening.
7. When routing the antenna cable inside the vehicle, keep the cable
away from noise sources, such as the ignition system, gauges, etc.
8. Exercise care to prevent cable damage. Make use of existing gaskets,
grommets and weather stripping to protect the cable along its route."
Good luck with your project.