What we found at the CA shootouts is that when the bottom
section runs closely parallel to the vehicle body, as it
does with a trailer hitch mount on an SUV, the field
strength is much lower than if that bottom section is
in the clear, e.g. mounted on the roof of the SUV.....

I often wonder about this myself, but never get around to trying a
bumper mount. In the past, I've always preferred to have the lower
mast and coil as clear of the body as possible. But on the other
hand, if I mounted the base on the bumper, I could have a longer
mast below the coil. It's hard to decide which would be better on
paper. But...On my "play" truck, I decided to go whole hog. I
mounted the base of the antenna on the rear pillar of my cab,
back behind my head. The base of the antenna is appx 64 inches
off the ground. Yes, it kicks butt... But I sometimes wonder how
it would do with the bumper mount, and longer lower mast. The
problem is I have campers on both of my trucks, and have always
been afraid to have the lower mast right up against the back tailgate,
and camper. It's hard to decide of the longer antenna would outweigh
the higher mount, and shorter antenna. I think really the only way to
know for sure is to actually try and compare both.
But in the past, and present, I'm a "high mounter" as far as mobile
whips.
BTW, I was out camping in Utopia TX about 2 months ago, and had
the chance to really give that truck and antenna a good workout.
It was browning the food. I was S9 plus to all TX stations, and even
S 9 to a Salt Lake City puter receiver listened to on the internet.
That was 80m...On 40m, it's even better. Course, that antenna when
parked is 14 ft tall, and has the coil at 8 ft from the base. It's 11
ft
tall in the driving mode. Even the short version is tall, when mounted
on the cab of that truck. The radio was the 706 barefoot. I use no
amp when mobile. I do know it's really bad news to have the coil
near body metal. But I've never had that problem yet on my various
vehicles.
MK