Mobile ground plane question

[Sabre]

I've read a lot of information about optimizing the location of a mobile antenna, including the excellent discussions on http://www.k0bg.com/. I'm getting ready to put a mobile dual-band set in the Montero and have planned on drilling a 3/4" hole for the NMO mount, using best practices to really minimize losses. My question is about ground planes and about the odd configuration of the Montero's roof in particular.

One of the cool things about the Monty is the enormous glass sunroof. It's 32" wide and a whopping 30" long. However, that leaves only 11" of roof between the sunroof opening and the windshield trim. I had planned on centering the mount in this space, but wanted to seek the input of folks with real-world experience. Putting the antenna aft of the sunroof would be problematic from two standpoints. First, that's where the glass sunroof lives when retracted, so there would be no access for an antenna for at least 30 inches, second, that's where my roof rack is, which is very often loaded with a second spare, fuel, water, or whatever other stuff I'm hauling. Hopefully a rooftop tent will live there some day.

The Generation III Montero is a unibody truck and I'm planning on spending the time to do the best job of bonding that I can. What do you think about positioning the mount in the center of this 11-inch span of roof between the windshield and sunroof?

 

[4x4junkie]

Should be fine there. It's up high and in the clear.

Besides, it sounds like that's the best you'd be able to do anyway, so why not?

IMO, I think the ground plane thing becomes more of an issue if the antenna is mounted toward a corner of the vehicle (meaning there's little or no ground plane under two adjacent sides). If the ground plane extends on two opposite sides, the losses are less (this is just my unscientific evaluation from working with various different band antennas on different vehicles, to include bicycles and ATVs).

 

[DaveInDenver]

I would put it right where you're thinking. It's not ideal but not very much of a compromise in the grand scheme and certainly better than just about anything short of the middle of a Econoline roof.

 

[Sabre]

Thanks, guys. Antenna just arrived today and I'll proceed as planned.

73s de Sabre :safari-rig:

 

[kb9yoj]

If you install the VHF/UHF antenna on the space you were talking about you should have no issue. Remember, at VHF/UHF the frequencies a physically short, and your antenna will be using the roof (and any other metal it is attached to) to create the image of the other side of the dipole. At frequencies below 30 Mhz is when it gets really challenging to create a "ground plane" on a car chassis.

Matt
KB9YOJ