http://www.pulseelectronics.com/download/3248/antenna_basic_concepts/pdf
I searched for that image to blow it up to find the dB loss. It's on page 8 of this PDF.
Thanks for finding that. I tried to process the pic again and get it down to a reasonable size for forum use (640x480) and it still makes the type too small to read really well. It looks OK on my screen, but not great. Others may have worse luck.
Just a caveat for everyone, even though the magnetic mount has the same minimal loss at the corner of the roof that it does in the center, the position will alter the radiation pattern. Center of the roof gives pretty much equal radiation in all directions. Corner or edge of the roof creates an offset pattern because there is less ground plane in one or more directions. A rough rule of thumb is that you strive for the 1/4 wave antenna's height in ground plane in all directions. I may not have phrased that very well so here is an example. If you're using a 17"
VHF quarter wave antenna, you ideally should have 17" of ground plane in all directions. If you're using a 6"
UHF 1/4 wave, you want at least 6" of ground plane in all directions.
Things get complex when you have multiple antennas of multiple frequency ranges on a single roof. Years ago, we were adding 800 mhz antennas for an MDT system to vehicles that already had 1/4 wave
UHF spikes in the center of the roof. The techs found that they needed to reposition the
UHF to accomodate the 800. I don't have the exact placements handy, but they told me it took a lot of experimentation to get it right.
I've got
VHF and
UHF quarter wave antennas on the roof of my F250 truck. Because of the construction of the cab, including side ceiling airbags, I did not want to drop the headliner to install them. But I was able to fish in from the
CHMSL far enough to place the
VHF antenna about 17" from the edge of the roof. The
UHF is behind it with 7" of roof clear in all directions. The antennas are about 9" apart. Since I only use the
UHF (GMRS) when traveling with the fifth wheeler and
VHF is strictly for
EMS operations when at home, I don't have interference or intermod issues. Others may have different results based in part on useage.