Doing some more work, and modeling the actual antenna (as opposed to the similar antenna mentioned above), I made two models, one sitting by itself 10 feet in the air, and another just as high with a metal sheet about a foot below it. I was able to get both antennas to tune using the gamma match. Also, for these antennas, the Y axis is along the direction the ends of the antenna points, and I am assuming that the are pointing "forward" on the vehicle.
One additional note, the red line in these models is horizontal gain, the blue line is vertical gain, and the green line the combination of the two.
To start with, horizontal plane (like we are looking down on the antenna). First the one in the air by itself, and next the one over a metal sheet.
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With the metal sheet under the antenna, we can see that we get almost 1 dB of additional gain to the front of the vehicle, at the cost of about 2 dB gain to the rear of the vehicle. I must point out that this gain is horizontal polarization, not the vertical polarization typically used for CB communication, so this will affect your ability to communicate further.
Looking along the Y axis on the vertical plane, again, antenna by itself first, antenna over metal sheet second.
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Here we see the directional effect from the side view. In the case of no metal sheet, peak gain is 55 degrees off of the horizon, and with the metal sheet peak gain is about 40 degrees off of the horizon.
Note: Horizontal gain here is the same as total gain. They are so close you can only see one of the two lines.
Now a look at the X axis.
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This is where we are seeing our vertical gain. With the metal sheet under the antenna, we have less vertical gain then without. In both situations, this gain peaks straight up.
The DB
