Q:

Raising height of inverted V by 2 meters

Now that I have a working G5RV Jr. and have had some good luck with DX (Croatia and Venezuela on 20m, Morocco on 40m from Michigan at 100w output), of course I want to mess with the configuration.  It's sort-of an inverted vee at about a 145 degree angle, the max height is a little over 23 feet and the ends are about 16 feet above ground (antenna is 52 feet long).  The ends of the antenna are well above my house and above all the neighboring objects except for trees of course.  I'm new at antenna modeling and I tried to model an inverted vee with MMANA just to see what the improvement would be if I increased the height in the middle by about 6 feet, and the ends by maybe a little higher, giving a sharper angle as well, but I wasn't impressed with the results.  Like I said, though, I'm new at modeling antennas.  
Also, I'm currently limited to 100w output.
So basically I'm looking for advice from more experienced people. I probably shouldn't expect a significant boost in performance for a 6 foot height increase at either 20m or 40m, should I?  I mean, it's not like I'll open up Japan or Australia by raising it by 6 feet, right?

A:

When a dipole is less than a 1/4 wavelength above ground, the image antenna formed by the reflection from the ground creates destructive interference at low angles, making the antenna quite useless for DX.
At 40 meters, a quarter wavelength is about 35 feet, more than the current 23 feet at apex. The effect will be diminished if you are not also able to raise the ends of the antenna, but still the effect will be more than nothing. Raising the antenna will direct more power at low angles useful for DX.
Furthermore, getting the antenna farther from the ground reduces ground losses. If your soil conductivity is low, this can be very significant.

