Ken Bessler wrote:
I was modeling a 5el 20m yagi. I changed antenna
height over realistic ground. As I expected, lowering
from 50' to 25' changed the take off angle from 18
degrees to 28 degrees.

What suprised me was the forward gain went from
14.44 dbi (@50') to 12.63 dbi (@25').

Is this normal?

Yes. A quick run of EZNEC with example file 20m5elYa shows just about
the same difference. Notice that the main lobe is much fatter when the
antenna is lower -- that's what happened to the gain. Everything else
being equal, a fatter lobe means less gain, since you have only so much
power to spread around.

I also ran a 3D plot so I could see the Average Gain. With no other
sources of loss, it indicates how much is lost in the ground reflection.
The lower antenna loses about 0.4 dB more than the higher one. That
accounts for 0.4 dB of the loss -- all the rest is because of the change
in pattern shape.

As a further educational exercise, try modeling a dipole at those two
heights and see what the gain difference is.

Roy Lewallen, W7EL