Antenna ?

Why are comm antennas oriented vertically and vor/glideslope antennas oriented horizontal?

Partly because the transmitting antennae are oriented that way. They have to match.

It's called vertical and horizontal polarization.

No expert, but the comm radio signal is vertically polarized. The nav is horizontally polarized. I know nothing about the science of that so perhaps someone else will chime in.

Here is a thread on another forum that had some good discussion about this:
http://www.homebuiltairplanes.com/forums/instruments-avionics-electrical-system/8729-antenna-placement-interference.html

With a vertically oriented antenna, the signal will propagate from the side of the antenna in a 360 degree pattern. The signal will be weakest in the direction that the tip of the antenna is pointing (i.e., straight down and/or straight up).

With a horizontally orientated antenna the signal will radiate out from the antenna the same as the vertical antenna. However, since it is horizontal the pattern will radiate vertically. I assume that the antenna on the ground would be set horizontal, perpendicular to the runway, with the strongest signal straight down the runway. Think of a tire cut in half, standing up.

Tracy.