James H
The attenna orientations is primary a matter of optimizing the transmitter-reception power transfer, and not really a part of em or signal analysis. The em waves have a rotating polarization. So any orientation works, you just design the attennas for a particular environment. Since a lot of modern cars don't have external attennas at all. They're embedded in the window glass.

lunchtime_browser
Historically, FM was introduced using horizontal polarization, intended for high quality sound reproduction in homes and fixed locations. When FM portables became practical, they were fitted with a whip with a 'knee-joint' at the base, so that they could be turned horizontal for best reception. Not particularly convenient. As you point out, cars tend to use a vertical whip. Nowadays, FM broadcast stations tend to use circular polarization. In the simplest case horizontal and vertical antennas fed with a signal with a 90 degree phase difference between them. The effect is an E component apparently rotating. This provides a good signal for a receiver antenna orientated in any direction.

JOHNNIE B
Most of natures static is vertical ,and many things like trees are polarized vertical. If U are horizontal and the station is vertical you should see about 30 dB loss. They absorber a lot of signal. If u are trying to shoot through a forest the horizontal will work best. You need an antenna with about 10 dB gain . and be able to rotate it to point at the station u want. If that is not enough get an amplifier to go at the top of your antenna and bring it down a coax cable to prevent radiation out of the amp. and causing oscillation.look on the inter net at Ar2 communications products. They have some very low noise amplifiers. Good luck

devilsadvocate1728
In the United States, FM and VHF television broadcast signals are horizontally polarized, so horizontally antennas are better for picking up their signals. The frequency at which FM is broadcast is similar to that for low-band VHF television signals so the same antenna can serve well for both. The signals from AM commercial radio broadcasts, on the other hand, are of a much lower frequency and are vertically polarized. These are dissimilar to the FM signals and the antennas are likely to be entirely separate. The big whip antenna you see on automobile bumpers and fenders usually isn't used for picking up FM signals. If you are so inclined, you may want to look a little more closely at your car radio to find out what and where the FM antenna is. On some cars, the FM antenna is built into the windshield. This kind of looks like a pair of thin vertical black lines a few centimeters apart coming from the bottom of the windshield and then bending away from each other into a horizontal line at the top. This forms a classic half-wave dipole antenna for horizontally polarized signals. On a few cars, the antenna is enclosed in a boomerang-shaped housing with a foot coming from the bottom by which the enclosure is attached to the trunk or the top of the passenger compartment.