When choosing between horizontal and vertical polarization for GMRS antennas, vertical polarization is generally preferred. This is because GMRS is primarily a mobile and portable radio service, and most mobile and portable antennas are vertically polarized. Therefore, repeaters are also typically equipped with vertically polarized antennas.
Using a horizontally polarized antenna to communicate with vertically polarized antennas on repeaters, mobiles, and portables can result in a loss of signal. This is due to the mismatch in polarization, which can lead to reduced signal strength and potentially poor communication quality. If you are considering a horizontally polarized antenna, it would be most effective if all other stations you are communicating with also use horizontal polarization, which is more common in fixed links rather than in mobile or portable setups.
In a line-of-sight situation with a direct, unobstructed path to the repeater antenna, a horizontally polarized antenna might offer more gain. However, this potential gain must be weighed against the likelihood of encountering mixed polarization in the field, which can negate any benefits. For general GMRS use, sticking with a vertically polarized setup is recommended to ensure compatibility and optimal performance with the existing infrastructure and other users' equipment.
If you are setting up a base station and are concerned about antenna polarization, it is also important to consider the height and placement of the antenna. Antenna height is often more critical than the length or type of antenna used. A higher antenna can provide better coverage and signal strength, which can be more beneficial than the polarization type in many scenarios.