WRYF638

Channels 1-22 are simplex channels, meaning there is no repeater offset unless you change the settings. Channels 23-30 are duplex channels, meaning there is a +5MHz offset for repeater use.

You can build a better antenna than any commercially manufactured one if you are handy. Search for homebrew ham antennas, and use a calculator to resize the elements for your preferred frequency. https://www.hamuniverse.com/wb3aywcollinear.html I built a 4-element collinear vertical antenna from copper tubing. It took a bit of tuning to get it perfect, but it was worth the effort. Total cost was about $60, and antenna gain is about 7 dBd (9 dBi). The difference between that and my dipole is amazing.

The amount of acceptable cable attenuation depends greatly on your antenna gain and its location. If you have an efficient antenna mounted high in the air, the antenna can make up for a good amount of cable attenuation. Conversely, if you have an inefficient antenna or the antenna is mounted poorly, you will need to almost eliminate cable attenuation in order to achieve any significant transmission distance. For short cable runs, less than 10 meters, almost any conventional cable can be used without major impact on the signal strength. For long runs, as would be expected for a base station, low loss cable is far more important than the radio output power.

From my days in CB radio (20 years ago), I recall that grounding the antenna and radio are critical to good transmission patterns, at least with standard mobile antennas. Grounding the antenna mount and radio also helps reduce RF interference from the vehicle. However, grounding will not change the incoming signal strength. One thing that will further improve reception is an RF signal booster. I have not seen any such products specifically designed for GMRS, but any UHF signal booster for 70cm/440mhz should do the job reasonably well.