The driver's side is less likely to hit tree limbs , unless you are on a narrow trail
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I put mine in the middle to try and avoid limbs . That's also why I went with the two foot Firestik . It works well for trail riding . The short antenna is fine for talking short distances like trail riding . For talking long distance , the taller the antenna the better . The antenna is the most crucial part of the system . A good properly working antenna with an inexpensive cb is much better than a high dollar cb with a crappy antenna . It's also very important to have the antenna tuned to keep the Standing Wave Ratio (SWR) as close to 1:1 as possible . This lets the radio transmit power through the antenna properly . Very few people actually use the cb for trails anymore . Most are using General Mobile Radio Service (GMRS) which shares some frequencies with Family Radio Service (FRS) FRS radios are the little inexpensive handheld radios you see at Walmart . The are fine for close proximity but only putt out .5 watts to 2 watts depending on frequency.
GMRS requires a license from the FCC and allows higher power on selected frequencies and allows for mobile radios along with handheld radios . The GMRS license is only $35 there is no test and it covers your immediate family . It is good for ten years .
With GMRS you can get by with a much smaller antenna and generally it is a superior radio for trail riding due to clarity and the availability and ease of use of the handhelds .