The piece of 75 ohm coax needed is to bring the impedance back down to 50 ohms. So some reading on what impedance is and how it works with antennas. It will explain a lot as to why you may need it. You can also use a 1:1 matching balun or about 12 mix 31 snap on ferrite chokes as well.
The reason you may be seeing the 2:1 VSWR readings is to due to an impedance mismatch. And the 75 ohm piece of of coax will bring the impedance back down to around 50 ohms. Also so will a 1:1 matching balun or a choke or even ferrite chokes as well. Read up on how dipoles work and how they need to be built to work properly. There is more to making an antenna than just an SWR reading. Just know that. I won't go into detail as it may really confuse you. But I suggest reading and researching how to build a proper dipole antenna for 11 meters or the CB band. Google will have tons of info on this subject. Hope this helps.