What is the signal carrier in a coaxial cable?
In coaxial cables, the RF signal travels in TEM mode both on the outside of the inner conductor and on the inside of the outer conductor(shield) if the input generator (antenna, sig gen, etc) and and the output load match the coaxial cable impedance. Waveguides have no center conductor and operate in either the TE or TM mode and have cutoff frequencies. If the coaxial cable is high quality and has a shield approaching 99% there is very little if any ingress and egress of the signal.  As the frequency goes higher, UHF and above the the Rf signal travels mostly on the outside surface of the inner conductor and on the inside surface of the outside conductor(shield). This is the so called "skin effect" where the Rf signal does not penetrate deep into the conductor. It is more pronounced as the frequency gets higher and higher. If input, cable and load are all matched the two signal paths are of opposite polarity at any finite point along the cable since RF is AC and AC current requires a return path from load to generator. The same is also true for  parallel open wire transmission lines. The coaxial cable impedance is determined by the diameter of the inside conductor, the inside diameter of the outside conductor, the distance between them and the dielectric constant of the insulating material. This is a calculator for the impedance of coaxial cable. impedance[https://www.pasternack.com/t-calculator-coax-cutoff.aspx] I have no connection to this link other than it is handy. 2010Hivoltage