Coaxial Transceiver – HF Signal & LF Signal on Same 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

The signal from the transmitter comes along the insulation layer between the shield and the center wire. Conductive metal has some induced current - it's needed to make the wave able to propagate in the insulation layer. The induced current exists on the inner surface of the shield and the outer surface of the center conductor.

When the wave meets the end of the line it meets actually a highly non-symmetric antenna. One piece of it is the red rod and the other piece of it is the grey rod and the outer surface of the shield. Quite far from anything balanced. But it can still do its job as an antenna. I have used portable HF radios with dipole antennas at frequencies 3...20 MHz. When there was coaxial feed with no balun the radio communication still happened, but the antenna cable became a part of the antenna and the whole radio did the same. If one touched loosely the metal of the radio or morse key when transmitting the skin started to smoke at the touching point. Harmful, but not serious due the low power (about 10W max total transmitter output). I guess the balun was not included because the radio did its job without one and every extra part increased system complexity and unreliability.

But you asked why the coaxial cable is unbalanced, not the why antenna construction is unbalanced. That's because its conductors are not equal. It's shield can collect substantially bigger noise voltage from the external electric fields than the center conductor which is surrounded by metal - nearly complete Faraday cage except the ends have holes.