As a very rough approximation, about the same diameter as loudspeaker cable. But, the diameter may be more if the wire is longer, depending on resistance and cross-talk interferance, and the length is dependant on the wavelength of the radio being used, the required and/or available range, the location on the globe, atmospheric conditions, and time of day. That's why you might see a 'doubled' antenna wire on some aircraft, and why many needed to also employ an antenna wire on a drum, as a trailing aerial.