Almost all old school car radios are AM only .Some are valve .They all have antenna trimmers which are quite touchey to set up for optimum AM reception.The only thing that AM car radios have going for them these days is distance which means that the antenna must be tuned up properly .The valve car radios are generally more critical .Old car radio antennas had a total capacitance of about 68pF . A customer has to buy a new antenna because the origional one is missing or broken.2nd hand units may be unreliable due to corrosion . The available new antennas purchased in CHCH NZ have a total capacitance of between 130 and 350 pF .So in english this means that Ye olde example PYE Air Ranger will not go on the new say REPCO antenna in the say 1964 EH Holden .If a classic car has a factory radio then it is relevant to restore it .I am sure that the REPCO antenna will work on a modern car radio with a broadband input needing no trimmer .I bench tested the new Antennas on other old car radios like FORD and PHILCO STUDEBAKER and found that the New Ant killed the OLD radio despite the radio running perfectly on 2 feet of wire stuffed into the Ant socket .I wont go into a lot of techobabble about buffer preamps etc on this site .Sure I can nail this with buffers but what do other people do ? Has anybody had this problem before ? Does somebody manufacture the old school low loss low capacitance car radio antenna cable ?
RG-62/U coaxial cable might be a reasonable match. The dielectric is air space polyethylene with a velocity factor of .86, impedance of 93 ohms 13.5 pf per foot. Outer dimension of 0.242 inches and a maximum operating voltage of 750 Volts RMS. This might be what you need for your vintage car radios.
This data comes from the Antenna Book (Amateur radio relay league) I don't know the year as the cover disintegrated years ago.....