Why do both Icom and SGC use those big ugly, expensive insulators at the base of their antennas?
A. When you use a tuner to load a physically short antenna like a 102-inch whip on low frequencies, the feed impedance becomes very high. The higher the impedance the higher the voltage for a given power output. Any length of feed line (even a few inches to a foot) will raise the impedance at the base of the antenna even more. With 100 watts output, it would not be unusual to have on the order of 1000 Volts at the base of the antenna. The big, ugly insulator is needed to ensure that it does not arc over, even with high humidity, rain, or dirt on the insulator. SGC advises against the use of the standard “CB” type ball mount which is designed for a 50-ohm feed impedance and much lower voltages. The ball mount may also exhibit excessive capacitance to the vehicle which can reduce the antenna system efficiency on the lower bands.
