Which of the following types of feed line has the lowest loss at VHF and UHF?

Air-insulated hardline is known for having the lowest loss at VHF and UHF frequencies due to its construction. Unlike flexible coax or other types of cable, the air-insulated hardline design minimizes dielectric losses because it utilizes air as the insulating medium rather than a solid dielectric material. This structure allows the signal to travel with minimal attenuation.

In addition, air-insulated hardline is typically built with a larger diameter, which helps to maintain a consistent impedance and reduces the impact of skin effect at higher frequencies. These characteristics result in superior performance for long cable runs, making it the preferred choice for many VHF and UHF applications, particularly in competitive amateur radio setups or broadcast environments.

Other types of feed line, such as flexible coax, may have higher loss due to their dielectric materials. Multi-conductor unbalanced cable can also incur losses due to its construction and is less efficient at maintaining the required impedance. Similarly, while 75-ohm coax is good for certain applications such as video transmission, it is not optimized for the power-handling and lower loss characteristics needed at VHF and UHF frequencies compared to air-insulated hardline.