Question T9B11
From subelement T9 - T9B
Which of the following types of feed line has the lowest loss at VHF and UHF?
Why is this correct?
Air-insulated hardline has the lowest loss at VHF and UHF because it uses air as the dielectric instead of foam or solid insulation. Air has virtually no signal loss compared to other dielectric materials. While 50-ohm flexible coax (A) is common and practical, it has higher loss due to its foam or solid dielectric. Multi-conductor unbalanced cable (B) isn't designed for RF transmission. 75-ohm flexible coax (D) has impedance mismatch issues with 50-ohm ham equipment plus higher dielectric losses than hardline.
Memory tip
Remember the pattern: air beats foam beats solid for RF loss. The thicker and more air-filled the cable, the lower the loss. This becomes critically important at higher frequencies where even small losses compound significantly over cable length.
Learn more
Air-insulated hardline represents the premium solution for commercial repeaters and high-power installations where signal loss directly impacts coverage area. The trade-off is installation complexity—hardline requires special connectors, can't bend sharply, and often needs pressurization systems to prevent moisture infiltration. Understanding this helps explain why flexible coax remains popular for amateur stations despite higher loss: the convenience factor often outweighs the efficiency gain for typical ham applications.
Think about it
Why do you think commercial repeater sites invest in expensive hardline installation despite flexible coax being much easier to work with?