ARRL Technician Practice Exam

Which type of feed line has the lowest loss at VHF and UHF frequencies?

1. Coaxial cable

2. Plastic insulated cable

3. Air-insulated hard line

4. RG-58 coaxial cable

The correct answer is: Air-insulated hard line

Air-insulated hard line is recognized for having the lowest loss characteristics at VHF and UHF frequencies. This is primarily due to its design and construction. The hard line consists of a rigid outer conductor, often made of copper or aluminum, which provides excellent shielding and minimizes signal leakage. The inner conductor is supported by a high-quality insulator, typically air, which reduces dielectric losses. At VHF and UHF frequencies, the factors impacting loss include the dielectric material used, external shielding quality, and the physical dimensions of the cable. Air insulation contributes to lower capacitance and, consequently, lower attenuation of the signal compared to cables with solid dielectric materials. In contrast, coaxial cables, including RG-58, utilize dielectric materials that can absorb some energy as heat, leading to increased loss, particularly at higher frequencies. While plastic insulated cables can be effective for many applications, their performance doesn’t match that of air-insulated hard line in terms of loss. Therefore, the choice of air-insulated hard line as the answer highlights its superior performance in maintaining signal integrity at higher frequencies commonly utilized by VHF and UHF communications.