Practical takeaway
Coax/feed-line types, attenuation, SWR, antenna tuners, RF connectors, and weather protection.
T9B · 12 pool questions
Feed Lines, SWR, Tuners & RF Connectors
Coax/feed-line types, attenuation, SWR, antenna tuners, RF connectors, and weather protection.
Technician exam focus
This group contains 12 questions in the current pool. The teaching below keeps the important tested terms visible while explaining what they mean in normal Amateur Radio use.
1. Most amateur coax is a 50-ohm system
The most common characteristic impedance for amateur-radio coaxial cable is 50 ohms. Coax is popular because it is easy to route and requires relatively few special installation precautions compared with open-wire/balanced feed line.
2. Feed-line loss grows with frequency
All practical feed lines have attenuation, and coaxial-cable loss generally increases as frequency rises. Larger cable such as RG-213 has less loss than RG-58 at a given frequency. Long runs on VHF/UHF are where this difference becomes especially noticeable.
3. SWR describes the match
Standing wave ratio (SWR) is a measure of how well a load is matched to a transmission line. An ideal match is 1:1. High SWR does not itself tell you exactly what is wrong; loose connections, water intrusion, antenna damage, or an off-resonance load can all be causes.
4. A tuner changes what the transmitter sees
An antenna tuner/coupler provides an impedance transformation so the transmitter can operate into the antenna/feed-line system. It does not magically remove loss already occurring in a feed line, and a low SWR at the transmitter does not guarantee the antenna itself is efficient.
5. Keep water out of RF connectors
Outdoor connectors — PL-259, BNC, Type N, and others — should be carefully weatherproofed. Water intrusion can corrode conductors, change impedance, and increase feed-line loss.
T9B checklist
- Most amateur-radio coax is 50 ohms.
- Coax is common because it is easy to use with few special installation concerns.
- Loss generally increases with frequency; RG-213 has less loss than RG-58 at the same frequency.
- SWR measures how well the load matches the transmission line.
- A tuner transforms impedance seen by the transmitter; it does not erase feed-line loss.
- Weatherproof all outdoor coax connectors.
- Erratic SWR can indicate a loose connection.
Exam scope: NCVEC 2026–2030 Technician Class (Element 2), effective July 1, 2026 through June 30, 2030, with February 19, 2026 errata. The current NCVEC pool is the exam authority; explanatory prose here is SPARKA teaching material. NCVEC pool