Practical takeaway
The feed line connects transmitter to antenna as an RF transmission system. Its characteristic impedance, loss, and mismatch determine how voltage and current vary along the line.
G9A · General question group
Feed Lines, SWR & Matching
The feed line connects transmitter to antenna as an RF transmission system. Its characteristic impedance, loss, and mismatch determine how voltage and current vary along the line.
General exam focus
The explanation below teaches the ideas behind G9A while keeping the important terms recognizable. The Quiz Me button uses the current question-pool wording for this group.
1. Characteristic impedance belongs to the line
Coax and balanced line are designed for a characteristic impedance determined by geometry and dielectric. A load equal to that impedance produces a matched line with no reflected wave in the ideal case.
2. SWR indicates mismatch, not antenna quality by itself
When load impedance differs from line impedance, part of the wave is reflected and standing waves form. A low SWR does not prove that the antenna radiates efficiently.
3. Tuners transform impedance at their location
An antenna tuner can present the transmitter with a suitable impedance even when the antenna/feed-line system is mismatched. It does not erase feed-line loss that already occurs on a lossy mismatched line.
G9A checklist
- Characteristic impedance is a property of the transmission line.
- A mismatch creates reflected waves and SWR.
- Low SWR does not automatically mean an efficient antenna.
- A tuner matches impedance at the tuner; it does not erase feed-line loss.
Exam scope: NCVEC 2023-2027 General Class (Element 3), effective July 1, 2023 through June 30, 2027, with February 4, 2026 sixth errata. The current NCVEC pool is the exam authority; explanatory prose here is SPARKA teaching material. NCVEC pool