Practical takeaway
Antenna gain describes direction, not free power. ERP and EIRP combine transmitter output, feed-line loss, and antenna gain to describe effective radiated strength relative to a reference.
E9A · Amateur Extra question group
Antenna Parameters, ERP & EIRP
Antenna gain describes direction, not free power. ERP and EIRP combine transmitter output, feed-line loss, and antenna gain to describe effective radiated strength relative to a reference.
Amateur Extra exam focus
The explanation below teaches the ideas behind E9A while keeping the important terms recognizable. The Quiz Me button uses the current question-pool wording for this group.
1. Gain needs a reference
Gain in dBi is referenced to an isotropic radiator; gain in dBd is referenced to a half-wave dipole. The numbers differ because the reference antennas differ.
2. ERP and EIRP include the whole transmit path
Start with transmitter power, subtract feed-line and other losses, then add antenna gain using the proper reference. ERP uses dipole-referenced gain; EIRP uses isotropic-referenced gain.
3. Efficiency is separate from directivity
Directivity describes how concentrated the ideal pattern is; efficiency accounts for real losses. Gain combines both, so a directional but lossy antenna may have less gain than its geometry suggests.
E9A checklist
- dBi uses an isotropic reference; dBd uses a dipole reference.
- ERP and EIRP include transmitter power, losses, and antenna gain.
- Directivity and efficiency are separate concepts.
- Antenna gain redistributes radiated energy rather than creating power.
Exam scope: NCVEC 2024-2028 Amateur Extra Class (Element 4), effective July 1, 2024 through June 30, 2028, with February 4, 2026 fourth errata. The current NCVEC pool is the exam authority; explanatory prose here is SPARKA teaching material. NCVEC pool