Antennas and Feed Lines

T9 — Antennas and Feed Lines

The last subelement covers the antenna itself (how it radiates and how gain/polarization work) and the feed line that connects it to your radio.

T9A — Antenna fundamentals

A beam antenna (like a Yagi) concentrates signal in one direction rather than radiating equally in all directions — of the choices typically given, the Yagi offers the greatest gain. Antenna gain is the increase in signal strength in a given direction compared to a reference antenna — it doesn't add power, it redirects/concentrates what's already there. Polarization is described by the orientation of the electric field, matching the T3B definition. A half-wave dipole radiates its strongest signal broadside to the antenna, not off its ends. Shortening a dipole raises its resonant frequency; lengthening it (or adding loading coils/capacitive tip-loading) lowers it. On 2 meters, a 19-inch vertical is popular because it's a resonant quarter-wave; a 5/8-wave whip offers more gain than a 1/4-wave, at the cost of a lower radiation angle. A handheld's short flexible "rubber duck" antenna trades efficiency for size compared to a full-size quarter-wave.

T9B — Feed lines and connectors

Coaxial cable is the dominant amateur feed line mainly because it's easy to use and needs few special installation considerations — not necessarily because it's the lowest-loss or cheapest option. 50 ohms is the standard coax impedance in amateur use. Loss in coax increases with frequency, and increases from water intrusion, high SWR, and additional connectors in the line. PL-259 connectors are common at HF/VHF but aren't watertight and aren't ideal above 400 MHz; Type N connectors handle frequencies above 400 MHz better. An antenna tuner matches the antenna system's impedance to the transceiver's output impedance — it doesn't change what the antenna itself resonates at. SWR (standing wave ratio) measures how well a load is matched to the transmission line; a loose connection is a common cause of erratic SWR readings. Air-insulated hardline has the lowest loss among common feed line types.