General Class License Study

Overview

Antennas and feed lines are arguably the most important topic for any HF operator. The best transceiver in the world is defeated by a poor antenna. This module covers HF antenna types, feed line characteristics, impedance matching, and standing wave ratio.

Fundamental Antenna Concepts

Half-wave dipole: The most fundamental HF antenna. Feed-point impedance approximately 72 ohms in free space. Close enough to 50 ohms for direct coax feed, though a balun is recommended. Length calculation:

Length (feet) = 468 / f(MHz)

Quarter-wave vertical: Fed against a ground plane. Feed-point impedance approximately 36 ohms over a perfect ground. Requires radials to form the ground plane. At least four radials are recommended; more is better.

Length (feet) = 234 / f(MHz)

Yagi Antennas

A Yagi (Yagi-Uda) uses a driven element plus parasitic elements to concentrate the radiation pattern in one direction. The reflector is behind the driven element; the director(s) are in front.

• A 3-element Yagi achieves approximately 7–8 dBi gain (≈5–6 dBd)
• Front-to-back ratio of a well-designed 3-element Yagi: 20–25 dB
• Gain is achieved at the expense of pattern — the antenna is less sensitive off the sides and back

Loop Antennas

Full-wave loop: Feed-point impedance approximately 100 ohms. Provides about 1.5 dBd gain perpendicular to the plane of the loop. Loops tend to be quieter on receive than verticals because they reject vertically polarized noise sources.

Small transmitting loop (magnetic loop): A highly resonant loop with a circumference of 0.1 to 0.25 wavelengths. Very high Q, very narrow bandwidth, requires precise tuning. Advantage: small physical size. Useful where full-sized antennas are not possible.

Feed Lines

Coaxial cable: Standard feed line for most amateur stations. The center conductor carries the signal; the outer shield is the return conductor. Most common amateur impedance: 50 ohms. Loss increases with frequency and with the SWR on the line. Common types:

• RG-58: Thin, lossy, inexpensive. Acceptable only for short runs at low power.
• RG-8/RG-213: Standard-diameter 50-ohm coax. Good for HF.
• LMR-400: Low-loss large-diameter coax. Good for VHF/UHF and longer HF runs.

Ladder line / open-wire line: Very low loss even at high SWR — typically less than 1 dB per 100 feet. Must be kept away from metallic objects. Cannot be routed through walls or around corners. Used with an antenna tuner for multi-band non-resonant antenna systems. The combination of a doublet antenna, ladder line, and a balanced tuner is one of the most efficient multi-band HF systems available.

Baluns

A balun (BALanced-UNbalanced transformer) connects a balanced antenna to unbalanced feedline. Without a balun, common-mode current flows on the outside of the coax shield, causing RF in the shack, distorted radiation patterns, and TVI/interference.

• 1:1 current balun (choke balun): Best choice for connecting a dipole to 50-ohm coax. Blocks common-mode current without affecting the differential signal.
• 4:1 balun: Transforms 200 ohms to 50 ohms. Used with center-fed antennas fed with ladder line.

Standing Wave Ratio (SWR)

SWR is the ratio of maximum to minimum voltage along a transmission line that is not perfectly matched. SWR of 1:1 is perfect. As impedance mismatch increases, so does SWR.

• Most transceivers begin reducing output power when SWR exceeds approximately 3:1
• High SWR on a lossy feedline causes significant power loss in the feedline itself
• An antenna tuner can give the transmitter a 1:1 load while the feedline still has high loss — the tuner solves the transmitter's problem, not the feedline's loss problem
• SWR is not the complete picture of antenna system performance — a 1.5:1 SWR on 500 feet of RG-58 may represent worse system efficiency than a 3:1 SWR on 50 feet of LMR-400

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Practice Questions