G9: Antennas and Feed Lines – Ham Radio General License Study Guide

G9 covers the transmission lines that connect your transmitter to your antenna, the fundamental antenna designs used in amateur radio, directional antenna systems, and specialized antennas for specific applications. Four exam questions come from this subelement, one from each group.

G9A addresses feed lines, SWR, and impedance matching: what determines characteristic impedance, how high SWR increases loss, the impedance of window line, what causes reflected power, how coaxial cable attenuation changes with frequency, how feed line loss is expressed, how to prevent standing waves, what a matching network does to SWR on the line, and how to calculate SWR from known impedances. G9B covers dipole and monopole antennas: radiation patterns of dipoles and verticals, how height and feed point position affect feed point impedance, radial wire placement, the advantage of horizontal polarization, dipole and monopole length calculations, and random-wire antenna considerations. G9C examines Yagi directional antennas: element sizes and functions, gain in dBi vs dBd, bandwidth improvements, front-to-back ratio, main lobe direction, stacking gain, optimizing parameters, beta/hairpin and gamma matching. G9D covers specialized antennas: NVIS, end-fed half-wave, halo, antenna traps, log-periodic, screwdriver mobile, Beverage, small loop, multiband, and inverted V designs.

G9A: Feed Lines and SWR

The characteristic impedance of a parallel conductor feed line is determined by the distance between the centers of the conductors and the radius of the conductors — not by length or frequency. Window line (ladder line with plastic insulating windows cut into it) has a nominal characteristic impedance of 450 ohms. Coaxial cable is typically 50 ohms (amateur use) or 75 ohms (video/cable TV). Feed line loss is expressed in decibels per 100 feet, and coaxial cable loss increases with frequency.

Reflected power at the antenna feed point is caused by a difference between feed line impedance and antenna feed point impedance. To prevent standing waves, the antenna feed point impedance must be matched to the characteristic impedance of the feed line. High SWR increases loss in a lossy transmission line — the energy bounces back and forth, experiencing more resistive loss in the conductors and dielectric with each pass. Conversely, higher transmission line loss reduces SWR measured at the transmitter input because the reflected wave is attenuated by the lossy line before it returns.

A matching network at the transmitter end that presents 1:1 SWR to the transmitter does not change the SWR on the feed line itself — if the line had 5:1 SWR before the network, it remains 5:1 on the line. SWR is calculated as the larger impedance divided by the smaller: 200Ω load on 50Ω line = 4:1; 10Ω load on 50Ω line = 5:1.

G9B: Dipole and Monopole Antennas

A half-wave dipole in free space (plane containing the wire) has a figure-eight radiation pattern at right angles to the antenna — maximum radiation broadside to the wire, nulls off the ends. When a horizontal dipole is mounted less than 1/2 wavelength above ground at elevation angles above 45°, the azimuthal pattern becomes almost omnidirectional — the ground reflections fill in the pattern.

The feed point impedance of a horizontal dipole steadily decreases as antenna height is reduced toward 1/10 wavelength. Moving the feed point from the center of a half-wave dipole toward the ends causes impedance to steadily increase — the center is the current maximum (low impedance); the ends are voltage maxima (high impedance).

A quarter-wave ground-plane vertical is omnidirectional in azimuth. Sloping its radials downward adjusts the feed point impedance toward 50 ohms. Ground-mounted vertical radials should be placed on the surface or buried a few inches below the ground. Horizontal polarization has the advantage of lower ground losses compared to vertical.

A random-wire antenna connected directly to the transmitter allows significant RF current to flow on station equipment — a common cause of RF interference and shock hazard, addressed by adding a good RF ground or antenna tuner.

Dipole and monopole length formulas: half-wave dipole = 468 ÷ frequency (MHz); quarter-wave monopole = 234 ÷ frequency (MHz).

• 14.250 MHz dipole: 468 ÷ 14.250 ≈ 33 feet
• 3.550 MHz dipole: 468 ÷ 3.550 ≈ 132 feet
• 28.5 MHz quarter-wave monopole: 234 ÷ 28.5 ≈ 8 feet

G9C: Directional Antennas

A Yagi antenna consists of a driven element (approximately 1/2 wavelength), a reflector (slightly longer than the driven element), and one or more directors (slightly shorter than the driven element). The reflector is behind the driven element; the directors point in the direction of maximum radiation. Adding more directors and lengthening the boom increases gain. Larger-diameter elements increase bandwidth.

Antenna gain can be referenced to an isotropic radiator (dBi) or to a dipole (dBd). Gain in dBi is always 2.15 dB higher than the same gain expressed in dBd — because a dipole itself has 2.15 dBi of gain over an isotropic radiator.

The front-to-back ratio is the power radiated in the major lobe compared to that radiated in the opposite direction. The main lobe is the direction of maximum radiated field strength. All three Yagi parameters — boom length, number of elements, and element spacing — can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth.

Two stacked three-element Yagi antennas spaced 1/2 wavelength apart provide approximately 3 dB more gain than a single three-element Yagi.

Two common Yagi feed matching methods: a beta (hairpin) match is a shorted transmission line stub at the feed point for impedance matching. A gamma match does not require the driven element to be insulated from the boom.

G9D: Specialized Antennas

A NVIS (Near Vertical Incidence Skywave) antenna radiates at nearly straight up to use the ionosphere for short-skip communication covering a few hundred miles. The best NVIS antenna for 40 meters is a horizontal dipole placed between 1/10 and 1/4 wavelength above the ground — this height produces a high-angle radiation pattern ideal for NVIS.

An end-fed half-wave antenna has a very high feed point impedance at its end — typically several thousand ohms — requiring a matching transformer.

A halo antenna for VHF/UHF radiates omnidirectionally in the plane of the halo — it is a bent half-wave dipole formed into a loop, providing horizontal polarization with omnidirectional coverage for mobile or portable use.

Antenna traps are LC circuits inserted into antenna elements whose primary function is to enable multiband operation — they electrically isolate the outer portion of the antenna at one frequency while allowing the full length to operate at a lower frequency. A disadvantage of multiband antennas in general is poor harmonic rejection.

Vertically stacking horizontally polarized Yagi antennas narrows the main lobe in elevation, concentrating radiation toward the horizon for long-distance DX work without changing the azimuthal beamwidth.

A log-periodic antenna has wide bandwidth as its primary advantage — element lengths and spacings vary logarithmically along the boom, allowing consistent performance across a wide frequency range. It has lower gain per element than a Yagi.

A screwdriver mobile antenna adjusts its feed point impedance by varying the base loading inductance — a motor-driven coil at the base changes the resonant frequency and thus the impedance.

A Beverage antenna is a long wire antenna (typically one to several wavelengths) used for directional receiving on MF and low HF bands. Its gain and directivity make it valuable for receiving weak signals and rejecting interference from unwanted directions.

An electrically small loop (less than 1/10 wavelength in circumference) has nulls broadside to the loop — pointing perpendicular to the plane of the loop. This bidirectional null pattern is used in direction-finding applications.

A dipole with a single central support — where both elements droop downward from the center point — is called an inverted V.

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