G9B: Dipole and Monopole Antennas – Ham Radio General License Study Guide

G9B covers the two most fundamental antenna types in amateur radio — the half-wave dipole and the quarter-wave vertical (monopole) — along with the practical considerations of height, polarization, radial systems, feed point impedance variation, and length calculation. The twelve questions in this group span radiation patterns, how antenna height changes the pattern and impedance, where ground radials should be placed, the benefit of horizontal over vertical polarization, and the formulas for calculating antenna length at a given frequency.

Topics include random-wire antenna characteristics, how to set a ground-plane vertical's impedance to 50 ohms, the radiation pattern of a quarter-wave vertical, the radiation pattern of a dipole in its own plane, the effect of height on a horizontal dipole's azimuthal pattern, where to place radials for a ground-mounted vertical, how dipole feed point impedance changes with height, how feed point impedance changes with feed point position along the element, the advantage of horizontal polarization, and three antenna length calculations.

Dipole Radiation Pattern and Feed Point Impedance

A half-wave dipole in free space has a well-defined radiation pattern. In the plane containing the wire (the H-plane), the pattern is a figure-eight at right angles to the antenna — maximum radiation comes from the sides (broadside), with sharp nulls off both ends. This bidirectional broadside pattern is the classic dipole characteristic.

The feed point impedance of a half-wave dipole is approximately 73 ohms at resonance when fed at the center. As the feed point is moved from the center toward the ends, impedance steadily increases. The center is the point of maximum current and minimum voltage — the low-impedance point. The ends are points of maximum voltage and minimum current — very high impedance. This is why center-fed dipoles are convenient: the 73-ohm center impedance is close to the 50-ohm characteristic impedance of coaxial cable.

Effect of Height on Dipole Pattern and Impedance

Antenna height above ground affects both the radiation pattern and the feed point impedance of a horizontal dipole:

Effect on Radiation Pattern

At elevation angles higher than about 45°, a horizontal dipole mounted less than 1/2 wavelength above ground has an azimuthal pattern that is almost omnidirectional. The reason is that at low heights, the ground reflections reinforce radiation in all horizontal directions at high angles, filling in the null off the ends of the dipole. This is desirable for NVIS (Near Vertical Incidence Skywave) propagation, where radiation straight up is the goal.

Effect on Feed Point Impedance

As a horizontal dipole's height is reduced toward 1/10 wavelength above ground, the feed point impedance steadily decreases. The presence of the lossy ground image antenna interacts with the real antenna, and at very low heights the ground coupling reduces the radiation resistance significantly. This makes very low dipoles harder to match and less efficient.

Vertical Antennas and Radial Systems

Quarter-Wave Ground-Plane Vertical

A quarter-wave ground-plane vertical has an omnidirectional azimuthal radiation pattern — it radiates equally in all horizontal directions. The radiation pattern in the vertical plane has a low-angle lobe well suited to DX communication.

The feed point impedance of an ideal quarter-wave vertical over a perfect ground plane is approximately 36 ohms. To raise it to approximately 50 ohms — a better match for coaxial cable — a common technique is to slope the radials downward at an angle (typically 45°). The downward-sloping radials change the effective ground plane geometry in a way that raises the feed point impedance.

Radial Placement for Ground-Mounted Verticals

For a ground-mounted vertical antenna system, the radial wires should be placed on the surface or buried a few inches below the ground. A buried radial system (typically 32 or more radials) provides an effective RF ground, reducing ground losses and improving efficiency. Elevated radials work differently and require only a small number (4 or more) to be effective, but ground-mounted systems benefit from horizontal radials at ground level.

Random-Wire Antennas

A random-wire antenna connected directly to the transmitter — without a proper ground or balun — allows significant RF current to flow on station equipment including the transmitter chassis, coax shield, and connected equipment. This is because the random wire is unbalanced and the station ground forms part of the antenna system. The result can be RF interference to audio, computer equipment, and a shock or burn hazard from RF voltage on equipment surfaces.

Polarization Considerations

Comparing horizontal polarization to vertical polarization for HF use:

• Horizontal polarization has the advantage of lower ground losses. Horizontally polarized antennas interact less with the lossy earth surface, particularly at the lower elevation angles used for short to medium-distance communication. This makes horizontal antennas more efficient over typical ground.
• Vertical polarization provides a lower-angle radiation pattern more suited for long-distance DX, and is often used when a low horizontal dipole would be impractical.

Antenna Length Calculations

Two formulas cover the exam length calculations:

G9B Practice Questions