G9D: Specialized Antennas – Ham Radio General License Study Guide

G9D covers antenna designs beyond the basic dipole and Yagi — systems optimized for specific propagation modes, installation constraints, or operational requirements. The twelve active questions in this group span a wide range of specialized antennas used across HF, VHF, and UHF amateur radio.

Topics include the best antenna type for NVIS short-skip propagation on 40 meters, the feed point impedance of an end-fed half-wave antenna, the radiation direction of a VHF/UHF halo antenna, the primary function of antenna traps, the advantage of stacking Yagi antennas vertically, the advantage and description of a log-periodic antenna, how a screwdriver mobile adjusts impedance, the primary use of a Beverage antenna, where an electrically small loop has nulls, the disadvantage of multiband antennas, and the common name for a dipole with a single central support.

NVIS and End-Fed Half-Wave Antennas

NVIS (Near Vertical Incidence Skywave)

NVIS propagation uses the ionosphere to provide reliable communication at short distances — typically from about 50 to 500 miles — by radiating at nearly straight up. The signal goes up, reflects off the ionosphere, and comes almost straight back down, covering a circular area around the transmitter. This is valuable for emergency communication and regional nets where HF skip distances are too long for regular coverage.

For NVIS on 40 meters during the day, the best antenna is a horizontal dipole placed between 1/10 and 1/4 wavelength above the ground. At this low height, the radiation pattern peaks at high elevation angles — close to vertical — which is exactly what NVIS requires. A dipole at 1/2 wavelength or higher would concentrate radiation at low angles, unsuitable for NVIS. A vertical antenna produces predominantly low-angle radiation and is a poor choice for NVIS.

End-Fed Half-Wave Antenna

An end-fed half-wave (EFHW) antenna is a half-wave element fed at one end rather than at the center. At the end of a half-wave antenna, current is minimum and voltage is maximum — creating a very high feed point impedance, typically several thousand ohms. Connecting this directly to a 50-ohm feed line would produce an extremely high SWR. End-fed half-wave antennas require a matching transformer (commonly a high-impedance unun, often 49:1 or 64:1 winding ratio) to step the impedance down to a usable level.

Halo, Antenna Traps, and Multiband Antennas

Halo Antenna

A halo antenna is a horizontally polarized antenna for VHF and UHF use. It is constructed by bending a half-wave dipole into a loop (halo shape) and feeding it at one end with the other end capacitively loaded. The radiation pattern is omnidirectional in the plane of the halo — radiating equally in all directions horizontally while providing horizontal polarization. This makes it suitable for mobile and portable VHF operation where horizontal polarization is preferred for repeater contacts but omnidirectional coverage is needed.

Antenna Traps

Antenna traps are parallel LC resonant circuits inserted in series with the antenna element. Their primary function is to enable multiband operation. At the resonant frequency of the trap, the LC circuit presents a very high impedance, electrically isolating the outer portion of the antenna. Below that frequency, the outer portion becomes part of the resonant length. A single antenna with traps can thus operate on multiple bands without requiring a separate antenna for each.

Multiband Antenna Disadvantage

Despite the convenience of multiband operation, multiband antennas have a significant disadvantage: poor harmonic rejection. An antenna resonant on one band often also resonates on harmonically related frequencies, allowing harmonic energy from the transmitter to radiate efficiently. This can cause interference on other bands and potentially violate FCC spectral purity requirements.

Log-Periodic and Stacked Yagis

Log-Periodic Antenna

A log-periodic dipole array (LPDA) is a broadband directional antenna in which the element lengths and spacings vary logarithmically along the boom. Each element resonates at a slightly different frequency, so the antenna maintains consistent gain, pattern, and impedance across a wide range of frequencies. The primary advantage of a log-periodic antenna is its wide bandwidth — it can cover an entire HF band or multiple bands without retuning.

Compared to a Yagi optimized for a single frequency, the log-periodic has lower gain per element — the broadband design trades peak gain for frequency coverage. The element length and spacing are both scaled logarithmically from front to back along the boom.

Stacking Yagi Antennas Vertically

When horizontally polarized Yagi antennas are stacked vertically (one above the other), the advantage is that stacking narrows the main lobe in elevation. The horizontal beamwidth remains the same as a single antenna, but the elevation pattern becomes more concentrated toward the horizon, improving long-distance (DX) performance by reducing radiation at high elevation angles where it would be wasted on NVIS-type propagation.

Screwdriver Mobile, Beverage, and Small Loop

Screwdriver Mobile Antenna

A screwdriver mobile antenna is a base-loaded HF vertical antenna for vehicle installation. Its distinguishing feature is a motor-driven coil at the base that allows remote adjustment of the loading inductance while in motion. The antenna adjusts its feed point impedance by varying the base loading inductance — changing the coil's number of active turns changes the resonant frequency and the feed impedance, allowing quick band changes from inside the vehicle.

Beverage Antenna

A Beverage antenna is a long-wire receiving antenna, typically one to several wavelengths long, installed low over the ground and terminated at the far end in a resistive load. Its primary use is directional receiving for MF and low HF bands. The Beverage is highly effective at rejecting signals from unwanted directions — its bidirectional null pattern can significantly reduce interference and noise. It is almost exclusively a receive antenna; its very low radiation resistance makes it impractical for transmitting.

Electrically Small Loop

An electrically small loop antenna (circumference less than 1/10 wavelength) has a distinctive figure-eight radiation (and reception) pattern. The nulls occur broadside to the loop — perpendicular to the plane of the loop, pointing outward from the face of the loop. Maximum radiation occurs in the plane of the loop (off the edges). This sharp null pattern makes small loops valuable for direction-finding applications — the null can be rotated to determine the bearing to a station by finding the angle at which the signal drops to minimum.

Inverted V

An inverted V is the common name for a dipole antenna with a single central support. Both halves of the dipole droop downward from the central support point, forming a shape like an inverted letter V. Advantages of the inverted V include requiring only one tall support (a mast or tree) instead of two, and a somewhat more omnidirectional azimuthal pattern compared to a flat-top dipole. The drooping elements also reduce the feed point impedance slightly below the 73 ohms of a flat dipole.

G9D Practice Questions