E9C: Practical Wire Antennas

Wire antennas remain among the most practical choices for HF operation. By varying length, feed point location, the presence of a terminating resistor, and the phasing between multiple elements, dramatically different radiation patterns can be achieved with simple wire.

This lesson covers phased vertical arrays and their radiation patterns, folded dipoles, long-wire and rhombic antennas, off-center-fed dipoles, the G5RV, Zepp and extended double Zepp antennas, and how ground type and antenna height affect radiation patterns.

Phased Vertical Arrays

Two quarter-wave vertical antennas can be combined with different spacings and phase relationships to create specific radiation patterns:

Long-Wire and Rhombic Antennas

An unterminated long-wire antenna produces a bidirectional pattern. As wire length increases, additional lobes form and the major lobes become increasingly aligned with the axis of the antenna (end-fire direction), making longer wires more directional along their length.

Adding a terminating resistor to a rhombic or long-wire antenna absorbs the backward-traveling wave, changing the radiation pattern from bidirectional to unidirectional. This provides a cardioid-like pattern with a usable front lobe and suppressed rear.

Off-Center-Fed Dipole (OCFD)

An off-center-fed dipole (OCFD) is fed at a point between the center and one end rather than at the exact midpoint. The purpose is to create a similar feed point impedance on multiple amateur bands, making it easier to operate on several HF bands with a single antenna and tuner. Because the feed point is off-center, the impedance is higher than a standard center-fed dipole but tends to remain in a workable range across multiple harmonically related bands.

Folded Dipole

A folded dipole is a half-wave dipole with an additional parallel wire connecting its two ends — effectively two parallel conductors forming a closed loop with a gap at the feed point in one conductor. The feed point impedance of a standard two-wire folded dipole is approximately 300 ohms — four times the impedance of a simple half-wave dipole (75 Ω). This higher impedance matches directly to 300-ohm twin-lead transmission line.

G5RV Antenna

The G5RV is a wire antenna that is center-fed through a specific length of open-wire transmission line (typically 34 feet of 450-ohm ladder line), which acts as an impedance transformer. This open-wire section connects to a balun and then to coaxial feed line running to the transmitter. The G5RV is a popular multiband wire antenna, though it typically requires an antenna tuner for best performance across all bands.

Zepp and Extended Double Zepp

A Zepp antenna is an end-fed half-wavelength dipole. The name comes from the Zeppelin airship, which trailed a wire antenna from one end. End-feeding a half-wave element produces a high-impedance feed point requiring an appropriate matching network.

An extended double Zepp (EDZ) is a center-fed 1.25-wavelength dipole. By making each leg 5/8 wavelength long (total 1.25λ), the antenna achieves approximately 3 dB gain over a half-wave dipole in the broadside direction, making it a useful gain antenna for fixed-direction operation.

Ground and Height Effects on Patterns

The ground beneath an antenna significantly affects the elevation radiation pattern:

• Vertical antenna over seawater vs. soil: A vertically polarized antenna mounted over highly conductive seawater produces more radiation at low angles than the same antenna over average soil. Seawater's high conductivity creates a near-perfect ground plane, enhancing low-angle reflection and strengthening the low-angle lobe.
• Horizontal antenna height: As the height of a horizontally polarized antenna above ground increases, the takeoff angle of the lowest elevation lobe decreases — lower angles favor DX propagation. Higher antennas favor lower radiation angles.
• Antenna over a downhill slope: A horizontally polarized antenna mounted over ground that slopes downward in one direction produces a lower main lobe takeoff angle in the downhill direction compared to flat ground. The slope effectively raises the antenna's electrical height in the downhill direction.

E9C Practice Questions