Dipole Antenna

Part of Radio

The dipole is the fundamental antenna — two equal conductors fed at the center — and the reference against which all other antennas are measured.

Why This Matters

Every antenna theory begins with the dipole. It is the simplest resonant antenna, the easiest to build, and it works remarkably well. Two equal lengths of wire totaling a half-wavelength, fed at the center, radiate and receive efficiently at the design frequency. No ground plane required, no complex matching network needed for common feedlines — just wire, a center connector, and a feedline.

For a rebuilding civilization, the dipole antenna is the entry point to effective radio communication. A half-wave dipole for the 40-meter amateur band (7 MHz) is 20 meters of wire cut in two — materials that are simple to obtain. Hung between two trees or poles, it becomes a capable antenna for both transmitting and receiving over hundreds of kilometers. Understanding the dipole is understanding all antennas that follow.

The dipole also serves as a calibration reference. Antenna gain is measured in dBd (decibels relative to a dipole) or dBi (decibels relative to an isotropic theoretical point source — a dipole is 2.15 dBi). When someone says an antenna has 6 dBd gain, they mean it concentrates energy 4 times more than a dipole in its favored direction. All antenna performance begins with the dipole as the baseline.

Dipole Dimensions and Calculation

A half-wave dipole resonates when its total length equals approximately half the wavelength of the target frequency. The relationship between frequency and wavelength: λ (meters) = 300 / f (MHz). For a 7 MHz signal, λ = 300/7 ≈ 42.9 meters. Half-wave = 21.4 meters total, each arm 10.7 meters.

In practice, the actual resonant length is shorter than the theoretical half-wavelength due to the “velocity factor” of the wire and end effects. For a bare copper wire dipole in free space, the resonant length is approximately 95% of the theoretical half-wave. A practical formula: total length in meters = 142.5 / f(MHz). Each arm = 71.25 / f(MHz).

Examples:

• 3.5 MHz (80m band): total 40.7 m, each arm 20.3 m
• 7 MHz (40m band): total 20.4 m, each arm 10.2 m
• 14 MHz (20m band): total 10.2 m, each arm 5.1 m
• 28 MHz (10m band): total 5.1 m, each arm 2.5 m

For broadcast reception (AM band, 0.5–1.7 MHz), a half-wave dipole would be 90–300 meters long — impractical. For AM reception, a simple long wire works better. Dipoles shine for amateur radio bands from 3 MHz upward.

These formulas give a starting point. The actual resonant frequency depends on height above ground, nearby conductors, and wire diameter. Always cut elements 3–5% longer than calculated and trim to resonance after measuring.

Building a Simple Wire Dipole

Materials needed: insulated wire (14–18 AWG copper, or any available conductor), a center insulator with connector, two end insulators, rope for support, and feedline (coaxial cable or twisted pair).

The center insulator is the mechanical and electrical heart of the antenna. It must hold both wire elements, support the feedline connection, and bear the mechanical load. A simple version: two stainless steel bolts through a block of polyethylene (HDPE cutting board material) or hardwood treated with weatherproofing. One bolt connects to each element; the feedline inner conductor and shield connect to opposite bolts.

Cut both wire elements 5% longer than calculated. Strip 2 cm of insulation from each end. At the center insulator, attach the inner conductor of the coax to one element and the coax shield to the other. Mechanical security matters — solder the connections, then wrap with self-amalgamating silicone tape for weather protection.

At the far end of each element, attach an end insulator: a short piece of plastic rod or even a dried hardwood dowel sealed with varnish. The support rope ties to the insulator, not the wire, preventing mechanical stress from changing the element length.

Hang the antenna as high as possible. The minimum useful height is λ/4 (one quarter wavelength) above ground. Higher is better — at one wavelength height, the radiation pattern shifts from a high angle (good for nearby contacts) toward a lower angle (better for long-distance contacts).

Feedline and Impedance Matching

At resonance, the feed-point impedance of a half-wave dipole in free space is approximately 73 ohms (resistive). Standard 50-ohm coaxial cable (RG-58, RG-8, or equivalents) presents a moderate mismatch — the SWR is about 1.5:1, which is acceptable for most transmitters and causes only modest power loss.

For receiving only, impedance mismatch is largely irrelevant — signal levels are far below the threshold where mismatch losses matter. Use whatever feedline you have: coax, twin-lead, twisted pair, or even a single wire with a proper balun.

A balun (balanced-to-unbalanced transformer) at the feedpoint is recommended when using coaxial cable. The dipole is a balanced antenna (both elements float symmetrically), while coax is unbalanced (one conductor is grounded). Without a balun, RF current flows on the outer shield of the coax, which radiates and receives, distorting the antenna pattern. A simple 1:1 current balun is a choke of 8–10 turns of coaxial cable wound on a ferrite toroid — the choke presents high impedance to common-mode currents without affecting differential (antenna) currents.

Open-wire feedline (two parallel conductors 15 cm apart, connected at the center insulator) is the traditional alternative. Its characteristic impedance is approximately 300–600 ohms depending on wire spacing and diameter. A 4:1 or 9:1 transformer matches it to 50-ohm coax or directly to the tuned input of a transmitter. Open-wire line has much lower loss than coax at high frequencies and allows the dipole to be used on multiple bands with an antenna tuner.

Variations and Enhancements

The folded dipole replaces the simple wire with a closed loop at the same total length, connecting to the feedline at one point only. Its feed impedance is approximately 300 ohms — a good match for 300-ohm twin-lead. It is more broadband (works over a wider range without retuning) and has better Q than a simple dipole. Easy to construct from 300-ohm TV twin-lead (folded back on itself and twisted at the end).

The fan dipole connects multiple dipole elements — each cut for a different frequency — to the same feedpoint. The element nearest resonance at any given frequency dominates. This allows one feedline to serve multiple bands. Elements at different frequencies are spread apart slightly to avoid coupling; 15–20 degrees separation is sufficient.

The inverted-V is a practical compromise: the center is elevated on a single support, and both arms slope downward at 30–45 degrees. This requires only one high support point instead of two, and the slight shortening of effective height versus a horizontal dipole is usually acceptable. The radiation pattern becomes slightly omnidirectional, losing the figure-eight broadside pattern of a horizontal dipole.

For directional operation, a dipole with a reflector element (5% longer, λ/4 behind) becomes a 2-element Yagi with approximately 5 dBd gain in the forward direction. Add a director (5% shorter, λ/4 in front) and you have the basic 3-element Yagi that served broadcast television antennas worldwide. Understanding the dipole is the foundation of all these designs.