Antennas: Principles, Types, and Patterns

What an Antenna Does

An antenna is a transducer that converts electrical energy traveling along a transmission line into electromagnetic waves radiated into space, and vice versa. Every antenna is reciprocal: the same element that transmits a signal will receive it with identical directional properties. Understanding how antennas work—and how to read their radiation patterns—is essential for selecting and installing RF systems that perform reliably.

Fundamental Antenna Principles

Resonance and length. A half-wave dipole resonates when its total length equals one half of the operating wavelength. The free-space formula is:

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Length (meters) = 150 / f(MHz)

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A practical correction factor of about 0.95 accounts for conductor diameter and end effects, giving the commonly used formula:

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Length (meters) = 142.5 / f(MHz)

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At resonance, feedpoint impedance is approximately 73 Ω in free space (often treated as 50 Ω for practical matching).

Gain and directivity. Gain describes how much an antenna concentrates radiated power in a preferred direction compared to a reference. When referenced to an isotropic radiator (a theoretical point source that radiates equally in all directions), gain is expressed in dBi. When referenced to a half-wave dipole, it is expressed in dBd. The relationship is: dBi = dBd + 2.15.

Polarization. The orientation of the electric field vector determines polarization. A vertical element produces vertical polarization; a horizontal element produces horizontal polarization. Cross-polarization mismatch between transmit and receive antennas can cause significant signal loss.

Radiation Patterns and How to Read Them

A radiation pattern is a polar plot showing signal strength in all directions, normalized to the direction of maximum radiation. Manufacturers publish two cuts:

• E-plane: the plane containing the electric field and the direction of maximum radiation.
• H-plane: the plane containing the magnetic field, perpendicular to the E-plane.

Key features to identify on any polar plot:

• Main lobe: the region of maximum radiation.
• −3 dB beamwidth: the angular width of the main lobe between the two points where power drops to half (3 dB below the peak). This is the most common specification for describing how "wide" or "narrow" a beam is.
• Back lobe: radiation directly opposite the main lobe. The ratio of front to back is the front-to-back ratio, expressed in dB.
• Nulls: directions of minimal or zero radiation. Nulls are sometimes used deliberately to reject interference from a specific direction.

Five Antenna Types

Half-wave dipole. Two quarter-wave elements fed at the center. The radiation pattern is a figure-eight (donut shape in 3-D), with nulls off the ends and maximum radiation broadside to the wire. Gain is approximately 2.15 dBi. Widely used as a reference and as a practical antenna for HF, VHF, and UHF work.

Yagi-Uda (Yagi). A dipole driven element with a reflector behind it and one or more directors in front. Each added director narrows the main lobe and increases forward gain. A typical 3-element Yagi achieves roughly 7–8 dBd. Used for point-to-point links, television reception, and amateur radio.

Parabolic dish. A large curved reflector focuses energy toward or from a small feedhorn at the focal point. Very high gain (often 20–40+ dBi) and very narrow beamwidth make dishes ideal for long-distance point-to-point microwave links and satellite communications. Precise alignment is critical.

Omnidirectional vertical. A vertical element (often a quarter-wave ground plane or collinear array) that radiates equally in all horizontal directions, producing a toroidal (donut) pattern in 3-D. Collinear designs compress the pattern toward the horizon, increasing gain. Common in cellular base stations, repeaters, and Wi-Fi access points where 360° horizontal coverage is needed.

Panel (sector) antenna. A flat array mounted on a back reflector. Typical horizontal beamwidth is 60°–120°, providing sector coverage with moderate-to-high gain. Used in cellular base stations (three 120° panels cover 360°), and in point-to-multipoint wireless networks.

Selecting the Right Antenna

Match the antenna's pattern to the coverage requirement:

| Scenario | Key need | Best choice |

|---|---|---|

| Point-to-point microwave link | Maximum gain, narrow beam | Parabolic dish |

| Cellular sector coverage | Fixed sector, moderate gain | Panel/sector antenna |

| Omnidirectional repeater | 360° horizontal | Vertical collinear |

| Home Wi-Fi access point | Low-cost, near-omnidirectional | Small vertical or dipole |

| GPS receive | Circular polarization, wide sky view | Patch (microstrip) antenna |

Building the 2-Meter Dipole — Safety First

When constructing and testing antennas:

• Soldering: Work in a ventilated area. Wear safety glasses. Use a suitable stand—never hold the connector with bare fingers while soldering.
• Antenna analyzer / SWR meter: These instruments use very low RF power for testing; follow the manufacturer's guidelines and keep the antenna away from your body during the sweep.
• Working at height: If antennas are raised for testing, follow fall-protection and ladder-safety practices. Never raise an antenna near power lines—maintain safe clearance distances per OSHA and local regulations.
• RF exposure: Even low-power test transmitters can exceed safe exposure limits at very close range. Keep distance from the feedpoint during transmission.

Connecting It All

Antenna performance is only as good as the entire system: antenna → feedline → connectors → radio. SWR (Standing Wave Ratio) quantifies impedance mismatch. An SWR of 1:1 is perfect; 2:1 means about 11% of power is reflected. A well-built, resonant dipole should show SWR below 2:1 across its intended band. The antenna analyzer sweeps frequency and displays SWR or return loss automatically, making it straightforward to find the true resonant frequency and compare it to your calculated target.