A practical guide for amateur radio, IoT, cellular, Wi-Fi, and point-to-point systems

A portable radio station using different antenna patterns for different coverage goals

Technical Information | September 2026 | BOOBRIE

Start With the Coverage Problem, Not the Gain Number

Antenna listings make gain look like a simple ranking: if 3 dBi is good, 9 dBi must be three times better. Real radio systems do not work that way. Gain describes how strongly an antenna sends or receives energy in a particular direction compared with a reference. It says nothing by itself about whether that direction reaches the other station.

A short handheld antenna, a rooftop collinear, a Yagi, and a panel antenna solve different geometry problems. The best choice depends on where the other end of the link is, how much the equipment moves, the terrain, polarization, cable loss, supported frequencies, and legal EIRP limits. A lower-gain antenna aimed into the useful coverage region can outperform a higher-gain antenna whose main lobe passes above, below, or beside the target.

Key takeaway: Choose the required coverage shape first. Gain is one property of that shape, not a universal score for antenna quality.

What Antenna Gain Really Means

A passive antenna is not an amplifier. It cannot create RF power. Instead, its geometry redistributes radiation: more energy goes into some directions and less into others. The familiar flashlight analogy is useful. A reflector does not create more light; it concentrates the available light into a narrower beam. An antenna does the same thing in three dimensions, with main lobes, side lobes, and nulls.

Directivity describes this concentration without counting antenna losses. Gain includes the effect of radiation efficiency. Realized gain goes one step further by including mismatch at the antenna port. That distinction matters when comparing a well-matched, efficient antenna with a compact or broadband design that loses energy in conductors, loading coils, dielectric material, or an imperfect ground plane.

Figure 1. Higher gain reshapes the radiation pattern; it does not create additional RF power

Key takeaway: A high peak-gain value can coexist with narrow bandwidth, low efficiency away from one frequency, deep nulls, or poor coverage outside the main beam.

dBi, dBd, and the Number on the Label

Gain needs a named reference. dBi compares the antenna with an ideal isotropic radiator, a theoretical source that radiates equally in every direction. dBd compares it with an ideal half-wave dipole in free space. The same antenna therefore has a dBi number about 2.15 dB higher than its dBd number. A claim of 5 dBd and a claim of 7.15 dBi describe the same reference gain.

Also check whether the value is peak gain at one frequency, typical gain across a band, or realized gain in the final housing and ground-plane configuration. For multiband antennas, one headline number may hide large changes between bands. A trustworthy comparison needs the gain-versus-frequency data and radiation patterns, preferably in both azimuth and elevation.

Gain (dBi) = Gain (dBd) + 2.15 dB

Important: Do not compare dBi with dBd as if they were the same unit, and do not assume a peak value applies across the full operating band.

Why a High-Gain Omni Can Create Coverage Holes

An omnidirectional antenna is usually described as 360-degree coverage, but that statement refers mainly to the horizontal plane. In elevation, a vertical dipole produces a doughnut-shaped pattern. A higher-gain vertical collinear generally squeezes that doughnut into a flatter shape: more energy travels near the horizon, while less goes steeply upward or downward.

That flatter beam can be valuable on open, level ground when both antennas are at similar heights. It can be a poor fit on a hill, close to a tall tower, inside a multistory building, or on a moving vehicle that tilts and changes elevation. A user directly below a high rooftop antenna may sit in a weak part of the elevation pattern even though the antenna has an impressive gain rating.

Directional antennas make the tradeoff more obvious. A Yagi, panel, or dish concentrates energy in azimuth as well as elevation. This can extend a fixed point-to-point link and reject interference from other directions, but alignment becomes more critical and moving users can leave the beam.

Gain Is Only One Line in the Link Budget

The transmitter does not deliver its rated power directly into free space. Feed-line attenuation, connectors, adapters, lightning protection, splitters, and mismatch all subtract from the power that reaches the antenna. EIRP combines transmitter output, losses before the antenna, and antenna gain in the strongest direction. On receive, the same passive antenna pattern normally provides the corresponding directional sensitivity through reciprocity.

Suppose a transmitter produces 30 dBm, the cable and connectors lose 3 dB, and the antenna provides 6 dBi in the target direction. The result is 33 dBm EIRP, not 36 dBm. Replacing the cable with a longer, lossier run can erase much of the advertised antenna advantage. At higher frequencies, cable selection and length become especially important.

EIRP (dBm) = Transmitter power (dBm) - Feed-system loss (dB) + Antenna gain (dBi)

Key takeaway: Measure or estimate the complete RF path. Antenna gain cannot compensate for every cable, connector, polarization, obstruction, or alignment problem.

The Right Tradeoff Changes With the Application

Frequency alone does not select the antenna, but it changes wavelength, practical antenna size, propagation, cable loss, and regulatory limits. The matrix below is a starting point, not a substitute for the equipment manual or a site survey.

Figure 2. Useful antenna choices depend on the band, coverage geometry, mobility, and link objective

For HF, local and regional work may benefit from a horizontal antenna that intentionally produces a high takeoff angle, while long-distance DX usually favors lower-angle radiation and a clear horizon. In VHF/UHF handheld and mobile use, a broad elevation pattern can be more reliable over uneven terrain than a very long high-gain whip. A fixed base or repeater on level terrain may benefit from a collinear, but the vertical pattern still has to cover users below the site.

For 433, 868, and 915 MHz IoT nodes, compact omnidirectional antennas suit devices with changing orientation or unknown gateway direction. A high gateway can use a moderate-gain omni for area coverage, while a directional antenna is better for a fixed corridor or remote site. Regional channel plans and radiated-power limits must be checked. In cellular and Wi-Fi systems, preserve the required MIMO ports and polarization diversity; one high-gain antenna is not a substitute for a properly configured multichannel antenna system.

A Practical Antenna-Selection Workflow

Compare antennas under the same conditions and in the actual installation. A bench measurement can confirm impedance and cable loss, but it cannot reproduce the final radiation pattern created by a vehicle roof, equipment enclosure, mast, wall, balcony, or nearby metalwork.

Figure 3. Compare antennas in the intended installation, with the same cable path and test conditions

1. Define both ends of the link: Record their locations, heights, movement, orientation, polarization, and whether the path is one direction or many.
2. Map the required coverage: Decide whether you need 360-degree local coverage, a sector, a corridor, a high-angle regional path, or a narrow point-to-point beam.
3. Confirm the entire frequency range: Check every operating band, not just the center frequency or the largest gain value on the label.
4. Estimate the RF budget: Include transmitter power, feed-line loss, connector and adapter loss, receive sensitivity, fade margin, and antenna gain in the useful direction.
5. Check the pattern and polarization: Read both azimuth and elevation plots. Identify nulls, side lobes, beamwidth, front-to-back ratio, and required alignment.
6. Verify compatibility and compliance: Confirm impedance, connector type, power handling, weather sealing, MIMO port count, and regional EIRP or ERP rules.
7. Install, test, and document: Measure VSWR and loss, then verify the real link with RSSI, SNR, packet error rate, throughput, or on-air reports as appropriate.

Important: Change one variable at a time. Keep cable route, mounting height, radio settings, and test location consistent when comparing antennas.

A Fast Decision Tree for Common Installations

The decision tree below deliberately starts with geometry rather than dBi. It cannot cover every antenna, but it prevents the most common category error: choosing a narrow or flattened pattern for a link that needs broad angular coverage.

Figure 4. Choose the radiation pattern from the link geometry before choosing the gain number

After selecting the antenna family, choose only as much gain as the site can use. On a fixed link, greater directivity may provide useful margin. On a moving or multipoint system, excessive directivity often reduces reliability because the target does not remain inside the strongest part of the pattern.

Five Myths Worth Retiring

Practical validation checklist

•  Myth: 10 dBi means ten times the range. Reality: 10 dB is a tenfold power ratio in one direction, but usable distance also depends on path loss, antenna height, noise, obstructions, receiver performance, and fade margin
•  Myth: a longer whip must have more gain. Reality: physical length alone does not reveal loading, efficiency, bandwidth, pattern, or the role of the ground plane
•  Myth: low VSWR proves high gain. Reality: impedance match and radiation gain are different measurements; a dummy load can have excellent VSWR and no useful radiation
•  Myth: high gain improves every direction. Reality: improvement in one region of the pattern is paid for by reduced radiation elsewhere
•  Myth: the same antenna is ideal for every band. Reality: multiband systems need documented coverage, efficiency, pattern, and MIMO behavior across every required frequency range

  Final Field Checklist

  Practical validation checklist

•  State the band, bandwidth, and regional radio rules
•  Name the gain reference and confirm whether the value is peak, typical, or realized
•  Examine elevation as well as azimuth radiation patterns
•  Match the beam to the target's location, height, movement, and polarization
•  Subtract cable, connector, adapter, and protection-device losses
•  Preserve all required LTE, 5G, Wi-Fi, or other MIMO antenna ports
•  Test in the final installation and judge the complete link, not only the dBi label

The right antenna is not the one with the largest number. It is the one that places enough energy where the other radio actually is, rejects energy where it is not useful, survives the installation, covers every required frequency, and keeps the complete system within its RF and regulatory limits. Once gain is treated as part of a radiation pattern instead of a quality badge, antenna selection becomes much more predictable.