What is the difference between dBi and dBd antenna gain?

dBi is decibels relative to a theoretical isotropic radiator that radiates equally in all directions, while dBd is decibels relative to a half-wave dipole. The conversion is G(dBi) = G(dBd) + 2.15, since a half-wave dipole itself has a directivity of 1.64, equivalent to 2.15 dBi.

What VSWR is acceptable for an antenna feedline?

Practical antennas should exhibit VSWR under 1.5:1 across their operating band, corresponding to a return loss greater than 14 dB. At VSWR = 2:1 (return loss 9.5 dB), approximately 11% of forward power is reflected back — manageable in most systems but significant on high-power transmit paths where reflected power can damage amplifiers.

What is the maximum acceptable PIM level for RF passive components?

3GPP specifies −153 dBc as the maximum acceptable passive intermodulation (PIM) level, measured at +43 dBm per carrier in a two-tone test. PIM is generated by nonlinear passive components such as loose or oxidised N-connectors, ferrous materials in antenna radiators, and damaged cables, and testing per IEC 62037 should be performed on all DAS antenna ports during commissioning, particularly where 4G/5G receive and transmit bands are closely spaced.

What antenna gain and beamwidth is typical for a macro-cell panel antenna?

A directional panel antenna typically provides 12–18 dBi gain with a beamwidth of 60–90° horizontal by 6–12° vertical, tilted for coverage shaping — a common example is 65° horizontal beamwidth at 17 dBi gain for a 3-sector macro-cell base station. Cross-polarised (±45°) variants of these panel antennas support polarisation diversity and 2×2 MIMO.

What antenna spacing is needed for MIMO to provide a capacity gain?

For MIMO to provide a true capacity gain rather than just diversity gain, the spatial correlation between paths must be low, which requires antenna spacing of at least 0.5λ or cross-polarisation separation of at least 20 dB. In-building DAS typically supports 2×2 MIMO at best because implementing it requires two independent antenna ports and two independent coax/fiber runs to each antenna location, roughly doubling infrastructure cost.

Engineering·10 min read·June 17, 2026

📡 Antenna Fundamentals and Selection for RF Engineers

A rigorous treatment of antenna theory: gain, directivity, radiation patterns, VSWR, polarisation, PIM, and MIMO — with practical guidance on selecting between omnidirectional, Yagi, panel, and parabolic dish antennas.

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The Isotropic Radiator and dBi

An isotropic radiator is a theoretical point source that radiates equally in all directions, producing a perfectly spherical radiation pattern. It is the universal reference for antenna gain. Antenna gain in dBi (decibels relative to isotropic) quantifies how much more power an antenna radiates in its direction of maximum radiation compared to an isotropic radiator transmitting the same total power. A 6 dBi gain antenna focuses four times as much power in its peak direction as an isotropic source, while radiating the same total power — gain comes from redistribution of energy, not amplification. The alternate reference, dBd (decibels relative to a half-wave dipole), is used in some land-mobile specifications. The conversion is simple: G(dBi) = G(dBd) + 2.15.

Topics covered

antenna gaindBidBdVSWRreturn lossradiation patternomnidirectional antennaYagi antennaparabolic dishMIMO antennapassive intermodulationPIMantenna polarisationhalf-wave dipole