Also known as: antenna, aerial
An antenna is a conductor that converts electrical signals into radio waves and, on receive, converts passing radio waves back into a tiny current.1 It sets the ceiling on everything downstream — no receiver can recover a signal the antenna never captured. Because a metal structure that radiates efficiently also receives efficiently, the same piece of hardware serves both roles; this symmetry is the reciprocity principle, and it means a scanner’s receive antenna can be understood using the same transmit-side theory found in any antenna handbook.2
How it works
An antenna is a resonant structure: it works best when its physical dimensions are a specific fraction of the signal’s wavelength. A quarter-wave whip is λ/4 tall; a half-wave dipole is λ/2 end to end. At resonance the current and voltage along the conductor stand in a fixed relationship, the reactive part of the feedpoint impedance cancels out, and the antenna presents a nearly real resistance — around 73 Ω for a free-space half-wave dipole, near 36 Ω for a quarter-wave monopole over a ground plane. A feedline (typically 50 Ω coax) delivers energy to or from that feedpoint, and how closely the two impedances match is measured by SWR and return loss.
The energy an antenna radiates is not spread evenly in all directions. The angular map of that energy is the radiation pattern; its concentration relative to an isotropic reference is the antenna’s gain, and the angular width of its main lobe is the beamwidth. The orientation of the radiated electric field is the wave’s polarization, which is set by the antenna’s geometry and must be matched at both ends to avoid loss. Together, four properties characterize almost any antenna:
- Resonance and bandwidth — the frequency where it is well-matched, and how wide a range stays usable around it.
- Gain and pattern — how sharply it concentrates energy, and in which directions.
- Polarization — the field orientation it favours.
- Feedpoint impedance and the resulting match to the feedline.
Variants
Antennas span a wide family. Resonant wire types such as the dipole and monopole are the simplest. Directional arrays like the Yagi-Uda add parasitic elements to focus a beam and raise gain at the expense of coverage. Broadband designs (discone, log-periodic) trade peak efficiency for a wide usable range — attractive for scanning across many bands. Aperture antennas (horn, parabolic dish) dominate at microwave frequencies, where a physically small structure can be many wavelengths across. The right choice depends on the target band, the directions of interest, and whether the goal is all-round monitoring or reaching one distant site.
Relevance to SDR
For a software-defined radio receiver, the antenna is the single most cost-effective place to improve results. Choosing one cut for the target band, matching its polarization to the traffic (vertical for most land-mobile and trunked systems), and placing it high with a clear path usually improves SNR more than any setting inside the radio. GopherTrunk is purely a receive/decode chain — it has no transmit hardware and no beamforming array — so it treats the antenna as a fixed front end and works with whatever signal arrives at the ADC. Understanding antenna behaviour tells the operator why a strong nearby signal may still fail to decode (often multipath or a polarization mismatch) and why a modest antenna on a rooftop routinely beats a high-gain one indoors.
Sources
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Antenna (radio) — Wikipedia, for the definition, reciprocity, and key properties of antennas. ↩
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Recommendation ITU-R V.573: Radiocommunication vocabulary — International Telecommunication Union, for standardized definitions of antenna and radiation terms. ↩