What changed
Antenna theory — edited Aug 31, 2026 by Unknown editor
This is the first recorded version, so every line is shown as added. Note: Imported from content/wiki
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## Radiation resistance and efficiency
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An antenna converts power into radiation and heat. **Efficiency** is the fraction that radiates.
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Short loaded antennas — mobile whips, magnetic loops, attic compromises — have low radiation
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resistance, so losses in coils, connectors and ground dominate. This is why a 20 m mobile whip is
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perhaps 10% efficient and a full dipole is over 90%.
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## Gain
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Gain is not amplification; it is redistribution. dBi is referenced to a theoretical isotropic
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radiator, dBd to a dipole; **dBi = dBd + 2.15**. Advertised gain figures without a stated reference
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are marketing.
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## Radiation patterns and takeoff angle
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For HF DX you want energy at a low elevation angle, which mostly means **height in wavelengths**. A
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dipole at 35 feet is high on 10 m and low on 80 m, and behaves completely differently on each.
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## Polarization
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Horizontal or vertical; cross-polarized stations lose roughly 20 dB. Use vertical for FM and
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repeaters, horizontal for VHF/UHF SSB and most HF wire antennas. HF ionospheric paths scramble
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polarization anyway, so it matters far less above the ground wave.
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## Matching
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Feedpoint impedance depends on the antenna type and its height above ground: about 50–70 Ω for a
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half-wave dipole in free space, near 36 Ω for a ground-mounted quarter-wave vertical, thousands of
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ohms for an end-fed half wave. Matching networks — baluns, ununs, gamma matches, transformers —
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convert those to the 50 Ω the radio expects.
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## Baluns and common-mode current
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Feeding a balanced antenna with unbalanced coax lets current flow on the outside of the shield. The
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feedline then radiates, distorting the pattern and putting RF in the shack. A **common-mode choke**
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at the feed point fixes it and costs almost nothing.