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