Half-Wave Dipole (λ/2)
The half-wave dipole is the reference antenna whose total length is approximately half a wavelength and each arm of which is λ/4 long. Its gain relative to an isotropic antenna is 2.15 dBi, and that value is the basis of the conversion between dBi and dBd. At 433 MHz its total length is about 33 cm, at 868 MHz on the order of 16 cm.
Because this structure is resonant, it leaves largely no reactive component at the feed point and can be used without an additional matching network. This simplicity makes it a natural reference.
- Gain: 2.15 dBi, that is 0 dBd. An antenna listed as 5 dBd in a catalogue is 7.15 dBi.
- Length calculation: the theoretical half wavelength is 150 / f(MHz) metres; in a real conductor it shortens somewhat because of the velocity factor.
- Bandwidth is noticeably wider than that of shortened helical antennas; it is more tolerant of frequency shifts.
In the field the importance of this shows up when comparing antennas. A shortened helical antenna is physically far below λ/2 and sacrifices efficiency. A full-length wire or telescopic antenna at the same frequency is typically a few dB better; by the 6 dB rule, that means a difference that translates directly into range.
Context where this term is used: 433 MHz or 2.4 GHz?