The center-fed half-wave dipole is the foundational reference standard in radio frequency communications (referenced as $0\text{ dBd} = 2.15\text{ dBi}$). When cut to electrical resonance, the dipole exhibits a purely resistive input impedance of approximately 72 Ohms in free space (or 50 to 54 Ohms when configured as an inverted-Vee), providing an optimal match to common 50-Ohm coaxial transmission lines without requiring lossy matching networks.
The Standard 468/f Antenna Formula
Imperial: Total Length (Feet) = 468 / Frequency (MHz)
Metric: Total Length (Meters) = 142.65 / Frequency (MHz)
Each 1/4λ Leg Length = Total Length / 2
The theoretical free-space half-wavelength is $492 / f$. However, radio waves travel slightly slower along insulated copper wires due to conductor capacitance and "end effects" where charges accumulate at the wire extremities. Applying the empirical 0.95 correction factor ($492 \times 0.95 = 467.4 \approx 468$) ensures true electrical resonance.
Coaxial Transmission Line Velocity Factor
Inside a coaxial cable, radio waves travel slower than in air because the dielectric insulator (polyethylene, PTFE Teflon, or gas-injected foam) has a higher refractive index. This propagation velocity ratio is called the Velocity Factor (VF). When building coaxial impedance transformers, 1/4-wave tuning stubs, or phasing lines, you must multiply the free-space wavelength by the coax VF:
Should I cut the wire slightly longer than calculated?
Yes! Always cut antenna wire 2 to 4 inches (5 to 10 cm) longer than calculated. It is easy to trim or fold back wire through end insulators to tune for minimum SWR with an antenna analyzer or NanoVNA, but impossible to stretch a wire that was cut too short.
Do I need a 1:1 choke balun for a dipole?
Yes. A dipole is a balanced load, whereas coaxial cable is an unbalanced feedline. Connecting coax directly to a dipole induces "common-mode RF currents" on the outer coax shield, causing high noise floor on receive and RF in the shack on transmit. An air-core choke (5 to 8 turns of coax around a 4-inch PVC pipe) or a ferrite toroid choke prevents this.
What is the best wire gauge (AWG) for a wire dipole?
14 AWG stranded copper wire (either insulated THHN or bare copperweld) provides the best balance of low RF resistance, mechanical tensile strength to resist ice and wind loading, and wide SWR bandwidth.