Free Half-Wave Dipole & Quarter-Wave Antenna Calculator
Calculate precision element wire lengths, end-effect velocity shortening (K-factor), and feedpoint impedance for HF, VHF, and UHF antennas.
📡 Frequency & Antenna Configuration
📊 Resonant Wire Cuts & Impedance
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The Physics of Half-Wave Resonant Dipole Antennas
The center-fed half-wave dipole is the benchmark standard for all radio frequency antenna design. In free space, radio waves travel at the speed of light ($c = 299,792,458$ m/s). However, when radio frequency currents flow along a real metal conductor, electromagnetic energy slows down slightly due to finite wire diameter, capacitive end-effects, and surrounding insulation.
1. The Traditional 468 / f Formula vs True K-Factor
For over 80 years, amateur radio operators have relied on the classic imperial rule of thumb:
The magic number 468 incorporates an assumed 5% end-effect velocity factor ($492 imes 0.95 approx 468$). However:
- Insulated Wire Shortening: Using standard PVC-jacketed THHN copper wire slows velocity down by an additional 2% to 4%, lowering the constant to ~455 / f.
- Thick Tubing / VHF / UHF: As conductor diameter increases relative to wavelength, the length-to-diameter ratio ($L/d$) drops, requiring a smaller K-factor.
2. Feedpoint Impedance Matching
- Horizontal Dipole (λ/2 above ground): Radiation resistance is approximately 73 Ω. Connected directly to 50 Ω coax, it yields a safe 1.46:1 SWR with negligible loss.
- Inverted-Vee (100°-120° Apex): Dropping the dipole legs toward ground lowers radiation resistance from 73 Ω directly down to 50 Ω, creating an absolute 1.0:1 perfect match for 50-ohm coax.
- Quarter-Wave Vertical Whip: Exhibits a feedpoint impedance of ~36 Ω. Sloping the ground plane radials downward at 45 degrees raises impedance up to 50 Ω.
3. Why a 1:1 Current Balun Is Mandatory
A dipole is an inherently balanced antenna (equal and opposite RF currents on each leg), whereas coaxial cable is an unbalanced transmission line (center conductor shielded inside ground braid). Connecting coax directly to a dipole without a 1:1 current choke balun forces RF currents to flow down the outside of the coax shield, causing high RF in the shack, microphone bite, and distorted radiation patterns.
Frequently Asked Questions
How high off the ground should my dipole antenna be mounted?
For optimum horizontal radiation and low take-off angle DX, mount the dipole at least 1/2 wavelength above ground (approx 35 feet on 20m, or 65 feet on 40m). Lower heights increase ground losses and create a cloud-warmer NVIS (Near Vertical Incidence Skywave) pattern, which is great for regional contacts under 400 miles.
Can an Inverted-V antenna be hung from a single center tree or mast?
Yes! That is the greatest mechanical advantage of the Inverted-V. You only need one elevated support in the middle. The two ends slope down and are tied off to ground stakes or fence posts via Dacron rope and end insulators.
What should the apex angle be on an Inverted-V dipole?
The included angle between the two sloping legs should never be less than 90 degrees (100 to 120 degrees is ideal). Closing the angle below 90 degrees causes the out-of-phase electromagnetic fields from the two legs to cancel each other out, drastically reducing antenna radiation efficiency.
How do I tune the antenna if the SWR dip is too low in frequency?
If the lowest SWR occurs at a lower frequency than desired, the antenna is physically too long. Shorten both legs equally by folding 2 to 3 inches of wire back upon itself through the end insulator and wrapping it with tape. Never cut wire off until final on-the-air testing is complete.