RF / Antenna Calculator

Design antennas for any frequency. Calculate element lengths for dipoles, Yagi arrays, and small loops. Includes wavelength converter, VSWR calculator, and coax reference.

Half-Wave Dipole Antenna

Each arm is ¼λ. The 468/f constant already includes the ~5% end effect (492/f is the free-space length). It is an empirical starting point for thin bare wire at typical HF heights; real shortening runs about 2–6%, so cut long and trim.

L(m) = 142.5 × VF / f(MHz) L(ft) = 468 × VF / f(MHz)

Frequency

Velocity Factor

0 – 1

Tip: Cut each arm 2% longer than calculated, then trim for minimum SWR. 468/f already allows for the end effect, so bare wire uses a factor of 1.00 — don't apply a velocity factor on top. Insulation slows the wave a little: for insulated wire use about 0.95–0.98 instead.

Quarter-Wave Vertical (Ground Plane)

Vertical radiator ¼λ long over a radial ground plane. Feed impedance depends on radial count and angle.

L(m) = 71.25 × VF / f(MHz) L(ft) = 234 × VF / f(MHz)

Frequency

Velocity Factor

0 – 1

Number of Radials

Tip: Radials should be at least ¼λ long. Elevated horizontal radials give ~36Ω feed impedance; drooping them 30–45° raises it to ~50Ω, convenient for direct coax feed. A ground-mounted vertical shows 36Ω plus ground loss, which falls as you add radials. 468/f-style constants (234/f here) already include the end effect, so leave VF at 1.00 for bare wire or tubing.

Yagi-Uda Antenna Element Lengths

Driven element, longer reflector behind, and shorter directors in front. Each director adds ~2–3 dBd gain.

Driven ≈ 0.473λ Reflector ≈ 0.495λ Director ≈ 0.440λ (decreasing)

Frequency

Number of Elements

Tip: Boom-to-element mounting lowers effective element length by ~2%. All lengths shown are half-element (one side); full element = ×2. Use aluminum tubing for HF/VHF elements.

Small Transmitting Loop

Electrically small loop (<0.1λ circumference). Requires a series-tuned variable capacitor for resonance. High Q, narrow bandwidth.

L = μ₀ × r × (ln(8r/a) − 2) C = 1 / (ω²L)

Frequency

Loop Diameter

Conductor Diameter

Tip: Use the largest diameter copper or aluminium tubing practical. 15–25mm tubing dramatically reduces resistive losses and raises Q. The tuning capacitor must handle high voltages (hundreds to thousands of volts at QRP power).

Wavelength ↔ Frequency Converter

Enter frequency to get wavelength, or wavelength to get frequency. Velocity factor adjusts for media other than free space.

λ = (c × VF) / f c = 299,792,458 m/s

Frequency

Wavelength

Velocity Factor

0 – 1

Common Amateur & ISM Band Wavelengths

Band	Frequency	λ	Dipole arm (¼λ)	Use
160m	1.8–2.0 MHz	161 m	40.2 m / 132 ft	Amateur radio, AM broadcast
80m	3.5–4.0 MHz	80 m	20.1 m / 66 ft	Amateur radio
40m	7.0–7.3 MHz	42 m	10.3 m / 33.6 ft	Amateur radio
20m	14.0–14.35 MHz	21 m	5.1 m / 16.6 ft	Amateur DX
10m	28.0–29.7 MHz	10.5 m	2.6 m / 8.3 ft	Amateur radio
2m	144–148 MHz	2.06 m	51 cm / 20 in	Amateur VHF, public safety
70cm	420–450 MHz	68 cm	17 cm / 6.6 in	Amateur UHF, PMR446
Wi-Fi 2.4 GHz	2400–2484 MHz	12.5 cm	3.1 cm / 1.2 in	WLAN, Bluetooth, Zigbee
Wi-Fi 5 GHz	5150–5850 MHz	5.8 cm	1.45 cm / 0.57 in	WLAN 802.11ac/ax
GPS L1	1575.42 MHz	19.0 cm	4.75 cm / 1.87 in	GPS / GNSS

VSWR & Reflection Coefficient Calculator

Calculate impedance mismatch between feedline and antenna. Lower VSWR = better match = more power delivered.

VSWR = (1 + |Γ|) / (1 − |Γ|) Γ = (Zₗ − Z₀) / (Zₗ + Z₀)

Line Impedance Z₀

Load Impedance Zₗ

Common Coax Cable Specifications

Type	Z₀ (Ω)	VF	dB/100 ft @ 100 MHz	Outer dia.	Application
RG-174	50	0.66	8.9	2.8 mm	Miniature, short patch leads
RG-58	50	0.66	4.9	5.0 mm	General HF/VHF, thin & flexible
RG-8X	50	0.82	3.6	7.6 mm	Low-loss portable / mobile
RG-213 / RG-8	50	0.66	2.2	10.3 mm	Base station runs, high power
LMR-400	50	0.85	1.2	10.3 mm	Low-loss base station runs
LMR-600	50	0.87	0.8	15.0 mm	Very low-loss, semi-rigid
RG-59	75	0.66	3.4	6.1 mm	Video, CCTV, legacy CATV
RG-6	75	0.82	2.0	6.9 mm	Cable TV, satellite, DOCSIS

Sources: typical losses from the W2VA attenuation table (RG-174, RG-58, RG-8X, RG-213, RG-6), Belden 8241 datasheet (RG-59), Times Microwave datasheets (LMR-400, LMR-600). Exact figures vary by brand and construction; check the datasheet for the cable you buy.

VSWR guide: <1.5:1 = Excellent (use it); 1.5–2:1 = Good; 2–3:1 = Marginal (check connections); >3:1 = Poor (antenna or feedline problem). Most modern radios tolerate up to 2:1 without auto-fold-back.

Folded Dipole

Same overall length as a standard dipole, but the looped conductor raises the feed impedance to ~300 Ω — a near-perfect match for 300 Ω twin-lead or a 4:1 balun to 75 Ω. Wider bandwidth than a plain dipole.

Length(m) = 142.5 × VF / f(MHz) Z ≈ 300 Ω

Frequency

Velocity Factor

0 – 1

End-Fed Half-Wave (EFHW)

A half-wavelength wire fed at one end through a 49:1 (or 64:1) impedance transformer. Very popular for portable/POTA work — one wire, no center support, multiband on harmonics.

Length(m) = 142.5 × VF / f(MHz) Z_end ≈ 2000–3000 Ω

Frequency

Velocity Factor

0 – 1

Quarter-Wave Vertical with Radials

A vertical radiator over a radial ground system. Each radial should be at least ¼λ. More radials lower ground loss and bring the feed impedance toward the ideal.

Radiator(m) = 71.25 × VF / f(MHz) Radial ≥ radiator length

Frequency

Velocity Factor

0 – 1

Number of Radials

G5RV Multiband Dipole

A classic 102 ft (31.1 m) flat-top with a matching section of ladder line, designed to work across multiple HF bands with a tuner. On 20 m the flat-top is about 1.5λ (three half-waves).

Full G5RV flat-top = 102 ft · Half G5RV = 51 ft

Version

Feeding wire antennas: a half-wave dipole presents ~73 Ω (use 50–75 Ω coax + 1:1 balun); a folded dipole ~300 Ω (4:1 balun); an end-fed half-wave ~2000–3000 Ω (49:1 transformer). Matching the feed to the antenna is what keeps your VSWR low.

Gain: dBi vs dBd

Antenna gain is measured two ways, and confusing them is a classic mistake.

dBi — gain relative to an isotropic radiator (a theoretical point that radiates equally in all directions). dBd — gain relative to a half-wave dipole. Because a dipole already has 2.15 dB of gain over isotropic, the two scales differ by a fixed amount:

dBi = dBd + 2.15 dBd = dBi − 2.15

Convert

value

From

Key Antenna Concepts

Term	What it means
Radiation pattern	The 3-D shape of how an antenna radiates. Shown as polar plots (azimuth = top view, elevation = side view).
Isotropic	A theoretical antenna radiating equally in every direction — the 0 dBi reference. Doesn't physically exist.
Front-to-back ratio	How much stronger a directional antenna (like a Yagi) radiates forward vs backward, in dB. Higher = better rejection of signals behind it.
Beamwidth	The angular width of the main lobe between half-power (−3 dB) points. Narrower beamwidth = more focused = more gain.
Takeoff angle	The elevation angle of strongest radiation. Low angles (DX) favor long distance; high angles (NVIS) favor regional.
Polarization	Orientation of the wave's electric field. Horizontal, vertical, or circular. TX and RX antennas should match polarization.
Balun	BALanced-to-UNbalanced transformer. Connects balanced antennas (dipole) to unbalanced coax, and can transform impedance (1:1, 4:1, 9:1, 49:1).
Velocity factor	How much slower a wave travels in a conductor or cable vs free space (0.6–0.97). Shortens the physical length needed for resonance.

Amateur Radio Band Plan (HF–UHF)

The most-used amateur bands and their frequencies. Wavelength sets antenna size — lower bands need much bigger antennas.

Band	Frequency	½λ dipole	Typical use
160 m	1.8–2.0 MHz	~78 m / 256 ft	Regional/DX night, "top band"
80 m	3.5–4.0 MHz	~40 m / 131 ft	Regional nets, night DX
40 m	7.0–7.3 MHz	~20 m / 66 ft	Day/night all-rounder
20 m	14.0–14.35 MHz	~10 m / 33 ft	Premier daytime DX band
17 m	18.07–18.17 MHz	~7.9 m / 26 ft	Quieter DX, WARC band
15 m	21.0–21.45 MHz	~6.7 m / 22 ft	Daytime DX, solar-peak favorite
10 m	28.0–29.7 MHz	~5 m / 16.5 ft	Sporadic-E, sunspot DX, FM
6 m	50–54 MHz	~2.8 m / 9.3 ft	"Magic band" — sporadic openings
2 m	144–148 MHz	~1 m / 39 in	VHF FM, repeaters, SSB
70 cm	420–450 MHz	~33 cm / 13 in	UHF FM, repeaters, satellites

Rule of thumb: halve the frequency and you double the antenna size. That's why 160 m antennas are huge and 70 cm antennas fit in your hand. Always verify your country's exact band edges and license privileges before transmitting.