Calculate physical dimensions for dipole, monopole, patch, and helical antennas at common drone frequencies.
Free-Space Wavelength
λ = c / f
Total Antenna Length
Half-wave dipole total length with velocity factor
Leg Length (each)
Each leg of the dipole
An antenna is a transducer that converts electrical energy into electromagnetic waves and vice versa. For maximum efficiency, the antenna must be resonant at the operating frequency — meaning its physical length is a specific fraction of the wavelength of the signal it transmits or receives.
λ = c / f
Where λ is the free-space wavelength in metres, c is the speed of light (299,792,458 m/s), and f is the frequency in Hz. At 2.4GHz, λ ≈ 125mm. At 433MHz, λ ≈ 692mm.
A resonant antenna presents a purely resistive impedance to the transmission line — no reactive (capacitive or inductive) component. For most drone RF systems, the target impedance is 50Ω, which is the standard characteristic impedance of coaxial cables and RF connectors used in the industry.
When an antenna is cut to the correct resonant length, it efficiently transfers energy between the RF circuitry and the electromagnetic field. An antenna that is off-resonance creates a reactive mismatch, causing reflected power (measurable as VSWR) that reduces radiated power and stresses the transmitter's power amplifier.
The half-wave dipole is the most common antenna in FPV and drone RC links. It consists of two equal-length radiating elements extending in opposite directions from the feedpoint, each cut to approximately λ/4. The total length with velocity factor applied is:
L_total = (λ / 2) × V_f
Where V_f is the velocity factor (≈ 0.95 for wire dipoles, accounting for end effects that slightly shorten the resonant length compared to the free-space calculation).
Dipoles have an omnidirectional radiation pattern in the plane perpendicular to their axis, with nulls at the tips. A vertically-mounted dipole on a drone radiates in a donut shape around the aircraft — ideal for keeping ground control and the FPV system connected throughout a flight. The gain of a dipole is 2.15dBi relative to an isotropic radiator.
Placement on VTX
Mount the video transmitter antenna pointing straight down from the bottom of the frame. This places the radiation lobe horizontal — toward your goggles. Keep at least 10mm clearance from carbon fibre.
Dipole vs whip
A true dipole has two radiating elements and a balanced feedpoint. Many "dipole" FPV antennas are actually unbalanced whips — monopoles using the coax outer conductor as the second element. Both work, but true dipoles have better pattern symmetry.
A quarter-wave monopole is a single radiating element one quarter-wavelength long, fed against a ground plane. The ground plane acts as a mirror, creating an image of the antenna and completing the equivalent of a half-wave dipole. The monopole length is:
L = (λ / 4) × V_f
Same velocity factor as the dipole applies. At 2.4GHz: L ≈ 29.7mm.
Monopoles are used extensively on flight controllers, RC receivers, and WiFi modules where the PCB itself serves as the ground plane. The SMA pigtail antennas included with most RC receivers are quarter-wave monopoles.
Without an adequate ground plane, monopole performance degrades significantly — the radiation pattern distorts and impedance shifts away from 50Ω. On a drone, mounting a receiver with a monopole on a large carbon plate provides a reasonable ground plane; mounting it on a plastic standoff in the open gives poor ground plane performance.
A microstrip patch antenna is a rectangular conductor etched onto a dielectric substrate above a ground plane. It resonates when its length equals approximately λ/(2√εr_eff), where εr_eff is the effective permittivity accounting for fringing fields at the edges. The dimensions are calculated using the standard microstrip patch design equations:
W = (c / 2f) × √(2 / (εr + 1))
εr_eff = (εr+1)/2 + (εr−1)/2 × (1 / √(1 + 12h/W))
L = c / (2f√εr_eff) − 2ΔL
Where h is the substrate thickness (fixed at 1.6mm for standard FR4), and ΔL is the end-effect correction. The substrate permittivity (εr) is the primary material parameter — FR4 ≈ 4.4, Rogers 4003 ≈ 3.55.
Patch antennas provide 6–12dBi of directional gain in a compact, flat form factor. DJI O3, DJI Goggles 2, and most long-range FPV goggles incorporate patch antennas pointing forward. The directional nature means they must be aimed at the aircraft — either manually or with an antenna tracker for long-range missions.
Substrate selection
FR4 (εr ≈ 4.4) is the standard PCB material — inexpensive and widely available. Rogers 4003 (εr ≈ 3.55) offers better performance and tighter tolerances. Higher εr materials shrink the patch dimensions but can increase losses.
Circular polarisation
Patch antennas can be made RHCP or LHCP by truncating two opposite corners or feeding at two points in quadrature. Circular polarisation improves multipath immunity significantly — important for long-range video where ground reflections are common.
An axial-mode helical antenna is a coil of wire wound around a cylindrical axis, designed so that the circumference equals approximately one wavelength. In this mode, the antenna radiates off the end of the helix (axially) with high gain and inherent circular polarisation:
C = λ (circumference equals one wavelength)
D = C / π (helix diameter)
pitch angle α ≈ 13°
S = C × tan(α) (turn pitch)
10 turns at 13° pitch angle is a practical starting design. More turns increase gain: G ≈ 11.8 + 10·log₁₀(N) dBi.
Helical antennas are used for long-range FPV video reception, satellite communication, and as high-gain receive antennas on ground stations. Their inherent circular polarisation (RHCP or LHCP depending on the winding direction) makes them compatible with circularly polarised video transmitter antennas and resistant to multipath fading.
The 10-turn design in this calculator produces approximately 21.8dBi of gain — enough to extend video range dramatically compared to a simple dipole. The trade-off is directionality: the 3dB beamwidth of a 10-turn helix is roughly ±35°, requiring the operator to keep the antenna pointed roughly at the aircraft.
VSWR (Voltage Standing Wave Ratio) quantifies how well an antenna is matched to the RF system it connects to. A perfect match produces a VSWR of 1:1 — all transmitted power radiates. Higher VSWR means some power reflects back toward the transmitter:
| VSWR | Return Loss | Power Reflected |
|---|---|---|
| 1.0 : 1 | ∞ dB | 0% |
| 1.5 : 1 | 14 dB | 4% |
| 2.0 : 1 | 9.5 dB | 11% |
| 3.0 : 1 | 6.0 dB | 25% |
| 5.0 : 1 | 3.5 dB | 44% |
For drone applications, target VSWR below 2:1. Antennas cut to the correct resonant length and mounted correctly will naturally achieve this. Common causes of high VSWR include incorrect length, proximity to metal or carbon fibre, damaged coax connectors, and operating significantly away from the design frequency.
Antenna polarisation describes the orientation of the electric field of the radiated wave. Mismatched polarisation between transmitter and receiver causes signal loss:
Linear
Dipoles and monopoles radiate with fixed linear polarisation. Alignment between TX and RX is important — 90° cross-polarisation causes up to 20dB loss. Subject to multipath fading from ground reflections.
RHCP
Right-hand circular polarisation. The electric field rotates clockwise when viewed from the receiving end. Most common FPV convention. RHCP TX + RHCP RX = best performance. RHCP + LHCP = ~20dB loss.
LHCP
Left-hand circular polarisation. Rotates counter-clockwise. Less common in FPV but used in some systems. LHCP TX + LHCP RX = best. Immune to orientation-dependent loss — ideal for aerobatic flight.
Circular polarisation rejects multipath signals arriving from different directions because they arrive with opposite handedness to the direct path signal. This suppresses the fading that linear antennas experience when direct and reflected signals cancel. For long-range FPV video where the aircraft is close to terrain, circular polarisation is strongly preferred.
Keep clear of carbon fibre
Carbon fibre is electrically conductive. Mounting an antenna inside a carbon frame or within 5–10mm of carbon arms causes severe RF shielding and pattern distortion. Always route antennas outside the frame with at least 10mm clearance from carbon.
Prop clearance
Carbon fibre propellers scatter and attenuate RF, particularly at 5.8GHz. Mount VTX antennas pointing down from the bottom of the frame to keep the antenna away from prop wash. Mounting above the props risks periodic signal interruption as the blades rotate.
Vertical orientation for omnidirectional coverage
A vertically-mounted dipole or whip has its radiation null pointing straight up and down. For FPV, where the pilot is at roughly the same elevation as the aircraft, vertical mounting ensures the pilot is always in the antenna's equatorial plane where gain is maximised.
RC receiver antenna routing
Place dual receiver antennas at 90° to each other for spatial diversity — one pointing rearward along the arm, one pointing down or sideways. This ensures at least one antenna always has good orientation regardless of aircraft attitude, eliminating pattern null dropouts.