Crossed Dipole
How Two Orthogonal Dipoles Synthesize Circular Polarization
The crossed dipole, described by Brown in the 1930s and named the turnstile for its rotating-field behavior, is the simplest practical radiator for circular polarization at HF through UHF. Each arm is an independent half-wave dipole, so each presents the familiar figure-eight pattern and roughly 73 Ω resistive input impedance at resonance. Mounting the two dipoles perpendicular in a common plane and driving them with equal current but a 90° time-phase difference makes one field component lead the other by a quarter cycle. The vector sum sweeps through a full rotation every RF period, and the locus seen along the axis perpendicular to the dipole plane is a circle.
Whether the rotation is right-hand (RHCP) or left-hand (LHCP) depends only on the sign of the 90° phase shift. Reverse the quadrature and the sense flips, a property exploited in dual-polarization and direction-finding systems. Away from the boresight axis the two dipoles no longer project equal lengths onto the observer's plane, so the polarization degrades smoothly from circular to elliptical and finally to linear in the plane of the dipoles. This is why the crossed dipole radiates clean circular polarization upward but largely horizontal polarization toward the horizon, an ideal match for tracking satellites that rise and set across the sky.
Manufacturing accuracy matters more than for a linear dipole. Amplitude imbalance and phase error between the arms both inflate the axial ratio, the single most important figure of merit. RF Essentials builds crossed-dipole and turnstile feeds with tuned baluns and trimmed arm lengths to hold the on-axis axial ratio below 1 dB across the operating band.
Quadrature Feed and Phasing Methods
The classic phasing trick avoids an active network entirely: one dipole is cut slightly longer than resonance so its impedance is inductive (current lags), and the other slightly shorter so it is capacitive (current leads). When each arm presents about +45° and -45° of reactance, the relative current phase is the required 90° and both arms can share a single feedpoint. Wideband or precision designs instead use a 90° branch-line hybrid or a quarter-wave delay line feeding the two arms separately, which holds the phase quadrature over a wider bandwidth than the self-phased method.
Governing Relationships
E = E0(◯ cosωt + ŷ cos(ωt − 90°)) = E0(◯ cosωt + ŷ sinωt)
Axial ratio from amplitude/phase error:
AR(dB) = 20 log10(Emajor / Eminor) → 0 dB = perfect circular, ∞ = linear
Half-wave dipole arm length (with end effect):
L ≈ 0.48 × λ = 0.48 × (c / f)
Self-phasing reactance condition:
Xarm1 ≈ +R0, Xarm2 ≈ −R0 (R0 ≈ 73 Ω)
Where ◯, ŷ = orthogonal unit vectors, ω = 2πf, AR = axial ratio, λ = free-space wavelength, R0 = radiation resistance. Example at 137 MHz: λ ≈ 2.19 m, each dipole ≈ 1.05 m tip to tip.
Crossed Dipole vs. Other Circularly Polarized Antennas
| Antenna | CP Method | Typical Gain | 3 dB AR Beamwidth | Bandwidth | Best Application |
|---|---|---|---|---|---|
| Crossed dipole / turnstile | Orthogonal dipoles, 90° feed | 1 to 4 dBic | ~100° | 5 to 15% | VHF/UHF LEO satellite, broadcast |
| Quadrifilar helix | 4 phased helical arms | 2 to 5 dBic | 140 to 180° | 10 to 20% | GPS, handheld satcom |
| Axial-mode helix | End-fire helix > 1λ circumference | 8 to 15 dBic | 50 to 90° | 30 to 70% | Telemetry, ground stations |
| CP patch (corner-fed) | Two near-degenerate modes | 5 to 8 dBic | 70 to 90° | 1 to 3% | GPS/GNSS receivers |
| Crossed Yagi | Two phased Yagi booms | 9 to 14 dBic | 30 to 50° | 5 to 10% | High-gain satellite tracking |
Frequently Asked Questions
How does a crossed dipole produce circular polarization?
Two half-wave dipoles set at 90° in the same plane are fed with equal amplitude and a 90° phase difference. The horizontal field from one and the vertical field from the other combine in space- and time-quadrature, so the resultant vector rotates once per RF cycle, tracing a circle along boresight. Leading phase gives RHCP, lagging phase gives LHCP. The quarter-cycle shift comes from a quarter-wave line, a 90° hybrid, or by detuning one arm long and the other short.
What is the axial ratio of a crossed dipole and how is it controlled?
Axial ratio is the major-to-minor axis ratio of the polarization ellipse in dB, near 0 dB for perfect CP. A crossed dipole reaches near 0 dB on boresight with exact amplitude balance and 90° quadrature; a 1 dB amplitude imbalance or 10° phase error each push it to roughly 1 to 1.5 dB. Off axis it worsens, usually exceeding 3 dB beyond 40 to 50° from boresight, giving a practical 3 dB AR beamwidth near 100° in free space.
What is the difference between a crossed dipole and a turnstile antenna?
They are the same structure: two orthogonal dipoles in one plane. Turnstile usually denotes the omnidirectional horizontal-plane use, as in stacked VHF broadcast or weather-satellite receive bays. Crossed dipole emphasizes the quadrature feed that yields circular polarization along the axis. A turnstile fed in quadrature radiates RHCP or LHCP upward and horizontal polarization toward the horizon, which is why it suits low-earth-orbit satellites moving from horizon to zenith.