LPDA: log-periodic dipole arrays

Wideband directional antennas, from straight plates to hairpin and meander arms

Hairpin arms with meander, tilt and tail short; sheet metal
Hairpin arms with meander, tilt and tail short; sheet metal

Overview

Wideband directional antennas for EMC pre-compliance testing, software-defined radio (SDR), radio monitoring, ham radio and FPV drones. A parametric model with up to 27 parameters optimised together, plus per-dipole corrections: from straight arms to a hairpin with a meander on both branches. The tail always ends in a short; on metal it is a block with a boom extension that also mounts the antenna.

Values given for a design below come from our own simulations (computed examples); they are not measured guarantees.

Typical performance

  • VSWR < 2 over 195–410 MHz (hairpin arms with tilt, 12 dipoles, pictured under the variants)computed
  • Band 2 : 1 … 10 : 1 by the dipole count, 8–24 dipolestypical
  • Directivity ≈ 7–11 dBtypical
  • Hairpin: span ≈ 0.57 m against ≈ 0.75 m with straight arms at 195 MHztypical
  • Linear polarisationtypical

Typical values describe the antenna class, not a guarantee for a unit; “computed” marks our own simulated example.

Arm shapes

Straight arms

The simplest design and the most tolerant to manufacturing spread: two plates, laser cut, no bending. The widest band per dipole.

  • Computed example: VSWR < 2 over 300–800 MHz; 8 dipoles, span ≈ 0.48 m, 2 mm aluminium.
  • Use: EMC pre-compliance, test ranges, radio monitoring.

Meander arms

The arm is folded into a zigzag: the same lowest frequency with a smaller span. Meander height and section count scale with every dipole.

  • Computed example: VSWR < 2 over 195–400 MHz; 14 dipoles, 7 meander sections, span ≈ 0.6 m (straight: ≈ 0.75 m).
  • Use: lower VHF and UHF where the span is limited by the mast, the room or the drone.

Folded loop arms

Two parallel traces with a slot, joined at the tip: a stiffer arm, a lower dipole Q, a smoother input impedance over the band.

  • Computed example: VSWR < 2 over 300–800 MHz; 8 dipoles, slot 5 % of the arm length, 2 mm aluminium.
  • Use: fixed and vehicle antennas under wind and vibration, test ranges.

Hats, tilt, reflector

T-hat end loading

A cross bar at the arm tip adds capacitance: the dipole resonance drops by 10–15 % for the same span, without bending or a meander.

  • Computed example: VSWR < 2 over 300–800 MHz; 8 dipoles, hat 25 % of the arm length.
  • Use: compact SDR and monitoring antennas keeping the same lowest frequency.

Tilted arms (V shape)

The arms lean towards the apex: a narrower antenna for the same arm length and a narrower pattern in the dipole plane. The tilt can vary from the largest dipole to the smallest.

  • Computed example: VSWR < 2 over 350–750 MHz; 9 dipoles, 20° tilt, 2 mm copper.
  • Use: direction finding and monitoring, drone and mast antennas with a width limit.

Tail reflector

A parasitic, electrically longer dipole behind the largest one, within the tail: the same boom length, less back radiation at the lower band edge.

  • Computed example: VSWR < 2 over 300–800 MHz; 8 dipoles and a hairpin reflector, the boom length unchanged.
  • Use: indoor and range measurements where reflections from behind interfere.

LPDA on printed boards

Double-sided board with meander

Arms A and B in copper on both sides of one FR-4 board: light, repeatable in series, no mechanical assembly.

  • Computed example: VSWR < 2 over 195–400 MHz; 13 dipoles, 10 meander sections, 1.5 mm FR-4.
  • Use: SDR, receiving antennas, series products for integrators.

Two boards at an angle

Two identical single-sided boards, the second one flipped; screws through dielectric inserts hold the gap and the angle. The feed line works as in a metal LPDA.

  • Computed example: VSWR < 2 over 340–800 MHz; 17 dipoles, 9 meander sections, 1.6 mm FR-4, 3D-printed inserts.
  • Use: measurement and monitoring antennas with a wider band than a single board.

Board with hairpin arms

Hairpin arms with a 26° tilt and rounded joints to the boom: the smallest board span for a 195 MHz lowest frequency.

  • Computed example: VSWR < 2 over 195–410 MHz; 15 dipoles, 1.6 mm FR-4, 1.5 mm fillets.
  • Use: compact FPV and SDR antennas at low frequencies.
View A
Hairpin arms with tilt, tail short block, spacers; 2 mm aluminium, 12 dipoles, 195–410 MHz
Hairpin arms with tilt, tail short block, spacers; 2 mm aluminium, 12 dipoles, 195–410 MHz

What we optimise

Pos.Parameter
P1τ, σ, number, length and width of dipoles
P2Arm shape and tilt
P3Feed line, tail, reflector

What you receive

Pos.Deliverable
D1Files for laser cutting or board manufacture
D2Drawings of the short block and spacers
D3A 3D model
D4A measurement protocol of S11 and antenna factor

Designs

Hairpin arms with tilt, tail short block, spacers; 2 mm aluminium, 12 dipoles, 195–410 MHz
Hairpin arms with tilt, tail short block, spacers; 2 mm aluminium, 12 dipoles, 195–410 MHz
Straight arms
Straight arms
Meander arms
Meander arms
Folded loop arms
Folded loop arms
T-hat end loading
T-hat end loading
Tilted arms (V shape)
Tilted arms (V shape)
Tail reflector
Tail reflector
Double-sided board with meander
Double-sided board with meander
Two boards at an angle
Two boards at an angle
Board with hairpin arms
Board with hairpin arms

Discuss your RF requirements