Antenna Building for SDR
The antenna decides between success and frustration. From the λ/4 formula through the simple V-dipole for weather satellites to the broadband discone — with build plans and DIY videos.
The most important formula: λ/4
Almost all simple antennas are based on the wavelength λ (lambda). The quarter wavelength determines the rod length:
Example 137 MHz (NOAA weather satellites): 300,000,000 / (4 × 137,000,000) = 54.7 cm per rod
Example 400 MHz (TETRA): 300,000,000 / (4 × 400,000,000) = 18.75 cm per rod
Example 1090 MHz (ADS-B): 300,000,000 / (4 × 1,090,000,000) = 6.9 cm per rod
Antenna types at a glance
| Type | Frequency range | Character | Use |
|---|---|---|---|
| Whip antenna (included) | Broadband | A compromise, short, little gain | First test |
| Dipole | Narrowband, resonant | Two rods of equal length | FM, specific bands |
| V-dipole | Narrowband (137 MHz) | Dipole in a 120° V shape | NOAA/Meteor weather satellites |
| Discone | 25 MHz – 1.3 GHz | Very broadband | All-rounder outdoor antenna |
| Yagi | Narrowband, directional | High gain in one direction | Aviation, satellites, targeted |
| Collinear | Broadband, vertical | Good omnidirectional gain | VHF/UHF, emergency-services monitoring |
| Magnetic antenna (mag loop) | HF 1–30 MHz | Small, tunable | Short-wave reception |
| QFH / turnstile | 137 MHz circular | Circularly polarised | Weather satellites (more involved) |
The simplest and most effective build for getting started: Adam 9A4QV's V-dipole. Two metal rods, a terminal block, coax cable — done. Ideal for NOAA and Meteor-M2.
📺 Adam 9A4QV — V-dipole build guide and dimensions (original designer)
Assembly: Spread both rods at a 120° angle (V shape) · one rod to the inner conductor, one to the shield of the coax · mount horizontally · north-south alignment (satellites orbit polar)
Advantage: Horizontal polarisation attenuates vertical terrestrial interference by ~20 dB
If you want to receive everything from 25 MHz to 1.3 GHz with one antenna: the discone. A bit more involved, but the best all-rounder for scanner use, ADS-B, aviation, TETRA monitoring and more.
📺 DIY portable discone antenna — build it step by step
Materials: 8× radial wires for the cone + 8× for the disc (e.g. galvanised fence wire), central hub, N connector · also usable as a chicken-wire/mesh variant
Design calculator: VE3SQB discone software (Windows) or an online calculator work out the rod lengths from the target frequency
Range: usable up to ~10× the design frequency (130 MHz → up to ~1.3 GHz)
Antenna-length quick reference (λ/4)
| Service | Frequency | λ/4 rod length | Recommended type |
|---|---|---|---|
| NOAA weather sat | 137 MHz | 54.7 cm | V-dipole (53.4 cm in practice) |
| 2 m amateur radio | 145 MHz | 51.7 cm | Dipole / collinear |
| AIS ships | 162 MHz | 46.3 cm | Dipole / discone |
| TETRA / emergency services | 390 MHz | 19.2 cm | Dipole / discone |
| 70 cm amateur radio | 435 MHz | 17.2 cm | Dipole / Yagi |
| PMR446 | 446 MHz | 16.8 cm | Dipole |
| ADS-B aircraft | 1090 MHz | 6.9 cm | Collinear / ground plane |
Important practical tips
50 Ω coax cable
Always use 50 Ω cable (RG-58, RG-213, H-155) — not 75 Ω TV cable! The wrong impedance costs a lot of signal.
Location & alignment
Higher = better. A clear view of the sky for satellites. V-dipole strictly north-south. Away from PC/laptop (sources of interference).
No iron/steel
Use aluminium, brass or copper for the elements. Ferromagnetic materials increase losses (skin effect).
Weatherproofing
For permanent outdoor use: seal solder joints with hot glue/heat-shrink tubing. Terminal blocks are only good for testing.
VF ≈ 0.95 for thin wire, 0.66 for coax (RG58).