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Building a 915 MHz Antenna

Designing a simple antenna for 915 MHz, such as a quarter-wave monopole or dipole, is practical for applications like LoRa or RFID in the ISM band. Below is a guide for a quarter-wave monopole antenna, with a dipole option mentioned later.

Quarter-Wave Monopole Antenna

A quarter-wave monopole is a single radiating element paired with a ground plane. At 915 MHz, the wavelength (λ) in free space is:

  • Speed of light (c) = 300,000,000 m/s (approx. in air)
  • Frequency (f) = 915,000,000 Hz
  • Wavelength (λ) = c / f = 300,000,000 / 915,000,000 ≈ 0.328 meters (32.8 cm)

For a quarter-wave antenna:
- Length = λ / 4 = 32.8 cm / 4 ≈ 8.2 cm (82 mm)

Materials

  • Coaxial cable (e.g., RG-58 or RG-316) or stiff wire (copper/steel, 1-2 mm diameter)
  • Connector (e.g., SMA or BNC, depending on your device)
  • Ground plane: Metal sheet, PCB, or coax shield (at least 8-16 cm across)
  • Tools: Soldering iron, wire cutters, ruler, optional multimeter

Steps

  1. Cut the radiator:
  2. If using wire, cut to 82 mm.
  3. If using coax, strip the jacket and braid, exposing 82 mm of the inner conductor (the "whip").
  4. Prepare the ground plane:
  5. Solder the wire or coax shield to a metal sheet perpendicularly.
  6. For coax alone, flare the braid out as a crude ground plane.
  7. Attach the connector:
  8. Solder to an SMA or similar connector.
  9. Center pin connects to the 82 mm radiator; shield connects to the ground plane.
  10. Tune it (optional):
  11. Length may shift slightly due to materials or surroundings.
  12. Test with a signal source or SWR meter, trimming 1-2 mm at a time to peak at 915 MHz.
  13. Mount it:
  14. Keep vertical and clear of obstructions for omnidirectional performance.

Dipole Alternative

A dipole uses two 82 mm arms (total ~16.4 cm, half-wave), no ground plane needed, fed in the middle:

  • Construction:
  • Use two 82 mm wires.
  • Connect one to the coax center, the other to the shield, spread 180° apart.
  • Secure with a non-conductive center (e.g., plastic or wood).

Notes

  • Impedance: Targets ~50 ohms. Monopole impedance varies with ground plane size (36-50 ohms).
  • Range: Several hundred meters in open air, depending on power and environment.
  • Safety: No high voltages; avoid shorts.

If you have specific tools (e.g., VNA) or materials (e.g., PCB), the design can be refined further.