Small “rubber-ducky” helical antenna for 868MHz

This simple Helical Antenna can be used for most LoRa (and other small wireless device) applications, where space constraints exist. The construction is simple, made from wound solid Ag plated copper wire taken from piece of coaxial cable.

Helical axial antenna for 868MHz

Two options how to use this antenna on the PCB modules

There are two general types of helical antennae:

  • Normal mode helical antenna
  • Axial mode helical antenna

Axial mode helix can be end-fire or back-fire and both types exhibit high gains and excelent performance. Unfortunately they are not suitable for miniaturisation, because the circumference of the helix and its pitch are in the range of the wavelength.

Normal mode helix is smaller. The circumference and the pitch of the helix is significantly less than a wavelength. he antenna acts similar to a monopole antenna, with an omnidirectional radiation pattern, radiating equal power in all directions perpendicular to the antenna’s axis. However, because of the inductance added by the helical shape, the antenna acts like a inductively loaded monopole; at its resonant frequency it is shorter than a quarter-wavelength long. Therefore, normal-mode helices can be used as electrically short monopoles, an alternative to center- or base-loaded whip antennas, in applications where a full sized quarter-wave monopole would be too big. As with other electrically short antennas, the gain, and thus the communication range, of the helix will be less than that of a full sized antenna. Their compact size makes “helicals” useful as antennas for small Lora device [Source].

 

How to make such helix antenna

    1. The material of the coil is solid silver plated copper wire taken from core of the RG214U coaxial cable.
    2. Straighten hhe copper wire by pulling strongly. Make short, fast, strong pulls. This will make copper wire stronger.
    3. Take 5mm dia metal rod or drill and wind the wire around the rod at least 7 times or more.
    4. Cut to exactly 7 turns and extend the coil to 14mm (2mm pitch). This step should be done cerfully: every turn must have exactly 2mm. Uneven pitch will result in poor quality.
    5. Now create the groundplane stub on one end: bend the wire 90 degrees to have wire in the axial direction.
    6. Mark 6mm from the helix base, which is soldering plane. You can bend wire at the mark 90 degrees if you wish to mount the antenna perpendicular to your PCB.

 

Measurements

I optimised the geometry of the antenna with th VNA measurements. Here are results:

Antenna VSWR measurements

Downloads

Drawing in PDF format

3D step file

 

Use

Some of the final products in use for measuring particulate matter in air:

PM sensors equiped with described antennas.

 

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