When circumference approaches one wavelength, fields from successive turns add along the helix axis.
Antennas · F1SYH
Helical antenna
An axial-mode helix is a directional circularly polarized antenna widely used for UHF, satellites and microwave work.
UHF
SHF
Polarisation circulaire
Operating principle
Main characteristics
Typical gain
Variable selon nombre de spires
Impedance
Environ 100 à 150 Ω avant adaptation
Polarization
Circulaire
Difficulty
Avancée
Dimensions and design
Circumference is generally near 0.8 to 1.2 wavelengths. Pitch and turn count determine gain and overall length.
Construction and tuning
Step 1
Calculate helix diameter.
Step 2
Select the number of turns.
Step 3
Maintain constant pitch.
Step 4
Install a rear reflector.
Step 5
Build impedance matching.
Step 6
Verify polarization sense.
Advantages
- Natural circular polarization.
- Gain controlled by turn count.
- Relatively wide bandwidth.
- Accessible UHF construction.
Limitations
- Long structure for high gain.
- Critical diameter and pitch.
- Matching required.
- Fixed polarization sense.
History and origin
John D. Kraus developed and extensively described the axial-mode helix from the 1940s onward.
Safety
Protect wire ends and firmly secure the helix to its reflector.