Axial-mode Helical
In the axial mode, the helix dimensions are at or above the wavelength of operation. The antenna produces radio waves with circular polarisation. The main lobes of the radiation pattern are along the axis of the helix, off both ends. Since in a directional antenna only radiation in one direction is wanted, the other end of the helix is terminated in a flat metal sheet or screen reflector to reflect the waves forward.
In radio transmission, circular polarisation is often used where the relative orientation of the transmitting and receiving antennas cannot be easily controlled, such as in animal tracking and spacecraft communications, or where the polarisation of the signal may change, so end-fire helical antennas are frequently used for these applications. Since large helices are difficult to build and unwieldy to steer and aim, the design is commonly employed only at higher frequencies, ranging from VHF up to microwave.
The helix in the antenna can twist in two possible directions: right-handed or left-handed, as defined by the right hand rule. In an axial-mode helical antenna the direction of twist of the helix determines the polarisation of the radio waves: a left-handed helix radiates left-circularly-polarised radio waves, a right-handed helix radiates right-circularly-polarised radio waves. Helical antennas can receive signals with any type of linear polarisation, such as horizontal or vertical polarisation, but when receiving circularly polarised signals the handedness of the receiving antenna must be the same as the transmitting antenna; left-hand polarised antennas suffer a severe loss of gain when receiving right-circularly-polarised signals, and vice versa.
The dimensions of the helix are determined by the wavelength λ of the radio waves used, which depends on the frequency. In axial-mode operation, the spacing between the coils should be approximately one-quarter of the wavelength (λ/4), and the diameter of the coils should be approximately the wavelength divided by pi (λ/π). The length of the coil determines how directional the antenna will be as well as its gain; longer antennas will be more sensitive in the direction in which they point.
Terminal impedance in axial mode ranges between 100 and 200 ohms. The resistive part is approximated by:
where R is resistance in ohms, C is the circumference of the helix, and λ is the wavelength. Impedance matching to the cable C is often done by a short stripline section between the helix and the cable termination.
The maximum directive gain is approximately:
where N is the number of turns and S is the spacing between turns.
The half-power beamwidth is:
The beamwidth between nulls is:
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