Helical antenna

In the normal mode or broadside helical antenna, the diameter and the pitch of the aerial are small compared with the wavelength.

In the axial mode or end-fire helical antenna, the diameter and pitch of the helix are comparable to a wavelength.

Axial mode operation was discovered by physicist John D. Kraus[1] If the circumference of the helix is significantly less than a wavelength and its pitch (axial distance between successive turns) is significantly less than a quarter wavelength, the antenna is called a normal-mode helix.

Their compact size makes helicals useful as antennas for mobile and portable communications equipment on the HF, VHF, and UHF bands.

This means that for example a ⁠ 1 /4⁠ wave antenna at 27 MHz is 2.7 m (110 inches; 8.9 feet) long and is physically quite unsuitable for mobile applications.

Many examples of this type have been used extensively for 27 MHz CB radio with a wide variety of designs originating in the US and Australia in the late 1960s.

To date many millions of these ‘helical antennas’ have been mass-produced for mainly mobile vehicle use and reached peak production during the CB Radio boom-times during the 1970s to late 1980s and used worldwide.

[citation needed] These popular designs are still in common use as of 2018[update] and the constant turn design originating in Australia have been universally adapted as standard FM receiving antennas for many factory produced motor vehicles as well as the existing basic style of aftermarket HF and VHF mobile helical.

Another common use for broadside helixes is in the so-called rubber ducky antenna found on most portable VHF and UHF radios using a steel or copper conductor as the radiating element and usually terminated to a BNC/TNC style or screw on connector for quick removal.

The antenna has a bandwidth of only 6-7%, so to make it adjustable to different frequencies the element is divided into multiple vertical "bays", with a phase-adjustment "collar" between each, to keep the phase constant along the length of the tower.

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.

In an axial-mode helical antenna the direction of twist of the helix determines the polarisation of the emitted wave.

When C < λ it operates more in normal mode where the gain direction is a donut shape to the sides instead of out the ends.

Terminal impedance in axial mode ranges between 100 and 200 Ω, approximately[citation needed] where C is the circumference of the helix, and λ is the wavelength.

[8] The optimal pitch that maximizes the gain for a flat ground plane is in the range 3–10° and it depends on the wire radius and antenna length.