Normal-mode Helical
Radiating at 90 degrees from the axis of the helix this design is efficient as a practical reduced-length radiator when compared with the operation of other types such as base-loaded, top-loaded or centre-loaded whips. They are typically used for applications where reduced size is a critical operational factor.
These simple and practical "Helicals" were primarily designed to replace very large antennas. Their reduced size is therefore most suitable for Mobile and Portable High-frequency (HF) communications in the 1 MHz to 30 MHz operating range.
Usually wound in a linear "spiroidal" pattern (constant parallel spaced turns) providing consistent uniform radiation as a reduced sized equivalent in respect to the standard 1/4 wave antenna. This concept was proven practical by an Australian design.
An effect of this type of concertinaed 'reduced size 1/4 wave' is that the matching impedance is changed from the nominal 50 ohms to between 25 to 35 ohms base impedance. This does not seem to be adverse to operation or matching with a normal 50 ohm transmission line, provided the connecting feed is the electrical equivalent of a 1/2 wave at the frequency of operation.
Another example of the type as used in mobile communications is "spaced constant turn" in which two or more different linear windings are wound on a single former and spaced so as to provide an efficient balance between capacitance and inductance for the radiating element at a particular resonant frequency.
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. Multi-frequency versions with plug-in taps have become the mainstay for multi-band Single-sideband modulation (SSB) HF communications.
Most examples were wound with copper wire using a fibreglass rod as a former. This flexible radiator is then covered with heat-shrink tubing which provides a resilient and rugged waterproof covering for the finished mobile antenna.
These popular designs are still in common use today (2010) and 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. The most common use for broadside helixes is in the Rubber Ducky antenna found on most portable VHF and UHF radios.
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