Rubber ducky antenna
A compact helical monopole used in portable handheld radios.
The rubber ducky antenna is a short monopole antenna that works in a similar way to a base-loaded whip antenna. It is built from a springy wire shaped into a narrow helix, which is then covered in a rubber or plastic jacket for protection. This design is a type of normal-mode helical antenna.
The antenna was invented by Richard B. Johnson. Before its creation, portable radios typically used quarter-wave whip antennas. At VHF and UHF frequencies, these whips were often 0.6 to 0.9 meters (2 to 3 feet) long, making them awkward to handle. Many were made of telescoping tubes that could be retracted. To make antennas more compact, electrically short antennas—shorter than a quarter wavelength—were introduced. These short antennas have high capacitive reactance, so a loading coil (an inductor) is added in series to make them resonant. When the coil is built into the base, the antenna is called a base-loaded whip.
In the rubber ducky, the inductor is built into the antenna itself rather than the base. The helical spring of wire acts as this inductor. The springy wire is flexible, which makes it less likely to break than a rigid antenna, and the rubber or plastic covering protects it. Rubber ducky antennas are typically 4% to 15% of a wavelength long, which is 16% to 60% the length of a standard quarter-wave whip.
The term "rubber duck" comes from the rubberized protective coating often seen on handheld and police radios. Another name for this design is the "stubby antenna."
Because the antenna is much shorter than a wavelength, its effective aperture (if 100% efficient) would be approximately 3λ²/(8π). Like other electrically short antennas, the rubber ducky has poorer performance and less gain than a quarter-wave whip due to losses. However, it performs somewhat better than a base-loaded antenna of the same length, because the distributed inductance allows greater current along the element.
Rubber ducky antennas have lower gain than a full-size quarter-wave antenna, which reduces the radio’s range. They are typically used in short-range two-way radios where maximum range is not critical. Their design is a compromise between gain and small size. They are difficult to characterize electrically because the current distribution along the element is not sinusoidal, as it is with a thin linear antenna.
- Inventor
- Richard B. Johnson
- Year of invention
- 1958
- Typical length
- 4% to 15% of a wavelength
- Frequency range
- VHF and UHF
- Impedance range
- tends towards 0 Ω with large diameter coils; up to about 70 Ω with small diameter coils
Lore & Background
Before the rubber ducky, antennas on portable radios usually consisted of quarter-wave whip antennas, rods whose length was one-quarter of the wavelength of the radio waves used. In the VHF range where they were used, these antennas were 0.6 or 0.9 m (2 or 3 feet) long, making them cumbersome. They were often made of telescoping tubes that could be retracted when not in use. To make the antenna more compact, electrically short antennas, shorter than one-quarter wavelength, began to be used. Electrically short antennas have considerable capacitive reactance, so to make them resonant at the operating frequency an inductor (loading coil) is added in series with the antenna. Antennas which have these inductors built into their bases are called base-loaded whips.
The rubber ducky is an electrically short quarter-wave antenna in which the inductor, instead of being in the base, is built into the antenna itself. The antenna is made of a narrow helix of wire like a spring, which functions as the needed inductor. The springy wire is flexible, making it less prone to damage than a stiff antenna. The spring antenna is further enclosed in a plastic or rubber-like covering to protect it. The technical name for this type of antenna is a normal-mode helix. Rubber ducky antennas are typically 4% to 15% of a wavelength long; that is, 16% to 60% of the length of a standard quarter-wave whip.
The term rubber duck stems from the rubberized protective coating commonly seen on handheld radios and police radios. An alternative name is based on the short stub format: the 'stubby antenna'.
Reader's Guide
The rubber ducky antenna became the antenna of choice for many portable radio devices, including walkie-talkies and other portable transceivers, scanners and other devices where safety and robustness take precedence over electromagnetic performance. Its flexibility makes it more suitable for handheld operation, especially when worn on the belt, than earlier rigid telescoping antennas. The rubber ducky has poorer performance (less gain) due to losses and thus considerably less gain than a quarter-wave whip, but it has somewhat better performance than an equal length base loaded antenna because the inductance is distributed throughout the antenna and so allows somewhat greater current in the antenna. Rubber ducky antennas have lower gain than a full size quarter-wavelength antenna, reducing the range of the radio. They are typically used in short-range two way radios where maximum range is not a requirement. Their design is a compromise between antenna gain and small size. In common with other inductively loaded short monopoles, the rubber ducky has a high Q factor and thus a narrow bandwidth. This type of antenna is often used over a wide frequency range, e.g. 100–500 MHz, and over this range its performance is poor, but in many mobile radio applications there is sufficient excess signal strength to overcome any deficiencies in the antenna.
Did You Know?
- The rubber ducky antenna was invented by Richard B. Johnson.
- It is a form of normal-mode helical antenna.
More in Antenna Types, Part 2 1-24
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