Rubber Ducky Antenna
The first antenna
The first Rubber Ducky Antenna was designed for operation on the six-meter amateur radio band by Richard B. Johnson, [1,3] while he was a student at a reform school, The Lyman School for Boys.  It was to substitute for a vertical antenna which would have been one-and-a-half meters long (one-quarter of six meters), too long to fit into Johnson’s locker when not in use. The resulting antenna was slightly less than one-third of a meter in length, and was simply a screen-door spring, with its length adjusted for electrical resonance, and covered with a synthetic rubber hose. In the period of its development, antennas on portable equipment usually consisted of telescoping rods that were extended for operation and later retracted for storage.
Electrically short antennas have considerable capacitive reactance, so to provide an approximate impedance match, it is usual to add an inductor in series with the antenna. Antennas that have these inductors built into them are called base loaded antennas. It is possible to make an antenna in which the entire length of the driven element is an inductor, configured much like a spring. In fact, if a springy material is used, the antenna becomes flexible and immune to damage. If the spring antenna is further enclosed in a plastic or rubber-like covering, it is called a Rubber Ducky Antenna.
Origin of the name
Several years after its invention in 1958, the Rubber Ducky Antenna became the antenna of choice for portable transceivers. Rumor has it that Caroline Kennedy gave the antenna its final name when she pointed to the flexible antenna on a Secret Service Agent’s transceiver and announced; “Rubber Ducky.” However, Dr. Thomas A. Clark, then Senior Scientist with NASA, claims to have named it in 1961 after listening to one of Vaughn Meader’s  comedies about the Kennedy family. In both cases, its name seems to somehow to relate to the Kennedy family.
Because the length of this antenna is significantly smaller than a wavelength, the effective aperture is approximately: [2,5]
As with many monopole antennas, the Rubber Ducky requires a ground-plane or counterpoise  with which to complete its electrical circuit. With handheld transceivers, this ground-plane is often only a small internal shield or the jackets of internal batteries. Modern construction techniques using nonconductive plastics for transceiver cases further reduce the effectiveness of this antenna by eliminating a conductive path to the user, which could have provided an effective ground-plane or counterpoise. The original antenna mounted on a paint can to which were soldered four brass radials.  This helped make the antenna quite efficient. Modern versions eliminate this counterpoise, using the electrically resonant spring only for convenience. This often reduces the usefulness of this antenna to where it is barely adequate for its intended use.
Shown is the equivalent electrical circuit of the Rubber Ducky Antenna:
The antenna must operate at a frequency on or near its electrical resonance for it to function properly. 
These are the observed electrical characteristics of an antenna constructed from a screen-door spring, resonant at 50 MHz.
Some are different
The design of some Rubber Ducky Antennas is quite different from the original design. One type uses a spring only for support. The spring has an electrical short across it. The antenna is therefore electrically, a linear element antenna. Some other Rubber Ducky Antennas use a spring of non-conducting material for support and comprise a collinear array antenna. Such antennas are still called Rubber Ducky Antennas even though they function quite differently (and often better) than the original spring antenna. The Rubber Ducky Antenna has recently become known as the Flagelliform Antenna as well.
1. A Brief History of the Rubber Ducky Antenna
2. Kraus, John D. (1950) Antennas McGraw-Hill Chapter 3, The antenna as an aperture, pp 30
3. Johnson, Richard B. (2006) Abominable Firebug, iUniverse, Chapter 20, Freedom on the Inside, ISBN 0-595-38667-9
4. Counterpoise definition
5. Federal Standard 1037, Electronic Terms
6. Vaughn Meader’s obituary
7. Basic Kirchoff
8. The Lyman School for Boys
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