Rubber Ducky Antenna

Richard Johnson
Rubber Ducky Antenna on a transceiver
The Rubber Ducky Antenna is an electrically short antenna, which functions somewhat like a base-loaded whip or monopole antenna. Electrically short antennas are often used in portable equipment because a one-quarter wavelength element, necessary for electrical resonance of a linear element over a ground-plane, is often too long for convenient portable operation.

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. [8]  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.

Construction
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 [6]  comedies about the Kennedy family. In both cases, its name seems to somehow to relate to the Kennedy family.

Efficiency
Because the length of this antenna is significantly smaller than a wavelength, the effective aperture is approximately: [2,5]  A surprising result is that even though the Rubber Ducky Antenna may be small, its effective aperture can be comparable to larger antennas. If one takes care in its design to produce a reasonably high radiation resistance, one can produce a useful antenna.

As with many monopole antennas, the Rubber Ducky requires a ground-plane or counterpoise [4]  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. [3]  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 label R represents the combination of the radiation resistance and the various loss resistances in the device. The label L represents the inductance predominately generated by the coiled spring. The label C represents the distributed capacity, including the capacity caused by proximity to nearby objects. Observe that the series-connected components complete the circuit through a virtual ground represented by the body of the transceiver. If there are any losses within this circuit such as body parts of the person using a transceiver that uses this antenna, they will exist effectively in series with the radiation resistance, reducing the efficiency of the antenna. This is a reason why antennas on portable equipment, such as handheld transceivers, do not often function well. They usually do not have a sufficient counterpoise. Rubber Ducky Antennas suffer losses greater than ¼ wave whip antennas because of the higher radio frequency currents flowing in the antenna and the parasitic resistance of the person using the device. This is a case where not all the currents in the series circuit appear the same, seeming to violate Kirchoff’s laws. [7]  Of course, there is not any violation; it is that the circuit diagram is not a circuit model. The radio-frequency current in the spring near the transceiver is very large relative to the current at its end, which is only the displacement current of the antenna’s capacity to the external world. Therefore, we seem to have an instance where the current into an inductor on a schematic is not the same as the current out of it, but the spring is much more than an inductor.

The antenna must operate at a frequency on or near its electrical resonance for it to function properly. [5] 
This is why production of these antennas is for a specific narrow frequency range. For instance, one cannot effectively use a Rubber Ducky Antenna made for an amateur radio band on an aircraft radio.

Electrical design
These are the observed electrical characteristics of an antenna constructed from a screen-door spring, resonant at 50 MHz. Rubber Ducky Antennas have reasonable performance, but they do not have either the gain or the aperture of larger antennas. Therefore, their performance will always be somewhat of a compromise. They are difficult to characterize electrically because the current distribution along the element is not sinusoidal as is the case of a thin linear array. However, there are a few “rules-of-thumb” that can be used to design these antennas:

  1. If the coils of the spring are wide (a large diameter), relative to the length of the array, the resulting antenna will have narrow bandwidth.
  2. Conversely, if the coils of the spring are narrow, relative to the length of the array, the resulting antenna will have its largest possible bandwidth.
  3. If the antenna is resonant, and the spring has a large diameter, the impedance will be well below 50 ohms, tending towards zero ohms with large inductors as the structure starts to resemble a series-tuned circuit with little radiation resistance.
  4. If the antenna is resonant, and the spring has a small diameter, the impedance will increase towards 70 ohms.
Therefore, from these rules, one can surmise that it is possible to design a Rubber Ducky Antenna that has about 50 ohms impedance at its feed-point but a compromise of bandwidth may be necessary. Modern Rubber Ducky Antennas such as those used on cell phones taper in such a way that few performance compromises are necessary.

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.

References
  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

External links
Author’s website

Notes
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