Sunday, January 30, 2011
The Twin Coil Antenna Patent
Let's continue our discussion of the ferrite loopstick, a most interesting device.
A curious animal appeared on the radio scene in March of 2003. It was called the Twin Coil Antenna. Its inventor was one Christopher Justice of the C. Crane Company.
A July 2001 application for United States Patent number 6529169, "Twin Coil Antenna", shows an "antenna for a radio receiver comprising a ferrite rod; spaced coils wound about the ferrite rod; a variable capacitor element connecting each coil together; and a combiner connecting each coil together." It was a souped-up ferrite loopstick.
In analyzing the standard tuned ferrite rod utilizing a single coil, Justice determined there is a "need for improved efficiency and a higher signal-to-noise ratio antenna that does not saturate at high frequencies", and it was to these ends that his experiments were directed.
Ferrite core antennas are widely used as antennas for radio receivers, particularly for AM broadcast band radios. This is done by winding a coil or loop, generally with tightly-spaced turns over one end of a ferrite rod or bar. The ferrite has the effect of concentrating and intensifying the received magnetic field inside the loop.
Unfortunately, the ferrite core tends to absorb some of the signal power. Additionally, the combined resistance or impedance of the antenna and ferrite core is typically just a few ohms or less. In operation, the antenna must be coupled to the larger input impedance of the receiver. This is generally accomplished by adding a capacitor to tune the ferrite loop to a certain frequency, creating a resonant circuit.
When a signal enters a ferrite rod antenna that is tuned to resonance with a coil and capacitor, the magnetic lines of flux begin to saturate. When this saturation occurs, the ferrite rod antenna takes on polarity, much like a magnet does. Ferrite antennas having only one pick-up coil result in loss of efficiency and limit the signal-to-noise ratio available to the receiver.
In the words of the patent application, "The invention provides an antenna for a radio receiver that has high efficiency, high signal-to-noise ratio and that does not saturate at high frequencies. This is accomplished by an antenna structure which employs a ferrite core having two (or more) coils coupled together and located on the ferrite core such that the magnetic fields coupled to the coils induce signals which combine to produce a resulting signal level equivalent to the combined signals in the coils. The coils may be coupled through a transformer where the combined signals of the coils are received in the primary of the transformer. The transformer coupling the antenna coils can dramatically narrow the bandwidth of the received signal. In addition, the antenna coils and the transformer windings may be connected to a capacitor to form a resonant circuit. The capacitor may be variable so the resonant frequency of the antenna may be set by tuning the capacitor. This results in an increased signal level and reduced interference and noise."
Read more about this fascinating antenna device on Google Documents' Twin Coil Ferrite AM Antenna. Justice's patent was accepted and issued on March 4, 2003.
CCrane sells their version of the Twin Coil Antenna on their website.
Next up in this series: The Vertically-Challenged Ferrite Loopstick