Automatic tuning AM transmitter

Details Automatic tuning AM transmitter Automatic tuning AM transmitter

1998-11-30

2001-09-25

Kuntz, Curtis (Department: 2643)

Telecommunications

Transmitter

Measuring, testing, or monitoring of transmitter

C455S120000, C455S121000

Reexamination Certificate

active

06295443

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to radio transmitters. More particularly, the present invention relates to a transmitter that automatically adjusts the inductance of the transmitter to match a selected output frequency of the transmitter in order to maximize the output radiated power.
BACKGROUND OF THE INVENTION
There are many instances where information needs to be transmitted quickly and cheaply. Amplitude modulation (or AM) radio transmission can be easily and inexpensively accomplished and, despite some deficiencies, is very attractive for many applications. In particular, new types of highly-specialized, direct radio advertising can be achieved using AM radios. One relatively new application for AM radios is in the sale of real estate.
Recently, real estate of various types, but particularly residential homes, has been equipped with a radio transmitter which broadcasts a pre-recorded message describing the real estate and its features. A prospective buyer may then drive to the property and tune his or her automobile radio to the broadcast frequency of the radio transmitter and listen to the pre-recorded message. The system can operate 24 hours a day, seven days a week until the property is sold. Thus, prospective buyers may gather information at any time, not just those times when the property is open for inspection or when a real estate agent is available to show the property. Furthermore, the system has the ability to reach casual buyers who may not presently be interested in purchasing real estate, but who, if attracted to a property they pass by, may listen to a prerecorded message in their automobile. This initial exposure may lead to a purchase in some cases.
While direct radio marketing broadcasts have certain advantages, such broadcasts must meet certain FCC restrictions related to signal frequency and strength. One of the FCC requirements is that direct marketing broadcasts not interfere with other radio signals such as commercial radio stations. Another consideration to be made when designing a transmitter for real estate sales is that several AM radios might be used in the same geographical area if multiple properties in that area are for sale at the same time. The proximity of many radio signals may cause interference, particularly if the radios broadcast on only one frequency. Thus, a direct marketing radio transmitter must be designed so that the frequency of its broadcast signal can be selected from a range of frequencies depending on the specific placement to be made. However, presently available tunable radio transmitter are inadequate. First, multiknob tuning systems used in some transmitters are cumbersome. Second, in single tuning knob systems the output signal strength varies significantly depending on the broadcast frequency selected. Accordingly, there is a need for an improved radio transmitter that can broadcast a strong signal over a wide range of frequencies.
OBJECTS AND SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide an improved AM transmitter that can broadcast a strong signal over a range of frequencies.
It is another object of the present invention to provide an improved AM transmitter that automatically tunes itself for maximum output by matching a user-selected broadcast frequency.
It is another object of the present invention to provide an improved AM transmitter that can broadcast over a broad range of frequencies, without requiring cumbersome efforts by the user.
These and other objects are achieved in a radio frequency transmitter having a tunable radio frequency signal generator and an amplifier coupled to the tunable radio frequency signal generator. An aerial or antenna is coupled to the amplifier and transmits the amplified radio frequency signal, which may then be received by various radio receivers. The antenna has an output voltage and an impedance. The impedance includes inductive and capacitive reactances.
In order to maximize the output voltage of the antenna, an adjustable inductor is coupled to it. The adjustable inductor includes a pair of wire coils with motor-driven ferrite cores. A sampler measures the output voltage of the antenna and the sampled output voltage of the antenna is converted to a digital signal in an analog-to-digital converter. A processing unit, which is coupled to the converter and the adjustable inductor, receives the converted signal and analyses its amplitude. The processing unit then sends a signal to the adjustable inductor to adjust the impedance of the coils by moving the ferrite cores in order to maximize the amplitude of the output voltage of the antenna.


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Dwayne L. Kincaid, “An Automatic Antenna Tuner: The AT-11,” QST, vol. 80, No. 1, p. 35 (Jan., 1996).

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