There is a well-known need for travelers to communicate. Traditional land line telephones have provided communication capability, but have a disadvantage in that the traveler must be near a telephone and stationary at the time of the call. Cellular wireless telephones solve these problems, but are expensive and have many dead zones where there are an insufficient number of line of sight cell towers to transmit the signal. Citizen band radios and other similar radio communication methods have a limited number of frequencies available and suffer from interference, short range, and radio shadows in tunnels, cities, and valleys. Satellite radios have fewer dead zone problems than cellular telephones, but are more expensive and require sensitive aimed parabolic dish antenna directed at the satellite being used. Satellite radio telephones are thus inconvenient for use in motor vehicles since the vehicle motion and vibration make aiming the antenna dish difficult.

It is known to solve the moving vehicle problem by the use of a parabolic antenna that continually adjusts the direction of the antenna by means of a feedback system and three axis gimbal system. This solution is very expensive, complex, and prone to breakdown.

The use of longer wave radio bands, known as high frequency (HF) band, which operate in the 3 to 33 megahertz (MHz) range, has an advantage over other known communication methods in that the long electromagnetic (EM) waves bounce off what is known as the ionosphere, and can therefore transmit to a receiver that is"over the horizon"and have longer range and fewer dead zones. Because of the ability to bounce off of the ionosphere HF radios do not need the large number of expensive line of sight retransmission towers to transmit the signal over long distances.

A known problem with HF radio communication is that the antenna length required to be an efficient RF radiator becomes longer as the EM wavelength becomes longer. Thus, the size of a standard quarter wave linear antenna that will

efficiently transmit and receive in the HF band would typically be in the range of 5 meters in length. Such a long antenna is heavy, flimsy and prone to vibration and wind damage, and is as inconvenient for motor vehicle use as a parabolic satellite antenna.

Further, although so-called whip antennae exist that can operate efficiently in the motor vehicle situation for certain radio frequency ranges, in addition to the above-noted problems, the antennae are unsightly and obvious. Thus, there are applications for the use of HF radio transmissions, such as stolen vehicle recovery systems or police work, where an inconspicuous loop antenna would be beneficial.

Therefore, it would solve a problem in the mobile radio communication field to provide an efficient, robust, inconspicuous and inexpensive antenna for a vehicle borne HF radio system for long-range two-way communication. Such an antenna would allow communication while the vehicle is either moving or stationary. Such an antenna would enable communication between the vehicle and either a base station, or another vehicle.

SUMMARY OF THE INVENTION The invention provides an antenna positioned around the underside edges of a motor vehicle chassis, for transmitting and receiving High Frequency radio signals in the 3 to 33 MHz wavelength region. In one embodiment, the antenna is a loop antenna, i. e., an open-ended, non-grounded radiating element, that extends around parts of three sides of the vehicle and uses the vehicle both as physical support, by attaching the high impedance antenna wire near the bottom edges of the vehicle chassis, and as part of the radiating portion of the antenna. Part of the radiating element of the antenna is formed by a wire positioned near the metal that forms the skin of the vehicle. This results in the metal of the vehicle becoming part of the antenna radiating element. In an embodiment of the invention the wire is positioned close to the bottom edge of the vehicle because the geometry of an edge results in higher electric fields and improved injection efficiency. Use of the bottom edge also results in higher capacitance between the antenna and the ground by minimizing the distance between what are effectively capacitor plates.

In a preferred embodiment of the invention the antenna comprises a motor vehicle with a loop of conductive material attached to the vehicle and connected to a radio transceiver inside the vehicle. The loop of conductive material is an insulated copper wire, preferably enclosed by a Teflon dielectric layer, and is not in direct electrical contact with the vehicle. The loop is attached underneath a part of the outside surface of the vehicle, and does not form a complete loop. The wire is not exactly at the bottom edge, but is preferably positioned 10-15 centimeters inside the perimeter of the vehicle and about 30 centimeters above the ground. For the HF radio band the insulated wire is approximately 5 meters long, and runs approximately half way along one side of the vehicle, along the backside, and about half way back along the second side of the vehicle, forming a three-sided open loop.

The loop is attached to an impedance matching tuner circuit connected to a radio transceiver inside the vehicle. With such an arrangement, the vehicle acts as part of an efficient antenna, both electrically and physically.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side view of a radio inside a motor vehicle, and an antenna in accordance with an embodiment of the present invention.

FIG. 2 is a schematic bottom view of the vehicle and antenna of FIG. 1.

FIG. 3 is a signal loss chart comparing a standard antenna to an antenna in accordance with the present invention.

FIG. 4 is a signal loss chart showing a radiation pattern for a standard antenna.

FIG. 5 is a signal loss charge showing the radiation pattern for an antenna in accordance with an embodiment of the present invention.

FIG. 6 is a bottom view showing a plurality of different length antennas in accordance with another embodiment of the present invention.

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The

drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a vehicle 10 that has a body at least partially electrically conductive. Typically, even vehicles having fiberglass bodies still have a steel frame and axles. The present invention requires only that the vehicle have some electrically conductive material along the lower edges of at least some of the body.

The vehicle shown may be any type of vehicle, including an automobile, bus or truck. The vehicle 10 contains a radio 12, that is grounded to the vehicle body at point 14. The radio also has an antenna lead 16 that connects the radio tuning circuit to an antenna 18, that is outside of the vehicle 10 by way of an insulated pass-through 20. The antenna 18 is typically in the form of a strip of conductive material or wire, and must be electrically insulated from the vehicle body. Thus, the pass-through 20, the lead wire 16, and the antenna 18 are typically coated with an insulating material. Since the antenna wire is outside of the vehicle and is likely to be exposed to abrasion due to dirt, water, rocks and chemicals, a tough insulating material such as Teflon might be a preferred material for insulation purposes.

The antenna 18 is secured to the body near the edge of the vehicle. The antenna may be positioned anywhere on the vehicle surface and still operate in accordance with the invention, but an antenna position close to the ground provides for a superior ground counterpoise and more efficient radiation. An antenna position that is close to an edge of the conductive material of the vehicle body also provides for better energy injection efficiency due to the geometry of the edge intensifying the electric field and, thus, also results in superior performance. The antenna 18 could also be positioned inside of the vehicle near a window or other portion of the body that is non-conductive and, thus, will not block or absorb the radio waves. Typically, interior vehicle locations do not provide as efficient antenna performance as exterior locations. In either case, it is important to prevent the antenna from electrically contacting the vehicle body.

The antenna 18 is shown in FIG. 1 as running along the lower edge of part of one side of the vehicle 10, and held in place and at a distance from the body by

holding fixtures 22,24, and 28. If the wire 18 is an non-insulated wire, the fixtures 22-26 would have to electrically insulate the wire 18 from the vehicle 10 body to avoid shorting out the antenna. It is preferred that the antenna 18 form what is known as an open loop antenna by running along at least part of one of the sides of the vehicle, then along the back portion of the vehicle, and terminating along the other vehicle side. The preferred length of the antenna 18 is dependent upon the particular radio frequency being used. For example, a radio operating at 11 MHz should have the antenna 5 meters long for best performance.

Note that the vehicle 10 is electrically isolated from the ground by the wheels 28 and 30. This is necessary because as noted above the vehicle and antenna wire represent one plate of a capacitor, and the ground represents the other capacitor plate. Thus, the closer the antenna is to the ground without making electrical contact, the higher the overall capacitance, and the better the efficiency of the antenna. The capacitor property also depends on how conductive the ground is.

Dry sandy soil will result in worse antenna efficiency than moist soil because the distance into the soil that the electric field has to penetrate to find enough counter charge effectively increases the distance between the plates of the capacitor and, thus, decreases the capacitance of the system. The wire 18 position is held a distance 32 above the ground. In a preferred embodiment of the invention, the distance 32 is 30 centimeters.

In FIG. 2, a bottom view of the vehicle 10 shows that the antenna wire 18 emerges from the vehicle 10 through the electrical pass-through 20, and forms a three-sided open loop antenna of a desired length. The wire 18 is held in place and at a desired distance from the body and the ground by a number of holding fixtures 40-56. FIG. 2 shows that the wire 18 is positioned a distance 58 inside of the edge of the vehicle 10. In a preferred embodiment of the invention the distance 58 is 15 centimeters.

In FIG. 3, a signal loss chart shows the difference between the antenna efficiency of an embodiment of the present invention, the closed curve 100, and a reference antenna, the closed curve 102, in terms of decibels (dB) of loss at an angle of 20 degrees above the horizontal. Both of the antennae are optimized and being operated at the same frequency, 11 MHz, and are over a poor dry sandy soil. The

reference antenna is a full-sied vertical monopole antenna having 8 ground radials, and is representative of a high efficiency static antenna, as is commonly used in the art.

The top of the chart is labeled 0 degrees and represents the direction of the open end of the three-sided open loop antenna. The closed end of the loop is shown at 180 degrees and has only about a 9 dB signal drop at point 104, as compared to the reference antenna. The loop antenna is less efficient toward the open end, but the worst signal loss is only 19 dB over the fixed monopole antenna at point 106.

The reason the chart shows the data from an angle of 20 degrees is that the HF radio band has an advantage of being reflected by the ionosphere. In order to hit the ionosphere, the radiation must leave the antenna at about 20 to 40 degrees above the horizon.

In FIG. 4, the radiation pattern of the reference antenna 102 of FIG. 3 is shown from a vertical cross-section, where the angle 0 degrees marks the horizontal direction in which the vehicle is facing, and 180 degrees marks the other horizontal directions opposite to the direction of the vehicle. The position 90 degrees marks the vertically straight up direction. It can be seen that the standard vertical monopole antenna radiates inefficiently at the horizontal and at the vertical directions. This is not a problem since, as previously noted, HF radiation should be broadcast at between 20 to 40 degrees above the horizon to most efficiently bounce off of the ionosphere. It may be seen that the reference vertical monopole antenna radiates most efficiently at 30 degrees, as shown at points 110 near 30 degrees, and 112 near 150 degrees. Taking FIG. 3 with FIG. 4 shows that the envelope of efficient radiation from a monopole antenna forms a sort of symmetrical donut shape around the vertical axis.

In FIG. 5, the radiation pattern 100 of the antenna in accordance with the present invention of FIG. 3, is shown from a vertical cross-section, where the angle 0 degrees marks the horizontal direction in which the vehicle is facing, and 180 degrees marks the other horizontal directions opposite to the direction of the vehicle. The position 90 degrees again marks the vertical direction. It may be seen that the vehicle antenna is radiating most efficiently in the direction of the closed portion of the loop and at an angle of 50 degree above the horizontal. This angle is

not as optimal for reflection off of the ionosphere for longest radio range as with the standard vertical monopole, but has been found to be sufficient for efficient vehicle based long-range communications. Note that in one direction the signal level is essentially the same as the standard monopole antenna at 30 degrees above the horizon, while in the other direction it is reduced by only 10 dB, which has been found to be an acceptable value in the mobile communications environment.

Because the efficiency of any particular antenna falls off as the HF frequency varies from the optimum for the antenna length, a number of different length antenna may be separately attached to the same vehicle, each antenna selected for transmission and reception of a specific frequency. Each one of the different antennae would be electrically insulated from each other and the vehicle body, as shown in FIG. 6, where a first longest antenna 200 exits the vehicle 10 at pass-through 202, a shorter antenna 210 exits the vehicle at pass-hrough 212, a yet shorter antenna 220 exits the vehicle at pass-through 222, and a shortest antenna 230 exits the vehicle at pass-through 232.

As noted previously, a HF antenna for a frequency of 11 MHz would use an antenna of about 5 meters. A HF antenna for a frequency of 33 MHz would be one third as long, or about 1 and 2/3 meters. Thus, an antenna tuned for each of the expected HF frequencies may be provided. It is preferred that the individual antenna wires be individually insulated. Thus, the same holding fixtures may hold all of the individual wires without interference since only one antenna at a time will be attached to the radio and powered up.

It is clear that other methods of placing differently tuned antennae may be used with the present invention. For example, there may be only one pass-through such as 202. The wire running from the single pass-through may be formed into different length antennae by simply using shielded cable until the desired position is reached to form the proper antenna length. This method has the advantage of only having a single hole in the vehicle and using the same holding fixtures for all of the cables. Thus, the single multi-stranded cable cluster would have better environmental resistance and simplified construction.

It has been noted that the HF antenna provided by the present invention are the most efficient in the higher parts of the HF frequency band, particularly above

11 MHz In addition, the higher the frequency the shorter the antenna needs to be.

Thus, it is likely that the present invention will be most useful in the higher portions of the HF band, specifically from 11 to 33 MHz.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.