Antenna formed within a conductive surface

Details Antenna formed within a conductive surface Antenna formed within a conductive surface

1999-09-11

2001-12-11

Ho, Tan (Department: 2821)

Communications: radio wave antennas

Antennas

Including magnetic material

C343S7000MS, C343S769000

Reexamination Certificate

active

06329958

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of antennas, and more specifically to low-profile, conformal, broadband platform-mounted antennas.
2. Description of the Related Art
The descriptions and examples included herein are not admitted to be prior art by virtue of their inclusion in this section.
A wide range of frequencies is currently used in military and commercial communications, from about 3 MHz to about 3 GHz. Although much of commercial cellular telephone communication uses frequencies of about 800 MHz and above, the lower-frequency portion of the above range is very important for applications including military and public safety communications. Commercial pagers also operate at a relatively low frequency of about 150 MHz. Advantages of lower frequencies include improved diffraction around and penetration through obstacles such as walls and foliage, and reduced path loss and attenuation in air, resulting in longer transmission lengths for a given power level. The frequency range from 3 MHz to 30 MHz, designated as the “high-frequency” (HF) communications range, and the 30 MHz to 300 MHz range, called the “very-high-frequency” (VHF) range, are of interest for the lower-frequency applications described above.
Wavelengths in the HF and VHF ranges are on the order of meters to tens of meters. For communications in these ranges, and particularly for mobile communications, it is thus generally necessary to utilize electrically-small antennas, or antennas with geometrical dimensions which are small compared to the wavelengths of the electromagnetic fields they radiate. Unfortunately, electrically-small antennas exhibit large radiation quality factors Q; that is, they store (on time average) much more energy than they radiate. This leads to input impedances which are predominantly reactive and in turn allows the antennas to be impedance-matched only over narrow bandwidths. Furthermore, because of the large radiation quality factors, the presence of even small resistive losses leads to very low radiation efficiencies. In particular, the radiation Q of an electrically-small antenna is roughly proportional to the inverse of its electrical volume, and is essentially inversely proportional to the antenna bandwidth.
It is desirable to communicate over broad frequency ranges, particularly for military communications, in which a wide range of frequency bands is used. Furthermore, military communications may involve bandwidth-intensive techniques such as frequency-hopping to avoid interception and jamming. The above-described constraints on electrical size vs. band-width suggest that physically large antennas and/or multiple antennas are needed to cover a broad frequency range, or large bandwidth, at low frequencies. In practice, large antennas in the form of tall, high-profile whips are frequently used for mobile communications at HF and VHF frequencies. These whip antennas have disadvantages, however. Their high profile makes the antennas observable and relatively fragile. In military applications, high observability is disadvantageous because communications systems are high-priority targets for an enemy. Even in non-combat situations, whip antennas can be damaged during travel in forested terrain, for example. The use of multiple relatively narrowband antennas in order to cover a broad bandwidth is undesirable because it increases system complexity. It would therefore be desirable to develop a low-profile, broadband electrically small antenna.
SUMMARY OF THE INVENTION
The problems outlined above are in large part addressed by a method of employing current-blocking, or “choke”, structures to channel current flow on a conducting surface in order to force the current into patterns more conducive to radiation. The choke structure may be in the form of a cord or belt which is arranged upon the surface of a conductor to define one or more lines or shapes. Current in the conductor is prevented from passing over, through, or under the choke structure. Alternatively, the choke structure may be in the form of plates or tiles arranged upon the conductor surface such that areas of the conductor are defined through, over and under which current is prevented from flowing. The current-blocking lines, shapes and/or areas described above may also be formed by broad-area deposition and subsequent patterning of a suitable current-blocking material. The choke structures recited herein, when applied to a conductive structure and combined with a suitable feed arrangement, are expected to result in extremely low-profile broadband electrically-small antennas.
Proper prediction of bandwidth vs. electrical size requires inclusion in the electrical size assessment of any images of the antenna resulting from ground planes or other conducting objects. For example, a large flat conducting ground plane can effectively double the electrical size of the antenna. In this case, the “image” of the antenna is an exact replica of the antenna. It has been shown that finite-sized conducting ground planes or other objects can enhance the operation of an antenna beyond that of an infinite (or electrically-large) ground, especially when the finite ground plane or object is near resonance. Thus it is possible to arrange a situation in which the image of an antenna is actually larger than the antenna itself. Electrical size limitations in the HF and VHF bands make it desirable to somehow utilize a larger-than-life image in order to extract reasonable impedance bandwidth out of a small antenna. This amounts to effectively “using the vehicle as an antenna”. In the extreme case, the “antenna” is an electrically-small, near-field probe which would, by itself, radiate very little energy. However, when coupled electrically or magnetically to another larger object, such as a vehicle, this probe excites currents in the other object, causing it to radiate. Both capacitive and inductive probes have been used in previous attempts to exploit vehicles or other objects as radiators: examples of inductive coupling include electrically-small coils or multi-turn loops (MTLs); examples of capacitively-coupled probes include planar inverted-F and inverted-L configurations. Rigorous approaches to utilizing vehicle bodies as radiators have been published which involve deriving the eigenmodes of the vehicle, synthesizing a desired radiation pattern from these modes, and then designing a probe/antenna to excite the modes.
Unfortunately, in all of the above-described previous attempts, only modest improvements in bandwidth have been obtained (over that obtained when the antenna or probe is operated over a ground plane). Attempts to utilize part of a vehicle or other platform as an antenna using an electrically-small probe generally fail because the probe must be large enough to excite currents over a large part of the vehicle. When the probe is nearly the same size as a conventional antenna, little size or observability reduction results. Even if currents could be excited over the entire vehicle, disadvantages are believed to be associated with using an entire vehicle as a radiating element because the vehicle's radiating characteristics could change as the vehicle encountered different grounding situations. For example, a rubber-tired vehicle would be essentially ungrounded when on dry land, while it would be quite well grounded when immersed up to its axles in water or mud. This and other environmental conditions could change both the radiation pattern and input impedance of the antenna, resulting in unpredictable radio system behavior. Furthermore, exciting RF currents over the entire skin of a vehicle could pose a serious radiation hazard to personnel on or near the vehicle.
The method and antenna structure described herein are believed to address these problems by “breaking up” the conductivity of the body of the vehicle in order to (1) force the currents excited by a probe to travel over a large part of the vehicle and (2) confine these currents so that they occupy only the

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