Communications: radio wave antennas – Antennas – Spiral or helical type
Reexamination Certificate
2000-06-23
2002-06-18
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Spiral or helical type
C361S328000
Reexamination Certificate
active
06407720
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention generally relates to antennas and more specifically to quadrifilar antennas.
(2) Description of the Prior Art
Numerous communication networks utilize omnidirectional antenna systems to establish communications between various stations in the network. In some networks one or more stations may be mobile while others may be fixed land-based or satellite stations. Hemispherical antenna systems, i.e., antenna systems omni-directional above the azimuth and having good front-to-back ratio in elevation direction, are preferred in such applications because alternative highly directional antenna systems become difficult to apply, particularly at a mobile station that may communicate with both fixed land-based and satellite stations. In such applications it is desirable to provide an omnidirectional (in the azimuth plane) antenna system that is compact yet characterized by a wide bandwidth and a good front-to-back ratio with either horizontal or vertical polarization (in the elevation plane).
Some prior art hemispherical antenna systems use an end fed quadrifilar helix antenna for satellite communication-and a co-mounted dipole antenna for land based communications. However, each antenna has a limited bandwidth. Collectively their performances can be dependent upon antenna position relative to a ground plane. The dipole antenna has no front-to-back ratio and thus its performance can be severely degraded by heavy reflections when the antenna is mounted on a ship, particularly over low elevation angles. These co-mounted antennas also have spatial requirements that can limit their use in confined areas aboard ships or similar mobile stations. The following patents disclose helical antennas that exhibit some, but not all, of the previously described desirable characteristics.
For example, U.S. Pat. No. 5,329,287 (1994) to Strickland discloses a device for use in a helical antenna having an antenna element wound about the periphery of a dielectric support post, the post being in the form of a tube or cylinder. The device has an electrically conductive member electrically connected to one end of said antenna element. The conductive member is of any appropriate shape or configuration and is operable to increase the loading on the antenna whereby standing waves on the antenna element are reduced and a more uniform electrical current is produced along the antenna element.
U.S. Pat. Nos. 5,485,170 (1996) and 5,604,972 (1997) to McCarrick disclose a mobile satellite communications system (SMAT) mast antenna with reduced frequency scanning for mobile use in accessing stationary geosynchronous and/or geostable satellites. The antenna includes a multi-turn quadrifilar helix antenna that is fed in phase rotation at its base and is provided with a pitch and/or diameter adjustment for the helix elements, causing beam scanning in the elevation plane while remaining relatively omni-directional in azimuth. The antenna diameter and helical pitch are optimized to reduce the frequency scanning effect, and a technique is disclosed for aiming the antenna to compensate for any remaining frequency scanning effect.
U.S. Pat. No. 5,701,130 (1997) to Thill et al. discloses a self phased antenna element with a dielectric. The antenna element has two pairs of arms in a crossed relationship to transceive a signal at a resonant frequency. A dielectric is disposed adjacent an arm to obtain a self phased relationship in the arms at the resonant frequency. The arms can form crossed loops or twisted crossed loops such as a quadrifilar helix antenna element. A dielectric collar on arms of the same loop causes currents to be equally spaced from one another. The antenna size is reduced and a cross section of the antenna element appears circular without degradation of a gain pattern when the dielectric is used on a certain arm.
In U.S. Pat. No. 5,721,557 (1998) Wheeler et al. disclose a nonsquinting end-fed quadrifilar helix antenna. In essence this patent uses a limited series capacitive loading along the antenna element length. The disclosed antenna is 4 wavelengths long and is an array. Each conductor of the antenna is fed with a successively delayed phase representation of the input signal to optimize transmission characteristics. Each of the conductors is separated into a number, Z, of discrete conductor portions by Z-1 capacitive discontinuities. The addition of the capacitive discontinuities results in the formation of the antenna array. The end result of the antenna array is a quadrifilar helix antenna which is nonsquinting, that is, the antenna radiates in a given direction independently of frequency.
Quadrifilar helix antennas having a diameter of between 0.1 and 0.25 wavelengths are good candidates for satellite communications since they have overhead cardoid shaped patterns of circularly polarized signals and reasonable front-to-back ratios. However, these antennas do have pattern limitations. For a practical, useful impedance bandwidth, each antenna element must be at least three-quarters wavelength long. For example, an antenna with elements of that length and a diameter of 0.125 wavelengths can be constructed with a pitch angle of 65°. For a higher pitch angle helix, i.e., greater than 50°, impedance bandwidth increases with element length, but much more slowly than, for example, a 40° helix which cuts in sharply near ¾&lgr; and then is well matched forever. If the 65° helix is to be well matched, e.g., near ¾&lgr; its impedance bandwidth, when translated to a characteristic impedance, e.g., a feed Z
0
of 50 ohms, is about 12%. If the effective length of the antenna is greater than three-quarters of a wavelength, the patterns start to multilobe and split above the horizon with the severity of the splitting in terms of the depth of the pattern nulls being determined by antenna element pitch angle. The observed nulls are less deep for sharper beam, lower pitch angle, helices. However, for any quadrifilar helix, the pattern does tend to flatten toward the horizon as frequency increases.
Stated differently, for all quadrifilar helix antennas, increasing the pitch angle broadens the pattern toward the horizon; lower pitch angles produce sharper overhead patterns. Normally the broader patterns near the horizon are desired for satellite communication so some flattening of overhead gain is permissible since the distance to the satellite is generally less overhead than near the horizon. While the impedance bandwidth can be increased by allowing the antenna elements to become longer as measured by wavelengths, this will also produce a multilobing problem above the three-quarter wavelength distance.
As described in the prior art, there exists a family of quadrifilar helices that are broadband impedance wise above a certain “cut-in” frequency, and thus are useful for wideband satellite communications including Demand Assigned Multiple Access (DAMA) UHF functions in the range of 240 to 320 MHz and for other satellite communications functions in the range of 320 to 410 MHz. Typically these antennas have (1) a pitch angle of the elements on the helix cylindrical surface from 50 down to roughly 20 degrees, (2) elements that are at least roughly ¾ wavelengths long, and (3) a “cut-in” frequency roughly corresponding to a frequency at which a wavelength is twice the length of one turn of the antenna element. This dependence changes with pitch angle. Above the “cut-in” frequency, the helix has an approximately flat VSWR around 2:1 or less (about the Z
0
value of the antenna). Thus the antenna is broadband impedance-wise above the cut-in frequency. The previous three dimensions translate into a helix diameter of 0.1 to 0.2 wavelengths at the cut-in frequency.
For pitch angles of approximately 30° to 50°, such ant
Dinh Trinh Vo
Lall Prithvi C.
McGowan Michael J.
Oglo Michael F.
The United States of America as represented by the Secretary of
LandOfFree
Capacitively loaded quadrifilar helix antenna does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Capacitively loaded quadrifilar helix antenna, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Capacitively loaded quadrifilar helix antenna will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2893398