HU191908B - Flat Archimedian Spiral Antenna with Closed Spot Cavity, Broadband Range Directed and Circularly Polarized Radiation Radius - Google Patents

Abstract

A self-complementary planar symmetrically fed Archimedean spiral antenna, closed with a stepped reflector cavity, realizing a broad-band directed and circularly polarized radiation field, characterized in that the prescribed operating band is divided into subbands corresponding to the ratio .....8:4:2:1...., satisfying the condition of the minimum number of steps, and the distance of each step plane from the plane of the spiral antenna is where \ is the wavelength calculated from the band center frequency of the frequency range covered by the i-th step plane.

Description

The present invention relates to the use of a stepped structure reflector (or reflector cavity) which provides broadband operation of a flat symmetrical powered Archimedean spiral antenna placed on the mouth of a specially shaped reflector cavity.

In the mid-1950s, the use of the Archimedean spiral antenna type introduced by Turner became commonplace in the frequency range of 500 MHz to a few GHz. The use of the antenna type above 500 MHz is limited by relatively large geometric dimensions and above 10 GHz by design problems.

In practice, with a single Archimedes helical antenna excited in the axial mode (without reflector), self-complementary (with the same width of the joystick inserts), a frequency response of 7: 1.8: 1 was achieved. However, the property of a helical antenna operating in its own axial mode of radiation to radiate in both directions along an axis perpendicular to the spiral plane severely limits the use of f.

The radiation can be directed (unidirectional) by means of a reflector placed behind the spiral, but in this case the bandwidth of the antenna is significantly reduced. In the literature, antenna solutions providing directional radiation are limited to a maximum of 2:

frequency range without substantially changing the antenna characteristics in the center of the band. There are also known solutions in which the bandwidth is significantly greater than 2: 1, but the antenna parameters (especially the gain of the antenna) change significantly in the given operating band.

The reflector design of the present invention allows the bandwidth to be expanded in any proportion, in principle, so that the antenna characteristics do not change much more than with commonly used antennas having a frequency band of 2: 1.

The extension of bandwidth in the manner described herein is based on the well-known (theoretically proven) fact that at a frequency from the surface of the spiral antenna (more specifically, the threads forming the surface), not only one thread and not a full aperture (surface). radial surface of a particular width radiates intensely. Current band theory, which explains the operation of spiral antennas, is known to change the active frequency of the antenna by changing the excitation frequency of the antenna. a radiating ring wanders (widens or narrows) between the inner and outer strands of the aperture.

The essence of the bandwidth extension method is to create conditions in which the radiating ring displaced by the frequency change always sees the reflector behind the antenna at the optimum tuning distance λ / 4 according to the current frequency.

The stepped design of the reflector cavity shown in the figure, if not at all discrete frequencies, but within each frequency band, can meet this requirement. because

908 B 2 provides a maximum bandwidth of 2: 1 with each step (similar to a traditional reflector-cavity or "single-step" antenna), thus aiming at a minimum number of steps to achieve a wider band than an ordinary antenna (since at this time, 8) 4: 2: 1: .... etc. should be divided according to proportion. If this division cannot be performed for the specified lower and upper cut-off frequencies, then the operating band must be arbitrarily widened so that this division can be made.

In this way, increasing the number of steps may, in principle, result in an antenna operating in any wide band. More precisely, bandwidth can be achieved as with a spiral antenna alone. The frequency band can be sub-divided into sub-bands in smaller ratios than the max 2: 1 ratio, but in this case the number of steps and thus the number of interfering edges increases. /

Advantageous features of the invention are particularly true for antennas designed for a minimum number of steps. These beneficial features are:

- The special reflector design makes it possible to create a very wide band directional and circularly polarized radiation with a simple construction solution.

- Its size is smaller compared to other types of antennas operating in the same frequency range and having similar characteristics (eg conical spiral). The reduction in size makes a major difference especially for antennas operating in the 50-500 MHz band. This difference in size is striking when comparing a prototype antenna operating in the 100-500 MHz band with a conical log spiral antenna manufactured by HTV and operating in the same band.

- A flat Archimedes spiral (or semicircular approximation) is simpler and less expensive than a cone-coiled log spiral antenna.

Disadvantages: At the frequencies that can be assigned to the stairs' front (edges) (and its low VAT), a slight distortion is expected with respect to the shape of the directional characteristic, with no significant reduction in profits.

As a more detailed mathematical examination of the inhomogeneous cavity has not yet been carried out, the differences cannot be determined by calculation at present. However, in the case of prototype antennas (2) made in order to justify the invention, we found a remarkable deviation in the frequency response of the antenna characteristics during the measurements.

As the prototype antennas were designed with no experience of the interference caused by the edges, we did not have any measurement results, therefore the location of the stairs (edges) of the sample antennas was determined on the basis of frequencies that are not used during the antenna operation. less significant. After measuring the antenna, we found that this caution was unnecessary.

Preparation of prototype antennas, sitting. experience gained during the operation can be summarized as follows:

-2HU 191 908 Β

The two working antennas have proven that, in principle, the broadband antenna can be constructed with the reflector solution of the present invention. In practice, bandwidth is a function of the balun transformer bandwidth used, as well as a greater variation in the characteristics of the spiral antenna, in particular of the input impedance with frequency. fluctuation limits. In fact, the Archimedean spiral antenna cannot be derived from a relation derived by V. H. Rumsay, which gives the geometry of strictly frequency independent antennas, such as the single-angle log spiral antenna.

This is the reason why the antenna characteristics of a stand-alone spiral antenna change with frequency, though slowly. Thus, in practice, the greatest problem with bandwidth expansion is to compensate for the variation of the input impedance over the widest band.

The frequency change of the antenna input impedance and other characteristics can be reduced (slowed down) by stepwise increasing the sword width per frequency band (step) by radially outwardly moving the aperture of the antenna so that in these ranges, i.e. (almost the same). The antenna thus realized can be seen as a discrete approximation of a tightly coiled (coil constant a <1; r-exp a <£>; where a is a helix), whereby the antenna characteristics become a slower-changing function of the frequency.

The external threads of a flat Archimedean spiral antenna mounted on the mouth of a stairwell are preferably covered with a material having dissipative properties. sealing the ends properly eliminates the detrimental effects of waves reflected from the ends. to reduce it. This solution allows you to reduce the size of the antenna geometry. It is not necessary to leave the outermost turns, whose role is merely to reduce the effect of the reflected waves.

Claims (8)

PATENT CLAIMS A self-complementary flat symmetric powered archimedes helix antenna with a stepped structure reflector cavity, providing a broadband directional and circularly polarized radiation field, characterized in that the specified operating range meets the minimum step ratio condition ... 8: 4: 2: 1. is divided into sub-bands corresponding to the ratio ... and the distance of each step plane from the plane of the spiral antenna where \ is the wavelength of the center frequency of the frequency range encompassed by the i-th step plane. (Ft) Antenna structure according to Claim 1, characterized in that the operating band is divided differently from the 2: 1 frequency coverage providing a minimum number of steps, so that none of the steps achieves a band coverage greater than 2: 1. Antenna structure according to Claim 1, characterized in that the width of the lever of the printed flat helix antenna above the reflector formed on the basis of the minimum number of steps is formed so that the number of passes remains constant over the step planes encompassing the 2: 1 frequency bands. 4. Antenna structure according to claims 1 to 5, characterized in that the planar archimedean spiral antenna placed in front of the step reflector is not self-complementary. 5. Antenna structure according to claims 1 to 5, characterized in that the planar archimedean spiral antenna placed in front of the step reflector is a single-arm (asymmetrical power supply). 6. Antenna structure according to Claims 1 to 3, characterized in that the ends of the outermost threads are coated with a dissipative material. closed so that the size of the antenna structure is smaller in diameter. 7. Antenna structure according to claims 1 to 3, characterized in that the cavity of the stepped reflector is closed by a tightly wound (exp a <p; a <l, where "a" is a helix constant) log spiral antenna. 8. Antenna structures according to claims 1 to 5, characterized in that planar antennas have different profiles (e.g. 0; 0; ςύ; etc) are made up of a leader.