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How do you design, develop and produce an airborne blade antenna to cover 30 MHz to 2000 MHz and achieve greatly improved performance, especially at the low end of the band for SINCGARS applications, while maintaining its small size for mounting on subsonic aircraft and helicopters? Astron has had a series of R&D/production programs over the last nine years achieving miniaturized antennas for communications and direction finding (DF) arrays for the United Kingdom, U.S. Army/Navy, Australia, and others for submarines, ships, UUV/UAV, aircraft and land mobile systems.

The resulting Astron antenna and direction finding array technology has evolved into Astron's HESA technology. This set of advanced techniques provides for high efficiency, sensitivity and accuracy in miniaturized and standard DF antenna systems. A major benefit of the HESA platform includes the ability to co-locate multiple antennas and their associated electronics in a small package; oftentimes exceeding but always maintaining performance parameters equal to their larger counterparts.

To operate at the lower frequencies (antenna height is much less than /4) we can expect lower gains. As an example, at 30 MHz /4 is 8.3 feet. We are allowed about 18 inches for the airborne blade height. The antenna being much shorter than /4 at 30 MHz will have its VSWR suffer and the required broadband matching networks will present attenuation and a decreasing gain.

It is obvious that, certainly at the lower frequencies, in order to increase the antenna gain, it is essential that the antenna be miniaturized. It was, therefore, important to examine the fundamental limits and parameter trade-offs involved in antenna size reduction. First we need to evaluate the size vs. operating frequencies and bandwidths of conventional antennas. This analysis is based on the work of the Astron and Virginia Polytechnic Institute (Professor Warren Stutzman) team performed on a recent DAPRA antenna miniaturization program.

The size of the antenna is measured by the radius, r, of a sphere that just encloses the electrical radiating area of the antenna and its ground plane mirror image^[1]. This size should be measured as an electrical size, which is the principal size relative to a free space wavelength, as shown in Figure 1.

The radiation characteristics of electrically small antennas (r=0.16) were first investigated by H.A. Wheeler^[2]. One year later, L.J. Chu^[3], derived an approximate expression for the minimum radiation Q of a small antenna. In 1960, R.F. Harrington^[4] extended Chu's theory to include circularly polarized antennas. Shortly thereafter, R.E. Collin and S. Rothchild^[5], and R.L. Fante^[6], derived exact expressions for the radiation Q based on evanescent energy stored around an antenna.

To provide closure on the history of these important concepts, it is to the credit of R.C. Hansen^[7] that he assembled, integrated and promulgated much of this information. These corrected and refined relationships are plotted in Figure 2.

The efficiency family of curves includes the dependence of antenna radiation efficiency,. The top curve is for 100% radiation efficiency. It shows that for a small electrical antenna, say for Kr<1, Q increases dramatically as size is reduced.

To use the curves of Figure 2, a point is located for the antenna Q (center frequency of the operational bandwidth) and the antenna's value of Kr at the center frequency of operation. The enclosed radius, r, of the antenna is based on the antenna configuration's effective aperture area (see Figure 1) and includes any image of the antenna in the ground plane.

Astron has been using these curves in many of its R&D programs for DoD and found them an excellent guide in determining the potential gain, bandwidth, Q and efficiency for advanced designs and recognition of the point during antenna developments at which further efforts will not provide any significant improvements in bandwidth and efficiencies unless dielectric and/or ferrite loading is used. The use of this type of loading is the only way to defeat Chu-Harrington criteria.

To illustrate the use of this powerful dielectric/ferrite antenna-loading tool, let us apply it to the external dielectric/ferrite loading of a monopole, as shown in Figure 3. A test was run on the Astron VHF/UHF calibrated range to verify the theory regarding the ideal location of dielectrics and ferrites on a monopole antenna to achieve maximum foreshortening. A special jig composed of a hollow 20-inch low RF loss fiberglass cylinder (1-inch O.D.) was devised for holding a 20-inch, 0.125-inch O.D. copper rod at the tube's center in a vertical position. The first test, 89A, involves getting the copper rod (operating as a monopole) RF response vs. frequency with the HP network analyzer set to sweep over the frequency range of 20 MHz to 160 MHz. The 20-inch rod monopole has a maximum response at its resonance frequency of 135 MHz, which is shown in Figure 3. Its efficiency at its resonance frequency should be 100% and indeed it is. Note the 100% efficiency mark at the 132 MHz mark obtained from the initial range calibration.

The next test, 89C, involved filling the lower five inches of the fiberglass cylinder jig (still holding the 20-inch rod at its center axis in a vertical position) with a powdered ferrite having a relative permeability of µ_r = 4. The powder surrounded the rod at its base, externally loading the bottom five inches. The maximum foreshortening expected would be the square root of 4 or 2:1. The 0.125-inch copper rod is only partially loaded with ferrite material. The effect of the ferrite was to lower the resonance frequency to 112 MHz (down from 135 MHz). This effectively achieved only a foreshortening of 1.24, the 20-inch rod's electrical length effectively being increased from 20 inches to 24.8 inches.

The next test, 89H, removed the ferrite powder and added five inches of dielectric powder (having a relative permittivity of = 4) at the top five inches of the rod. The result was similar to test 89C, a decrease of the resonance to 112 MHz and an effective foreshortening of 1.24.

The last test, 89P, left the five inches of dielectric at the top five inches and returned the five inches of ferrite powder at the bottom of the 0.125-inch rod. The resonance dropped to 98 MHz, a foreshortening of about 33%, and increasing its effective electrical length to 28 inches. In all tests the efficiency was essentially 100%.

These tests showed that the optimum position for the ferrite is at the base (where the current is the highest) while the optimum position for the dielectric is at the top (where the voltage is highest).

The data in Figure 4 clearly shows the expanding gain/bandwidth for a constant height antenna (12.5 inches) while varying its width incrementally to 12 inches, 5.8 inches and 0.125 inches. It was the combining of the previously discussed HESA technologies that enabled Astron to achieve the desired JTRS/SINCGARS airborne blade antennas, an example of which is shown in Figure 5. Other HESA technologies are available for direction finding arrays, SATCOM and UHF/VHF/UHF communications antennas.

1. H.A. Wheeler, “Small Antennas,” Chapter 6, pgs 6-9; R.C. Johnson, “Antenna Engineering Handbook,” McGraw Hill, 1993.

2. H.A. Wheeler, “Fundamental Limitations of Small Antennas,” IEEE, vol. 69, pgs 1479-1484, December 1947.

3. L.J. Chu, “Physical Limitations of Omnidirectional Antennas,” J. Appl. Phys., vol. 19, pgs 1163-1175, December 1948.

4. R.F. Harrington, “Effects of Antenna Size on Gain, Bandwidth and Efficiency,” J. Res. Nat. Bureau of Standards, vol. 64-D, pgs 1-12, January/February 1960.

5. R.E. Collin and S. Rothchild, “Evaluation of Antenna Q,” IEEE TAP vol. 12, pgs 23-27, January 1964.

6. R.L. Fante, “Quality Factor of General Ideal Antennas,” IEEE TAP vol. 17, pgs 151-155, March 1969.

7. R.C. Hansen, “Fundamental Limitations in Antennas,” IEEE, vol. 69, pgs 170-182, February 1981.

Joseph R. Jahoda, Astron's chief technology officer, founded Astron 27 years ago. He graduated from College of the City of New York in 1950 with a B.E.E. and from Polytechnic Institute of Brooklyn in 1954 with a M.E.E. He has been involved in R&D for ECM and communications systems ever since. Jahoda can be reached at phone number: 703-450-5517; email: jjahoda@astronwireless.com; and fax: 703-450-9753.