By incorporating advanced techniques and an effi cient loop antenna
confi guration, a direction-fi nding antenna array has been achieved, which is
small and portable for 2 MHz to 30 MHz applications.
Direction finding (DF) at high frequencies (HF) of 2 MHz to 30 MHz has the basic problem of dealing with multiple signals due to long-range reception. The higher frequencies (above 30 MHz) have short-range operation, primarily line of sight. But at HF frequencies communication is not only line of sight, but also groundwave (hundreds of miles) and skywave (thousands of miles). See Figure 1. Thus, depending on the DF mission, the sensitivity of the antennas used must be carefully selected. The major consideration in establishing antenna gain requirements is the basic noise levels in the areas in which the DF system will be operating.
Figure 2 will establish the man-made noise power anticipated. The antenna gain required should be sufficient to achieve reception within the noise power to be encountered. In addition, it is important to know the signal density for the overall HF band. Indeed, according to the international frequency list of the IFRB, many of the frequencies are assigned to several users, especially for national services. This results in serious crowding of occupied frequencies in the lower portion of the HF band, up to about 10 MHz, within which about 62% of all allocated frequencies are. Figure 3 shows the percentage of the shortwave band for each 1 MHz bandwidth. The extremely heavy occupation of the available band, coupled with the large radiation range of HF shortwave requires careful design of the overall DF system, including the receivers and software. A typical high-frequency (HF 2 MHz to 30 MHz) direction-finding system is configured in a series of untuned loop antennas of three feet to four feet in diameter arranged in either a linear or circular arrangement. Three-foot diameter loop antennas are typically used because of their portable smaller size as compared to a monopole[1,2,3]. The monopole antenna height should be less than, say, eight feet to assure portability. Thus, at 2 MHz it would be about 0.016 wavelength. This equates to an efficiency of less than 1%. See Figure 4 for an 8-foot whip[4,5]. A loop antenna will be more efficient, and much shorter in height, making it more portable. Using multiple turns will further increase its efficiency. However, regardless of which antenna is used, it will require a large ground area; the spacing between each antenna will be at least four feet. What is required is a compact portable DF system based on advanced techniques using a minimum of ground space. Astron's initial design to achieve a portable DF system is shown in Figure 5.
Since early measurements of the original antenna configuration were taken, improvements were made in the antenna gain levels and decreased size. The techniques investigation included broadband antenna designs using dielectric/ferrite loading to miniaturize the antennas and increase portability.
Several designs were tested with the overriding concern being with the antenna mechanical size and complexity for a portable system, with acceptable set-up and tear-down times. Other designs have been investigated, prototypes built, and tested. The best approach for follow-on work was with the “Eggbeater” configuration (Figure 6).
Astron has built many versions of the “Eggbeater” antenna and range tested to provide measured results instead of modeled results. Attempts to improve performance by designing matching networks have yielded limited success because they often introduce resonances due to coupling interactions between elements. Modeled results have been unreliable due to problems with the modeling software. This problem has been held tightly within the HF NVIS community and suggests that results in error are obtained for antennas with NVIS capability — i.e., small HF antennas with horizontal currents in close proximity to the ground.
Astron, after considerable R&D efforts, developed the highly portable single compact array, of two mutual orthogonal rectangular loops, nicknamed the “Eggbeater” for obvious reasons (Figure 6). It has multiple feed points to eliminate resonances and minimize mutual coupling effects down to the smallest elemental levels. The Eggbeater has an upper section with feeds, a middle section that is passive, and a lower section with multiple feeds and a small mast extension to attach the antenna to the electronics enclosure and tripod assembly. The diameter of the overall Eggbeater is about 26 inches and its height is about eight feet. The weight of the antenna elements are readily handled by one person and are detachable in convenient-length segments.
The omnidirectional and sine gain are shown in Figure 7 along with the gain specifications. The gain tracking of the omni to sine gain is shown in Figure 8 and is ±6 dB over the band. Additional improvements in gain tracking may be achieved with a frequency-compensated attenuation circuit on the omni port in the mid-band region improvements.
Cross-coupling between antennas
Cross-coupling between all of the antennas within the small DF antenna array can cause distorted RF patterns, and LOB inaccuracies, if not seriously considered. Some of the techniques used to assure an accurate DF array are:
relative position and shape of the antenna elements;
current distribution in all of the antenna elements relative to each other;
co-location of all pertinent components in each sub-band;
interconnections between ground planes optimized; and
use of ferrites to decrease RF coupling fields.
Astron Wireless uses state-of-the-art computer simulation software, laboratory, and field testing, while incorporating the Astron HESA (high-efficiency, sensitivity, accuracy) antenna, technology beam-former network, and other component technology. This process results in cost-effective, active and passive DF antenna systems for diverse applications. It exhibits optimum and enhanced performance against its larger counterparts, yet occupies the smallest form factors available to date.
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, often times exceeding but always maintaining performance parameters equal to their larger counterparts.
H.A. Wheeler, “Small Antennas,” Chapter 6, pp. 6-9, R.C. Johnson, “Antenna Engineering Handbook,” McGraw Hill, 1993.
H.A. Wheeler, “Fundamental Limitations of Small Antennas,” IEEE, vol. 69, pp. 1479-1484, December 1947.
L. J. Chu, “Physical Limitations of Omnidirectional Antennas,” J. Appl. Phys., vol. 19, pp. 1163-1175, December 1948.
R. F. Harrington, “Effects of Antenna Size On Gain, Bandwidth and Efficiency, “J. Res. Nat. Bureau of Standards, vol. 64-D, pp. 1-12, January/February 1960.
R. C. Hanson, “Fundamental Limitations in Antennas, “IEEE, vol. 69, pp. 170-182, February 1981.
ABOUT THE AUTHOR
Joseph R. Jahoda, Astron's chief scientist, 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 military DoD R&D for ECM and communications systems ever since. Jahoda can be reached by phone at: 703-450-5517; e-mail: firstname.lastname@example.org; and fax: 703-450-9753.