RA-NS VHF Antenna Arrays
The RA-NS Precision Direction Finding Antenna series is designed for maximum accuracy and repeatability in direction finding with minimum field maintenance. The RA-NS series employs two beam antenna arrays, critically separated by a lightweight metal cross boom (model RA-CB). The received signals from both antennas are combined precisely out of phase, producing a pattern as shown in Figure 2. The deep forward looking null (-10 to -30 dBV) in the center is bound on either side (+ or - 15 to 30 degrees), by a point of maximum gain (peaks). When this system is combined with a compass rose assembly and digital data processor (TDP-2), a repeatable accuracy of ± 0.5 to 2.0 degrees can be attained when working with animals in conditions of relative inactivity and line of sight (LOS) conditions exist.
The RA-NS series should not be confused with the classic peak-null systems that have been employed for many years in the wildlife field. Under careful technical examination, the peak-null system, as used for precision location work, has fallen far short of its promised performance in obtaining consistent bearing accuracy of ± one degree over sustained periods of time and varying field conditions. There are sound technical reasons for the variable performance achieved with such systems. First, peak and null antenna patterns are obtained by attempting to control the phase of the radio signals received by each antenna through the use of coaxial cables of various lengths and impedance. Impedance transformation is accomplished in a like manner. The use of coaxial cables to accomplish phasing and the presence of an inline switching and control box create standing waves along the coaxial transmission lines. The waves result from improper impedance termination at, and within, the control unit. The standing waves along transmission lines effectively make the coaxial cables a portion of the antenna array itself, and result in a variable incongruity in the system which is most dramatically amplified under the changeable environmental conditions encountered in the field. Specifically, the bearing accuracy degrades if the cables change in relative position to one another, the mounting mast, cross boom, and to ground objects. This problem is further amplified if cables, cable connections, or antenna terminations become wet with precipitation, moist snow, or dew. In general, the performance of such a system is best under mild, dry conditions and when animals are located at relatively short distances from the receiving antenna site (one mile typically). Consistent performance degrades rapidly as distance increases.
The RA-NS Antenna Systems do not rely on cabling to accomplish critical multi-purpose functions. Phasing and impedance transformation are controlled by a model TAC-5 precision phase combiner, which presents a 50 ohm impedance at any input or output port over the frequency range of interest regardless of the input impedance characteristics. The problems associated with the standing waves in coaxial transmission lines are alleviated by proper termination. The system is therefore insensitive to disturbances such as wet or randomly position cabling encountered under everyday field conditions. The system is very forgiving even if the entire array is damaged and the antenna elements must be reformed into shape. Eyeball alignment of the elements and confirmation of the location through the use of a truthing or beacon transmitter is adequate to maintain the accuracy of the system. Cabling does not have to be cut to critical lengths as long as the lengths of the two input cables are kept equal.
- 1 degree bearing accuracy is possible using the deep forward null in the pattern.
- Accuracy is not dependent on the gain of the individual antennas selected.
- Selecting lower gain antennas and using horizontal polarization minimizes the received signal bounce.
- The use of the TAC-5 assures repeatable accuracy not possible with "old peak null" technology.
- The array has been mounted on vehicles for portability as well as permanent installations at fixed sites.
|Each RA-NS array includes:|
|2 ea.||Antennas (see choices below)|
|1 ea.||RA-CB cross boom|
|1 ea.||RA-TC "Tee" clamp (connects boom to mast)|
|1 ea.||TAC-5 Precision two port combiner|
|2 ea.||RW-3-17 cables
(17 foot length accommodates mast heights up to 12 feet above ground level.)
|1 ea.||RW-2 cable (connects Combiner to Receiver)|
|1 ea.||Set of drawings for mast base, compass rose assembly, antenna mounting and cable dress|
|User Must Provide:|
|1 ea.||1.25 - 1.7 inch O.D. Mast, 17 feet long|
|1 ea.||Mast Base with Thrust Bearing and Compass Rose Assembly|
|Please Specify Choice of Antenna:|
Mounting: Fixed Site or Mobile
It is critical that large arrays such as the RA-4A or RA-3 be mounted on a mast 20-30 feet above the ground to avoid biasing the results due to adverse proximity effects. In the case of either of the above, the antenna arrays can be end-mounted on the cross boom in either vertical (see Figures 4 and 5) or horizontal polarization (see Figures 3 and 6). Mounting the antennas in horizontal polarization with respect to the earth increases the array's immunity to reflected signals (bounces) entering the capture area of the antenna ray from the sides. Horizontal polarization is the preferred mounting technique for mountainous terrain where signal bounce is a major problem. It is critically important that both antenna beams are mounted in exactly the same way, not as mirror images (see Figures 3, 4, 5 and 6). A non-conductive cross-member (PVC or varnished wood) can be placed on the forward end of the antenna to assure that the physical relationship between beams remains constant. If this parallel relationship is disturbed, the main forward facing null will be shifted to one side or the other, and re-calibration of the system will be required.
Figure 3. RA-NS-3 in Horizontal Polarization
Figure 4. RA-NS-3 in Vertical Polarization
Figure 5. RA-NS-4A in Vertical Polarization
Figure 6. RA-NS-4A in Horizontal Polarization
The RA-NS RA-2A array can be utilized for both mobile and fixed site applications. The array is mounted as shown in Figure 7, when vertical polarization is desired. The array is small, lightweight, and presents little wind resistance. Further, only a 12 foot (approximately four meters) mast is required for mounting the array. The array is frequently mounted in the bed of a pickup truck or on the roof of other vehicles. If extensive or frequent movement of the array is required, it is best to develop a mount that can lie down in a truck bed to avoid vibration damage to the antenna.
The choice of antenna is determined by the system gain required. Unlike single antenna arrays where beam width (and therefore bearing accuracy) is a function of the number of elements in the array, the accuracy of the RA-NS precision system is independent of antenna gain. The RA-2A system two element antenna is preferred for short range fixed-site location work or mounting in the bed of a pickup truck or the roof of a car. In the RA-NS system, the small antennas provide the same bearing accuracy as the larger arrays. The user is therefore free to enjoy the reduced weight, wind drag, ice loading, and reduced physical dimensions of the small system. Spacing between two antennas remains the same, regardless of the type and size of antenna employed.
Antenna choice should be based on desired system sensitivity (operating range and signal strength). Always use the smallest antenna possible to avoid confusion due to reflected signals. All Telonics Receiving Antennas are trimmed to frequency and tested for bandwidth and VSWR. When ordering, specify lowest and highest frequencies that must be covered by antenna system (i.e. 164.0 to 166.0 MHz). Please contact our laboratory for further information.
The TAC-5 Null Combiner, shown in Figure 8, is a precision instrument. The TAC-5 forces a 180 degree phase shift between the paired antennas and thereby positions a narrow deep null directly in front of the array (Figures 9 and 10). The null is bound on either side by a point of maximum gain (peaks). The depth of the null or area of minimal signal reception, -10 to -30 dBV), is easily detected by the human ear. Precision direction finding (bearing location) with repeatable ± 0.5-2.0 degree accuracy can be obtained with little training. The RA-NS series is far more effective for precision direction finding work than classic single Yagi beam arrays. As previously mentioned (unlike single beam arrays), bearing accuracy is not a function of antenna gain.
Figure 9. RA-2A Null Polarization Plot
Figure 10. RA-4A Null Polarization Plot
The RA-NS Antenna Array does not employ a switching box to switch between a null mode and a peak mode. If a peak mode is desired a TAC-4 or peak combiner can be used to replace the TAC-5. The antenna pattern in the peak mode is shown in Figure 11. Although the system was initially fielded with both TAC-4 and TAC-5 combiners, it soon became apparent that the 3 dB additional gain by the TAC-4 was almost undetectable to the human ear and was capable of adding only 15 to 20 percent increase in range performance. For this reason, the TAC-4 is seldom used in actual field situations.
Although the TAC-5 is sealed, it is advisable to protect it from direct prolonged exposure to moisture. The unit should be mounted at the bottom of the mast, where it can be connected to the coaxial antenna feed lines, when in use. If the study area receives a great deal of precipitation, a drip loop should be made just in front of the TAC-5 from the antenna cables, cover the BNC cable connectors with CON-BNC-CAP-F connector caps to avoid mechanical damage and corrosion.
Coaxial cable connection to the driven element is shown in Figure 12. Note that the center conductor of the feed line is attached to the UPPER side of the driven element. It is critical that BOTH coaxial cables connected between the driven element of both antennas and the antenna ports of the TAC-5 Combiner be exactly the same length (within 0.25 in.), although the actual length is not critical as long as they are equal. Coaxial cable placement should be as close to the center beam of each antenna, the horizontal cross boom, and the vertical mast as possible. Particular attention should be paid to keep all cables away from the tips of the antenna elements; cables should never be allowed to dangle between elements. The outer braid or shield of coaxial cables is at RF ground potential and will electrically detune the antenna when placed between antenna elements. This results in improper (and sometimes erratic) feed impedance, changes in the phase relationship between elements, and a shift in the operating frequency of the array.
Note: Use Telonics type RW-3 cable for lengths less than 25 feet and type RW-5 cable for lengths from 25 to 100 feet.
If metal guy wires are to be employed to stabilize the antenna mast, it is critical to attach such cables to the mast at a point two wavelengths below the tip of the lowest elements to avoid adversely biasing bearing measurements as a result proximity effects in the near field of the antennas. If guy wires must be attached to the mast at a point closer than 12 feet (approximately three meters) from the elements, nonconductive guys should be used.
Caution: Choose nonconductive guy material carefully. A material such as nylon is generally chosen, as it has the ability to wick water quickly and dry rapidly. Damp rope and twine is electrically conductive at VHF RF frequencies. Cotton and other soft fiber materials are undesirable as they remain damp for extended periods of time. Hollow rubber/fiber tubing cord with a steel wire in the center is unacceptable as well (i.e. clothesline cord).
A compass rose assembly, as shown in Figures 13, is critical to obtaining fast, accurate bearing locations and for calibration of the system. This assembly is designed for use without thrust bearings, but can be modified to include the model RA-TB thrust bearing, thus allowing smooth rotation of the RA-NS Array during manual direction finding.
Note: In general, electrical antenna rotors are accurate to only ± 5 degrees in addition to other system errors. They are therefore unsuitable for precision work.
Calibration for Bearing Location
Electrical calibration of the RA-NS System requires deployment of a beacon reference transmitter(s), such as the model TBT unit. The array can be referenced to the beacon transmitter bearing at relative 0 degrees, or the entire array may be corrected to true north through the use of a compass.
Exacting electrical calibration of the RA-NS System is essential to obtaining high accuracy directional bearings. Mechanical and electrical bearing location may be slightly different. This could occur if the antenna elements of both antennas are not exactly parallel as a result of the initial attachment of the antennas to the cross boom, high winds, or other abuse. Electrical calibration and periodic re-calibration at multiple points will provide assurance that the system is performing optimally and with repeatability.
Deployment and Positioning of Antenna Sites
No direction finding antenna array can compensate totally for signal bounce, multi-pathing, or the inability to receive signals in a certain location when the animal is out of range or behind a major point of geographic relief. Antenna sites must be carefully chosen, especially if the site will be permanent and require a considerable investment of time and money to install. It is an extremely unique study area that can consistently obtain accurate bearing locations for triangulation from only two antenna sites. Virtually every study area contains locations where the animal/transmitter is either out of range, hidden behind a geographic prominence, located at a point where a bounce (reflected signal) gives false information to one or both sites, or located in an area directly between the two antenna sites. The simplest solution is often accomplished by moving the antenna sites closer together. However, error polygons may be unacceptably large in some part of the study area. Specifically regions at the outer perimeter of the range performance of the antenna sites may be especially problematic. Continuing to assume the use of a system with fixed sites, the most economical means of substantially reducing error polygons throughout the study area is the deployment of a third antenna site as indicated in Figure 14.
It is critically important to remember that this very simple analysis does not include additional errors resulting from reflected signals. Guidelines are simply established for maximization of system performance. Ultimately each study area must be carefully evaluated and completely ground-truthed in order to establish the most efficient antenna site positioning. Three sites are the minimum number required to do serious precision location work on most species. (Even with this number of sites, areas of ambiguity can exist). If the data collected is critical, and bearing accuracy and high locational resolution must be obtained, a total of five antenna sites are recommended. In many study areas a minimum of five sites are absolutely required for conducting location work with the precision necessary for contemporary studies of habitat utilization. Following the establishment of fixed receiving sites, the entire study area must be ground-truthed again with a rabbit test transmitter. The transmitter is moved throughout the entire study area and its known locations are compared with the bearing locations obtained from each antenna site. Given sites will provide discrepant information in the form of incorrect bearings when the "rabbit" transmitter is located in certain parts of the study area. It is necessary to note the antenna sites that cannot be relied upon when the animal is in a particular area. The investigator must then avoid using those sites as information is being collected throughout the study. It is advisable to map these areas of inaccuracy for each site early in the study. Any directional fixes taken subsequently which fall within these areas are then automatically excluded in the data reduction phase.
More information on recommended aircraft search patterns: Telonics Quarterly V10, N1, Spring & Summer 1997, "Aircraft Tracking"
Additional links to the following Telonics Quarterly articles: