Mountain RF Sensors Direction Finding (DF) Systems
System Description
The direction finding (DF) system is based on an amplitude-based method. An antenna with directional characteristics is electronically rotated, normally at 150 Hz. The RF output from the antenna is fed to the receiver/processor. The electronic commutation is synchronized with the receiver/processor. The signal is processed digitally to isolate the baseband commutation modulation. From the amplitude variations, the processor can determine a line-of-bearing (LOB). The LOB solutions are provided at 150 per second, the same as the rate of commutation.
DF Antenna
The antenna employed in the MtRF DF system is a cavity-backed annular slot antenna. This antenna displays a cardioid directional pattern. By placing multiple taps around the periphery of the slot element, the pattern can be electronically rotated by sequentially selecting the various tap points.
For applications requiring an omnidirectional pattern, the antenna is configured to sum 3 or more tap points to produce an omnidirectional pattern. When employed as a DF antenna, a minimum of 3 taps is required to produce a bearing solution, with 4 taps being the industry norm. The Mountain RF DF system normally configured with a 6-tap antenna, resulting in lower peak RMS errors than the 4-tap configuration. The DF type can also serve in an omnidirectional mode.
The slot antenna is unique in the fact that it can be flush-mounted while providing 360 degrees of azimuth coverage. The antenna is vertically polarized. Optimum vertical angle is +/- 30 degrees, but it will function at angles exceeding 45 degrees, at reduced accuracy.
The slot antenna has extremely wide bandwidth, approaching 2 decades. For example, a 12” diameter slot element is useful from 10 MHz to 400 MHz. The upper frequency limit is reached when the slot diameter is equals ½ wavelength. Above the limit, the cardioid pattern develops secondary lobes. There is no sharp boundary on the lower limit. The sensitivity drops as the frequency is lowered, and the accuracy will begin to roll off.
Since there is a trade-off between sensitivity and maximum frequency, users should avoid overspecifying the upper frequency range since there will be a sensitivity penalty. The slot elements can be nested inside of each other, offering the flexibility to extend the upper frequency range. There is some interaction between elements, so separate elements are preferred, where possible.
The system accuracy can approach 3 degrees RMS in an ideal environment, but factors on the platform will reduce this number. This is common to all DF systems, to some extent. Strategies such as dual-symmetrical antennas can negate some of the platform influence. Where required, the system can be calibrated to the platform, something that is not oftendone due to expense and time constraints.
In addition to the low drag and small diameter, the slot antenna is very light. The complete antenna (element, FRP mount & electronics) is only about 2 lbs. The mounting depth is 2”-3”.
Receiver/Processor
In the Mountain RF DF system, the digital receiver also processes the LOB. The commutation at the antenna is controlled by a microprocessor at the antenna. This architecture simplifies the interface between the receiver/processor and the antenna. The antenna and processor communicate via an RS-485 link. Only power, ground and sync are needed in addition to the serial link, so the interconnect cable requires only 5 wires.
The receiver is a direct-conversion SDR for frequencies up to 55 MHz. Models to 520 MHz employ a single conversion an IF frequency of 45 MHz or 160 MHz. The combination of a 16-bit A/D and processing gain give a dynamic range of 120 dB. There is no need for analog AGC in this situation, something that less-capable SDR systems still require.
2 MHz – 55 MHz SDR/DF Processor (Eurocard Module)
Wideband Slot Antenna, 6-Tap DF Type
MtRF-6124 5 to 200 MHz, 24”
MtRF-6118 7 to 270 MHz, 18 “
MtRF-6112 10 to 400 MHz, 12”
MtRF-6110 15 to 500 MHz, 10 “
MtRF-6105 30 to 1,000 MHz, 5”
The listed slot antennas are the “standard” sizes, but they can be easily fabricated for almost any frequency range.