HARDWARE

The hardware consists of a remote antenna-preamp assembly and signal analysis package. The antenna assembly should be mounted clear of metal objects for several loop diameters. In most cases, several feet above a building roof will be satisfactory. Generally, the interconnect cable between the antenna assembly and analysis package can be up to 100 feet long. The system requires a power source of 12 VDC at about 25 milliamperes. The RS232 data format is used to transfer sferics data to the PC serial port.
Two concentric loops are used for the antennae. Obviously, the loop construction should be sturdy since the direction resolution is related to physical loop arrangement and stability. The remote preamps should be mounted directly below the loops. As an example, the loops can constructed with PVC pipe and copper tubing by threading the tubing thru two sets of perpendicular holes in the pipe about 18 inches apart. At the top of the pipe, each loop tubing can be bent slightly to eliminate any contact between the loops. A PVC pipe splice (secured with several screws rather than glue) can be used below the loops to provide access to the loop connections and remote preamp.

Either individual or a multipair individually shielded pair cable connect the loop assemble to the analysis package. All shields are connected together only at the lower end. Each signal channel and power lines are isolated pairs in the cabling to reduce noise pickup.

Signals from the loops/preamps are applied to converter circuitry which is controlled by microcontroller to provide direction-of-arrival, pulse polarity and pulse width data. Briefly, the two loop signals are integrated by U2-C and U2-D during a specific portion of the sense signal as detected by the discriminators (U7-A, U7-B). After this integration, the two integrators are connected in a quadrature oscillator arrangement by the microcontroller. The time it takes the oscillator to reach a predetermined condition is proportional to the direction-of-arrival.

The peak amplitude circuitry (U6, U7-C and U7-D) generate trigger pulses which are analyzed by the microcontroller to determine the polarity and pulse width from the last two polarity excursions.

A parts list has been included to help securing the components. Most components are not critical and reasonable substitutes are acceptable. The microcontroller must be programmed which will be discussed.

The initial setup aligns the loop-sense tuned circuits. After these initial adjustments, the test generator can be connected to the converter/discriminator circuitry (subsitute for preamp). The generator provides simulated antennae signals which can be used to evaluate and calibrate the system. The high rate (1/SEC switch open) is best for viewing waveforms with a scope while the low rate (switch closed) provides a better rate for setting the calibration factor via the PC software. The high rate is a good test for the PC software but may introduce a few errors in the accumulated data. The op-amps outputs are sensitive to capacity loading, therefore, a low capacity probe should be used for all scope testing. The SYNC signal provides a negative going signal for scope triggering.

For relatively accurate direction calibration, use the WEST signal (switch closed) and no SOUTH signal. The direction calibration (about 200) is set with the "K" command to the microcontroller. The final calibration must be done by observing the data over a period on time and comparing with the actual weather location.

The microcontroller (PIC 16C73) program generates a three character serial data string for each input event. In addition, several functions are available to control the direction calibration , timing factors, and signal sensitivity.

Format for the three character event follows:

First character:

Second character Third character

Several commands can be sent to the controller in a three character sequence. The first character is a "don't care", second character is the command, and the last character is either a "don't care" or binary data. When a response is required, the data is the binary value of the returned character.

Format for the commands follow:

The PC control program includes provisions to read and set the above factors.

A self-extracting file may be downloaded hardware.exe which contains the above schematics, parts lists and PIC program.

Question or comments?

Please contact

Dick Fergus W9DTW

rfergus@theramp.net

Revised 11-13-98