Nonlinear transmission lines keep developer on the fence
Frank W Bell, Kybernetix LLC - January 17, 2013
In 1982, I was developing a phone and telemetry system to operate on New Zealand farmers’ electrified fences. The controller that generated the jolt was usually a mains-powered pulse generator, and better units produced mostly unipolar pulses of up to 5 kV for a duration of about 20 msec, repeated about once a second—not lethal, but enough to kill any notion of touching the fence again. The high voltages were even effective on unshorn, woolly sheep.
Such a fence behaves as an earth-return transmission line with an impedance of around 600Ω, depending on construction. The system polarity is decided based on resistance to lightning, which usually consists of negative current into the fence from a direct strike, though positive current may come from a strike nearby. The system I was developing used biased blocking diodes to prevent the pulse from frying the electronics (not to mention avoiding the loud click in the ear). Detection of a dc bias on the fence turned on a DTMF decoder to select the receiver.
Field trials were essential. On one farm, the DTMF decoder provided an output burst a couple of times and then died. Testing with a blocking diode and oscilloscope showed a positive pulse of 12 μsec after the negative controller pulse. Assuming a velocity factor of 0.7, that gave a range of 1.25 km, or about three-quarters of a mile. When I checked the fence leakage to ground, however, it was more than 3 kΩ for the whole farm. The controller included a circuit to eliminate positive pulses. How could a low impedance have inverted the pulse polarity?
I drove out in a truck to the 1.25-km distance on the odometer. I didn’t see any stray wires going into the ground, a common problem. When I got out of the truck, though, I noticed an audible click with a 1-sec interval.
I took a look at the fence. The farmer had used steel posts and had cut lengths of plastic hose, with a lengthwise slit, for insulators. A thin wire tied the hose to the post. Though there was insulation, the air gap was breaking down with the pulse from the controller and producing a positive polarity reflection. I explained the problem to the farmer, who later installed proper insulators and told me the fence worked “much better now.”
After diagnosing the cause of the failure of my prototype electric fence telephone/DTMF controller, I installed additional clamping so that positive pulses would not reach the DTMF decoder. Normally, farmers did their testing without any instruments—though perhaps with rubber gumboots—by touching the fence and a blade of grass, but testing in this environment required battery-powered oscilloscopes, as the voltage on the grounding points could easily exceed the rated insulation of the mains-powered equipment. The educational material for the controller installation made it clear that the controller ground had to be well separated from the ground of the ac mains because of such voltages.
On another farm, connections between phones along the fence were intermittently dropping out between controller pulses. I checked out the fence, which was so rusty that I had to work the alligator clips—with insulated pliers, of course—to get a connection to the metal underneath. The connections between wires were simply loops holding loops. These also were rusty connections, and the arc of the pulse temporarily welded the wires together. Poor connections also cause AM radio interference when they get rusty. The farmer replaced the rusty fence, which was becoming unreliable anyway.
In order to examine the high-voltage pulses, we modified a Heathkit TV high-voltage probe using a capacitor made from car-ignition cable with a metal conductor, a braid on the outside, and a divider and a compensation circuit to correct the square waves from a calibration source (tube scope) of higher- than-normal voltage. The Tektronix EHT probe would have been better, but it wasn’t affordable.
The patent for the telemetry system eventually expired without resulting in a successful product.
Frank W Bell is president of Kybernetix (Clifton, NJ).