[aprssig] SWER Testing
Bob Bruninga
bruninga at usna.edu
Sun Jun 28 21:31:02 EDT 2009
Single Wire - Earth Return Power Distribution.
On the way home from Field Day, I finally had a chance to do the test I have been wanting to do for decades... See how far I can deliver power over a single wire. (for eample, to get power out into a wilderness station) from my car back in the parking lot.
I will need this for the upcoming APRS Appalachian Trail Golden Packet attempt on 26 July because my station is on top of a 70 foot observation tower which is itself on top of a several hundred foot hill, which is about 3000 feet from the nearest place I can get my car.
See www.aprs.org/hamtrails/GD-hill.html
Of course, plan A is to use SOLAR panels and small batteries, but if it is cloudy, then the only way to maintain a 50 Watt packet station for 6 hours is via some really heavy batteries, or this SWER system.
Having picked up a 6" spool of #20 wire (about 3200 feet) for next to nothing, I couldn't wait to see what we could do.
To test it, we found a rural road, stuck a broom handle out the window and let the wire fly. Cut a normal 8' ground rod into three pieces and drove them into the ground about 4 feet apart. That was one end. Then at the other end (the hiking end) I used four very small 2 foot rods 3/16ths inch) spaced about 3 feet apart.
Path length was about .45 mile or 2500 feet between ground rods (though I had 3200 feet of wire but just ran out of straight road). The wire resistance is about 35 Ohms.
Using only 214 VDC input (conveniently from the Prius), I got a 100W light to normal brilliance (measured 115 volts across it) meaning there was 100 volts loss in the ground loop system. This implied a 100 Ohm loop system (32 ohms was in my wire). SO I achieved about 65 Ohms ground system with my little rods. Doubling the depth of the rods or the number of rods would halve the resistance.
Performance of 4 rods yielded 114 volts to bulb
Performance of 3 rods dropped voltage to 108
Performance of 2 rods dropped voltage to 82
Performance of 1 rod dropped voltage to 47.
The 100 Watt bulb of course is very non-linear, so I will have to bench test it to find out what the currents were drawing at the other 3 test points. But because of the spacing of the rods being farther than the depth of the rods, the resistance should have been relatively independent yielding a linear response to the number of rods.
This test was entirely satisfactory. Especially since in practice, I will not use the Prius battery, but will use a 150 watt 115 VAC inverter from the vehicle 12v system so that I have complete isolation of the SWER loop from the vehicle. I will use 2 caps and 2 diodes to double that to 330 VDC which will further reduce the loss in the loop resistance. In any case, I will have more than enough power to power my 50 Watt station on the mountain top.
See the web page (still a work in progeress):
www.aprs.org/aprs-swer.html
SWER systems can be done just as safely as other power distribution systems if done correctly. They can also be done dangerously by idiots. Idiots, please do not try this at home.
Bob, WB4APR
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