A remote microwave relay - and the power of solar. I use solar to power a digital microwave relay at 10,400 feet on Bulldog Ridge in Utah's Henry Mountains. Two times, the system shut down when heavy snow caked the solar panels or clouds prevented recharge of the tower-mounted batteries. Similar difficulties forced removal of a remote cellular site on an adjacent peak. Last year we modified the site to add a 300-watt solar panel alongside the 260-watt panel, which had powered the load since 1994. The Gel tower-mounted cells were replaced with a pair of 375-a/h valve regulated lead acid Absolite 12-volt batteries. I wanted a more temperature controlled housing for the batteries. Where to locate them? Burial would require drilling and blasting. Too costly. Traditional housings didn't seem sufficient. So for $600, we bought a surplus mini-military communications shelter and set it alongside the tower. We had lots of room in the shelter for the batteries - plus room for accessory stuff to monitor and provide control of the site that was never before possible. A digital memory unit now stores bunches of data, including temperatures. We installed a spread spectrum 128K digital link to the site for command, control and downloading data. We thought to again use heat to melt snow off solar panels. So how much current is pulled by auto rear window defrosters? Frank held a compass next to the heat elements in a car, while Chuck (with the defroster on) pulled fuses until the compass dial moved in sync with the removal of that circuit's fuse. He then put a .01-Ohm 2-watt resistor in place of the fuse. A parallel volt meter indicated a quarter volt drop. Ohm's law said we were thus pulling 24 amps. Not enough watts to melt snow on our big panel! Back to the drawing board. RAY CHEM'S PTC Next came the discovery of positive temperature coefficient (PTC) plastic heat cable. It yields six watts of heat per foot. Self-regulating. To heat that stuff, we went to the nearby Flying J truck stop and bought off the shelf one of those 12-volt in, 1750-watt 115-volt AC output inverters. Truckers use them to power their mini-kitchens. Through a timer, the 115-volt output is remote relay routed to one of four PTC lengths along the backsides of the primary solar panel. It says here, this should melt any snow packed on the front side. To assure a tolerable smoke level, the current from that battery goes through a 200-amp fuse. We needed a shunt to measure current into the inverter. Looked in the catalogs. Dumb. Use your head! Make a shunt. One foot of #10 wire in series with the fuse provided exactly the .001 Ohms needed. Very accurate. Very cheap. After foam insulation was sprayed on the backside of the solar panel, cut sheets of aluminum were applied to protect the foam. Except the foam turned to liquid. Ugh. Second time let the foam dry. Cut off the bumps. Then the aluminum was placed over the hardened foam. When the 115 VAC is applied to any one of the four loops of the cold cable, the inverter pulls over 100 amps and stabilizes down to about 15 amps in less than a minute. An auto-shut down was incorporated in the site logic. We separated the four loops to separate feed lines off the power supply, and the self inducing regulation kept the current down to acceptable levels when we wanted heat to de-ice the single bed-sized solar panel. EFFICIENCY We have learned that ice and snow will stick to a solar panel with more enthusiasm than it will to panels with heavy insulation on the back side. Adding insulation can increase efficiency about 15%. This bit of information came as a complete surprise. We also learned that alignment of the panels to due south by GPS line up is an absolute necessity. Each degree of mis-alignment will degrade total performance by one percent of optimum output of the panel. Voltage and charge currents of each of the two solar power panels are logged to RAM every five minutes. To collect winter heat, the roof of the shelter is painted black. It is not uncommon for winter inside air temp to be 70º.   Copyright 2001 by A. W. Brothers and Americas Network magazine. All rights reserved.   © Beehive Telephone Co.