I last blogged about last January.
What has proven to be the largest challenge so far has been getting access to a site where we have line of sight for most of our sites of interest. This challenge was finally overcome when we signed a site agreement with Lehigh Southwest Cement to build a sector site on an abandoned water tower on the northern edge of their property, which is on the western edge of Cupertino.
Permanente Quarry. Lehigh Cement wasn't able to provide us power this far from their plant, so it looks like we need to run the site off-grid.
As for the hardware up the tower, we're using a Mikrotik SXT SAr2. (Yes, it's really there; it's tiny!) This gives us a relatively wide 90 degree beam width, which for this site covers all of Cupertino with a single radio. All wisdom about deploying point-to-multi-point says that trying to cover this much of this area is going to be a bad time. Frankly, I'm going to be relatively shocked if we make it very far with just this one sector antenna, but getting to the site and adding more radios is so easy that I figured we would give this a wag and see how far it gets us. We're not looking for a huge amount of bandwidth (~5Mbps), so we can tolerate lower levels of quality than if we were trying to operate a consumer WISP.
CdTe thin film solar panels (BP Solar 980L). You might say 320W of solar panels for a single 5W router might be a little over-kill, and I wouldn't disagree with you. We are going to have a very comfortable power budget (which is going to keep our batteries really happy), and this leaves us plenty of room for growth as this site gains additional sectors, point-to-point links, etc over time without us needing to revisit the power system.
I deeply, deeply regret having gotten frameless thin film solar panels for this project. It's the timeless story: guy finds guy on Craigslist. Guy offers guy solar panels at $0.75/W. Guy gets great deal on solar panels before entirely appreciating what a pain it is sourcing 6mm frameless panel clamps. Getting the edge clamps for these panels to mount them to the unistrut was a huge pain, and I'm never dealing with it again.
100Ah 12V deep cycle lead-acid batteries, wired in series to make a 100Ah 24V bank for the solar charge controller.
The middle shelf holds an EPSolar Tracer 2215BN solar charge controller, a 24V-12V DC-DC regulator to provide some 12V power for the network switch and fan, and an unmanaged workgroup switch with a PoE injector for the access point on the tower. The solar controller warrants some particular attention: this is by far the least expensive solar controller I've seen which actually uses Maximum Power Point Tracking (MPPT), which is a big deal. Lots of $20 charge controllers on eBay will claim that they are MPPT, but they simply aren't. MPPT means that the controller is going to make an effort to hold the panel array at the correct voltage/current to extract the maximum amount of power available from it given the current solar conditions, which isn't a trivial task since this magic point moves throughout the day based on the amount of sunlight and temperature, and the fact that the panels are wired as an 80V array while trying to charge a 24V battery bank. This takes a serious DC-DC buck converter, which is why MPPT controllers need such a beefy heat sink compared to the cheaper PWM controllers.
The top shelf is currently mostly empty, waiting for more equipment or a second battery bank. The only thing there now is an MT-50 control head for the solar controller. This is another feature of the EPSolar Tracer series controllers that I absolutely love: their user interface is over RS-485 MODBUS, so you can mount your controller as close as possible to your batteriy bank and load, and all you need to do is run CAT-5e to wherever you want your nice user interface to the controller. The control head lets you set the battery charge mode, etc, as well as review all the collected statistics like current panel/battery/load voltage/current and cumulative power generated/consumed in kWh.
SHAMELESS PLUG: I'm giving a talk on solar panels and designing these sorts of solar power systems next month in San Jose on April 13th, 2016 at the West Valley Amateur Radio Association. If you're in the Silicon Valley and interested, I encourage you to come see.
Of course, a huge thanks again to our great team of volunteers, the city of Cupertino for supporting us on this project, and a HUGE thanks to Lehigh Southwest Cement for entertaining some random group of volunteers setting up some equipment on their property. The Cupertino ARKnet wouldn't be possible without everyone's help.
(Thanks to Marcel AI6MS and Jim KN6PE for letting me use their photos in this blog post)