Category Archives: SCADA

MPPT Solar Battery Charging in the Arizona Desert

I’ve got a 125 W solar panel feeding a 12 Vdc 85 Ah deep cycle battery at a comms site in the southwest Arizona desert. I’m using a Rich Solar MPPT-20 solar charge controller (SCC), which so far appears to be doing a great job. The load on the battery is a Raspberry Pi with two NESDR Smart radio dongles, running software called RWMon, and decoding ATCS indication messages being sent from railroad control points along the I-8 corridor, but that functionality isn’t the subject of this article. It’s the battery management that’s the thing!

Figure 1 – 24-Hour Battery Voltage

It’s a tough environment out there in the desert, and determining an appropriate energy storage technology is part of the challenge. I settled on a lead-acid battery, mainly because it tolerates higher working temperatures, and since there are plenty of charge management devices that understand lead-acid chemistry really well.

In Figure 1, battery voltage is shown over a 24-hour period. This particular 24-hour period was dead clear during the daylight hours, so the curve is very clean.

Figure 2 – Manufacturer’s Chart on Battery Charge Management

Figure 2 is the chart right out of the Rich Solar MPPT-20 charge controller manual. From about 0700 (the sun begins to directly illuminate the panel) to around 1130 the SCC is in the fast charge mode. From 1130 to about 1330 the SCC is in the sustained charge mode; from 1130 to 1730 the SCC is in float charge mode. It’s nice to see that the real-world situation replicates the manual pretty accurately.

There are other nice things to discuss about this SCC, including very low RFI generation, that future notes will touch on.

UPDATE

I realized that I also have for the same setup the voltage characteristics for the PWM SCC that I had out there last month before switching to the MPPT unit.

In Figure 3, the units are wrong, but the curve is the important bit.

Figure 3 – PWM SCC Voltage Curve

Here, it’s a much cruder controller, but the same kind of curve is evident. The drop off from boost to float is a very slow ramp, that’s probably not as good as what the MPPT does.

Nice filter for 1090 MHz ADS-B site

Over a year ago I decided that I needed some good filters for ADS-B reception on mountaintops. Not that I had an immediate need, I didn’t have anything on a mountaintop, but I suppose I had a little extra cash and felt excited to be able to imagine a good ADS-B receive site atop an Arizona mountaintop (or hilltop) location. So, I drafted up what I thought would be a reasonable spec and went into Alibaba to find a filter manufacturer to build one. I ended up with 5 filters, all exactly to my spec, and for a bargain price (well, relatively speaking).

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Above is the finished ADS-B receiver assembly, complete with Raspberry Pi, RTL-SDR 1ppm TCXO SDR dongle, a eBay-purchased LNA, an eBay-purchased 12 vdc to 5 vdc DC-DC converter, and some coaxial cabling (also from eBay). The black square in the center of the image is the 1090 MHz filter, and it’s a quite good one.

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It’s a straightforward cavity filter, a little aluminum brick with fine performance.

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Solid out of band rejection, and I suspect around -100 dB ultimate rejection. The SA just doesn’t have the range to see it.

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While the signal of interest is only a MHz wide, I wanted a filter that was wider so that temperature and mechanical variation would never haunt me, and I wanted a low bandpass loss (the above shows less than 1 dB loss) across the band.

Behind the filter is a run-of-the-mill eBay wideband LNA with a 1 dB NF, and somewhere around +30 dBm IP3. The RPi is running the most current version of FlightAware’s PiAware, rev 3.0.4, and supports just about any off-the-shelf USB SDR dongle.

After setting it up, it looked like I needed to reduce the overall gain a bit, so I discovered how to go into dump1090 and change the gain from “automatic” (really not, I think it’s just max) to 42 dB. That gave me best range and most received a/c.

The antenna for the site is a FlightAware fiberglass stick, about 12′ above the ground, mounted on the side of the tower.

HeyWhatsThat_30aug16Coverage seems to be pretty close to the model generated by HeyWhatsThat.com (above). The blue line is the 40,000′ contour, while the orange line is the FL300 contour.

24 hours or so of actual flight logs produces the following plot, which is more or less pretty similar to the HeyWhatsThat plot.

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The primary notch in the pattern, in the SE, is the higher part of the ridge on which the radio site sits. It ends up blocking any coverage of flights in and out of Tucson, over 110 miles away, until said flights get to FL300 or so.

It will be interesting to see how the coverage shapes out over the next few weeks – I hope that it will get up near the top of all the local receive sites in performance.