Category Archives: Solar power

volt/amp/watt/energy meters

Got another two of the inexpensive volt/amp/watt/energy meters from mfreelaa on eBay. These are pretty nice, especially for the $12 each including shipping.

I put two of them in series to see how well each tracked the other. There should be a very small drop in voltage for the second on in series as opposed to the first. That does appear to be the case.

The unit under test is pulling approximately 1-1/4 amps at 12 volts. Assuming these ammeters use 100 A/75 mV shunts (since the meters without built in shunts do the same and it’d be easier to keep them all the same) this would imply that the second meter should read about 0.001 volts lower than the first. However, the display precision is only 0.01 A so it’s not practical to see the difference with this small load.

After two hours now, both meters are showing 50 w-h, I’ll have to wait longer to see if they diverge from one another at all. The voltage (refreshed once per second) is generally within 0.02 volts, which is 0.15% agreement. The current reading is looser, maybe 0.2%. The power reading is quite close, but one would expect that the power reading comes from the multiplication of the v and i values. The energy reading is the time cumulative sum of all the power measurements.

Next project is to measure the efficiency of my new MPPT solar battery charger. Just need to have a sunny day!


Here it is the next morning and the DUT has now pulled a total of either 261 or 262 W-h, depending on the meter read. That’s still pretty good internal consistency between meters, well under 1% difference.

So, relatively speaking, these meters are pretty consistent in their performance. Perhaps one of these days I’ll set up a calibrated load to get a better handle on the absolute accuracy. But for now, just knowing that the meters are consistent allows me to do the solar MPPT charger testing.

Update on the rooftop amp

Last time we visited the roof, the amp followed by the FM BCB notch filter was now in the die-cast enclosure, but not actually attached to anything.  Now it finally has a home, at least for now, on the tripod leg. It required a visit to Artie’s Ace Hardware in Phoenix at Tatum and Thunderbird, which until about 8 hours ago was unknown to me as a purveyor of a near infinite number of different kinds of metric fastener! Only 4 miles away, it’s a treasure to know that I can get an M4x8 mm pan head screw even late in the afternoon.

The metric hardware was required to install the steel mounting ears on the die-cast enclosure; those mounting ears accept the muffler clamps that hold the whole thing to the leg of the tripod. Later on this winter I’ll bend up some 0.032 Al sheet to act as a sun shield and remount the box on the north leg with the shield to keep it cooler during the summer. I still need to do something permanent about the power for the amp, it’s currently the solar power setup I made a couple weeks ago.

Left is input, right is output. Runs on any voltage up to about 32 vdc and down to about 7 vdc. The internal dc-dc converter keeps the amp supplied with an even 5.0 volts.

With this amp in place, my stack’o-scanners is just bangin’ along. I’ve got great reception, and no FM BCB interference. And, there’s space in the enclosure for a future Arduino or Raspberry Pi, as well as the necessary network connection.

Next step on the rooftop LNA setup

This past weekend I finally started building the ultimate case to house the LNA and FM BCB filter for the rooftop multireceiver project.

Last night I did the final bit of wiring, installed a DC-DC buck converter to take +12 vdc and knock it down to 5.0 vdc.

Here’s some pics of the project.

The two separate die-cast enclosures are the LNA (lower) and the FM BCB notch filter (upper). There’s a LM2596 DC-DC converter in the lower left, and a weathertight Ethernet connection in the center-right wall. All external RF connectors are N. All internal connectors are TNC. All coax is RG316 double-shielded. I used an Ethernet connection to get power to the box and to allow the future addition of either an Arduino or an Raspberry Pi for telemetry purposes.

The overall enclosure with installed components. Got this particular enclosure for about $22 delivered – someone in Santa Maria CA didn’t want it. Was missing the base plate upon which I mounted all the components – fortunately, I had a piece of Al in the garage that was a near-perfect fit! Would have cost nearly $70 new.

The left N connector is the antenna input, the right N is the assembly output to the shack. The Ethernet connector is for future expansion and dc power.

Installed on the roof temporarily (will be mounted on the tripod to right ultimately) with power supplied by batteries in the Tupperware container.

The batteries (Eneloop AA x 10) are charged with a newly modified solar panel install. Again, over the weekend I cut some Al extrusions to replace the old way I’d attached a single 5 w solar panel, now it supports two 5 w panels for a total of 10 w. This almost guarantees sufficient charge for long winter nights.

In addition, the box mounted on the tripod already had a PoE Ethernet connection from the main rooftop unit. I use that 12.6 volt PoE  through a single 1N4001 rectifier to source power to the LNA box when the solar panels can’t provide enough charge to keep the batteries up. I want to add some telemetry (through a future Arduino or similar on the roof) to measure voltages, temperatures, etc.


One weird trick for making the amp behave

Alright. Things are looking much better again. Moved the LNA to its rightful position just 2 m of LMR-195 behind the antenna. Put the FM BCB band stop filter behind the LNA. Since I’m not sending dc up the coax right now, and since the BCB stop filter is in the way anyway, I bit the bullet and built myself a quick and dirty solar charging battery supply for the LNA!

5 w solar panel, purchased a decade ago from Harbor Freight; 8 NiMH AA cells in a series holder; LM2596 buck converter to reduce the battery voltage to 5.0 v; a Tupperware container to hold the batteries and converter. Impressive, no? %^) Let’s see how it survives the night.

Here’s the new block diagram – note the PCR-1000 is on one leg of the first splitter, so it should have no more than about 4 dB additional loss than when directly connected to the coax from the roof.

001 – 100 MHz, w/LNA, w/filter, w/o-3dB-splitter:

001 – 100 MHz, w/LNA, w/filter, w-3dB-splitter:

Here, the added loss from the splitter is apparent from DC to 80 MHz or so. It apparently doesn’t pass low frequencies well at all. Since the antenna is not rated that low anyway, it’s good to get rid of additional interfering signal…

100 – 200 MHz, w/LNA, w/filter, w/o-3dB-splitter:

100 – 200 MHz, w/LNA, w/filter, w-3dB-splitter:

It seems that the splitter has greater than 3 dB additional loss until up around 140 MHz. The aviation band (118 – 136 MHz) is about 7 dB worse than without the splitter. But that’s it. No other weirdness. I can live with this.

Am much more satisfied now. LNA is directly behind antenna. Have a new experiment to see how solar battery charging works out.