Figuring out how to make the antenna track the sun was fun and interesting. I started out with a book called "Sundials: Their Theory and Construction" by Albert E. Waugh (ISBN: 0-486-22947-5). It takes you through the basics of how the sun appears to move through the sky. Remember that it's the Earth that's rotating... In the appendix of the book are several helpful tables of data. I ended up using some of that data in the PIC firmware.

The key to all of this is something called The Equation of Time (EOT), which describes how the position of the sun at noon varies throughout the year. From that equation, one can calculate the Solar Azimuth Angle and the Solar Elevation Angle.

The problem is that both require calculations of sin, cos, tan, and their inverse functions. I'm trying to do this in a relative small PIC processor. The only way to get there is to figure out how much precision I really need.

One Aside: I know I could run some planetarium software on my PC and just send standardized messages to the antenna driver to move it. That would be fine if I was willing to always have the application running on my desktop. I like to build stuff that requires less direct control by my PC. In this case, I want the antenna system to track the sun by itself, with higher level status and control to/from the PC.

Anyway, the half power beamwidth of the HD-6000 at 108 MHz is about 70 degrees, so that gives a fair amount of leeway in Azimuth. I don't have any data for the vertical beamwidth, but I decided that 5 degree steps in each direction should be fine. I worked from there and decided that I needed to move the antenna every 20 minutes to keep it reasonably well aimed at the sun.

To do all of this with integer math, I decided to scale the trig functions by 1000: 0-0.999 becomes a three digit integer from 0-999. To conserve memory, I have one table that converts integer angles from 0 to 90 degrees into the sin times 1000: sin(45) = 707. From this one table, I calculate the sin of any angle by inverting the angle and/or result. I use similar tricks to get the cos of any angle. The tricky bit is figuring out when you need to divide by 1000 to stay in scale.

Here's how it ends up working in the software.

Get the time from the RS-485 port. See my Precise Time and Frequency Page for details of where that comes from. Convert it from UTC to Local Mean Solar Time(LMST) which corrects for the difference between solar time and my exact longitude, and adjust it for today's equation of time. The EOT correction is done every morning at 2:30 AM.

Every 20 minutes, calculate a new position for the antenna:
Look up today's solar declination angle
Calculate the hour angle from the LMST
Calculate the Solar Elevation using the trig functions
Calculate the Solar Azimuth using the trig functions
If the sun is above the horizon, move both motors.

I have one place where I need to fudge to make up for the limitations of the integer math. When the hour angle is zero, right at noon LMST, the azimuth equation really needs more digits of precision to avoid bad results. I just check for the angle = zero and set the azimuth to zero. Much simpler than dealing with bigger variables throughout.

The code uses about half the available ROM and RAM. That leaves room for future improvements, but I'm really sure I couldn't have fit the floating point and trig routines from the C compiler.

It's working well, and most of the tweaks I've made have more to do with finding home and dealing with the limit sensors.

Drop me a line if you want to see the actual code.

Cheers,

Keith

This is my semi-homemade alt-az mount with broadband VHF antenna attached, ready to go on the roof. Well, almost ready... I still need to do some testing on the ground. It's a lot more convenient than climbing a ladder to fix something.

It all starts with a pretty generic tri-pod mount. On top of that is a cheap used ChannelMaster rotator I got from eBay. That gives me the azimuth rotation part of the alt-az mount. Above that is a short section of pipe/mast which holds the antenna. So far, pretty standard.



Here you can see a closeup of the part above the rotator. Instead of using the standard "U" bolt to attach the antenna boom to the mast, I located the balance point of the antenna and drilled a hole there. A combination of threaded rod and nylon bushings allows it to tilt up and down from below horizontal to about 80 degrees. I'm at 38.5 deg north, so that's enough to track the sun to its highest point in the summer. The altitude drive is a 200 step/rev stepper motor. The shaft is extended through the vertical mast to a toothed pulley. The matching belt wraps around an acrylic disc from Tap Plastics and is fixed to the disc at a point opposite the stepper pulley. The large pulley gives me higher resolution than 1.8 degrees per step I would get with a direct or 1:1 drive.

The antenna is a Winegard HD-6000 I got from Wholesale Electronics



I got a little bit lucky with the driver electronics. The ChannelMaster control box had a PIC16C57 processor in a socket. I reverse engineered it and then designed a board to plug in to the PIC socket. That's what you can see here in this inside view of the control box that mounts below the rotator. The original rotator control board (minus the LED display) is on the bottom, and my board is on top. I used a newer PIC16F87 and added a stepper driver for the altitude step motor. The heatsink (from an old PC Power supply) has the driver chip (salvaged from an old printer) and a 7805 5V regulator. I also added an RS-485 serial port so that I can tell the PIC the date and time as well as getting status back from it.

The small board with terminal block in the lower left of the picture is the power supply. The transformer is a Class 2 type mounted to the side of an electrical box in the attic so that no 110V needs to go outside.

The software that tracks the sun using only PIC level integer math is another story...

Cheers,

Keith

I'm really looking forward to the upcoming Western Regional Conference of the Society of Amateur Radio Astronomers coming up next month at Stanford. It's just too much of a trek for me to get from Sonoma County, CA to the big SARA meeting at Greenbank in the summer. I do hope to make that pilgramage one year. SARA had a meeting at Owens Valley Radio Observatory several years back, and it was great.

I'm sure I'll learn some things I can apply to my projects here, and I'll meet some interesting people too!

Cheers,

Keith

Now that the seismometer has been delivered, I can get back to work on my Multi-Frequency Monitor project. The basic idea is to monitor radio activity in several parts of the the spectrum, from VLF up to Microwaves. I was inspired to do this because of the long, deep, solar minimum we just experienced. I want to see what happens as the sunspot numbers go up through this next solar cycle.

My plan is to have several antennas covering the bands of interest, backed up by receivers for each. The receiver outputs will be conditioned, displayed on bar graphs (I LOVE blinky lights!) and then digitized with a MAX187 A/D converter. Radio SkyPipe will handle the data acquisition and display.

First up will be 111MHz (modified aircraft receiver kit), 20MHz (RadioJove), and 24.8KHz (HP3586C SLM).

I designed and built the bar graph cards last year. Here's a photo of one of the prototypes:



I searched all over the net trying to find a kit or module that would do a bargraph. When I couldn't find one, I resorted to building my own.

These will be mounted vertically in a 3U (5.25") card cage, with the LEDs poking out through a panel for each board. This prototype has the wrong Red and Yellow LEDs: they will all be T1 size. In the lower right are the input and output jacks. Next to them are the gain/offset/filter amplifiers. The pots allow me to adjust offset and gain for both the display and the signal going to the A/D. In the middle is the LM391x bar graph driver. They have three variants with either linear, log, or VU scaling. The upper right has the power input and voltage regulators.

These work great. I had only a couple of minor problems with the protos, but otherwise they work great.

Next up, the A/D board.

Cheers,

Keith

Well, here's the finished seismometer ready for delivery. It was picking up the heavy surf off the Sonoma County coast for the last week, and also recorded a 3.0 at The Geysers.
To get here, I changed the upper suspension to be mostly rigid. It's hard to tell in the photo, but the angled part is 1/2" x 1/16" aluminum angle. I only have a couple of inches of exposed wire. This really killed a rotational movement about the boom axis that was cluttering up the results.

You can also see the cast lead masses at the end of the boom. I did these with a simple wooden form, and melted the lead on my Weber barbecue over some charcoal briquettes. I ordered the lead, a ladle, and some casting putty from Rotometals. The putty was very nice to smooth out the inside corners of the mold. Rotometals has a bunch of great stuff for low-melting temperature metal casting work.

The electronics for this setup all came from Larry Cochrane at the Redwood City Public Seismic Network. I bought a sensing coil, amp/filter board, and 16 bit A/D board all in a nice box. They are working great.