Posts Tagged Weather
It didn’t take long after my first successful attempt at receiving weather satellite broadcasts for me to realize that I would need a much better antenna. I had been using a 1/4 wave whip with a 4-wire ground plane. There performance out of this antenna was poor. I read up on QFH (Quadrifiliar Helix antennas) from many of the high quality posts from around the world. I took what I could from these implementations, and did my best with the supplies I had available.
I ultimately started with whatever PVC pipe I had lying around; it ended up being 1-1/4″ outside diameter garden-variety PVC. Next, I went onto the excellent QFH antenna calculator. This site already has the defaults for a good 137 MHz antenna, and I didn’t change them. Next I measured the diameter of my pipe (in mm) and printed out the drilling templates using their drilling template generator.
Pick a point for the top line of the template and scribe a line perfectly parallel with the axis of the pipe. This is easy if you use the “estes door jamb trick.” For those that haven’t built a model rocket recently, you put the pipe in a door jamb crease. This right-angle will make the pipe perfectly square and you can mark a line. Also, you can use a piece of angle stock, as this guy did. Make sure that all your template strips line up with this line. Next mark the holes’ letter with pen on the pipe. This is your reference for where the wires need to go. Finally, drill the holes. I only drilled the top and bottom holes, and I didn’t make them as big as the template said. The template (and the calculator) suggest using 3/8″ soft copper tubing, but I didn’t have any of that around, and it costs a lot more than the #10 ground wire that I used.
I knew I would need spacers half-way through the helical loops, but I didn’t have any of the tubing that most people were using, so I used some acrylic sheet. I just took the width of the loop and marked those points along a line. In the center I cut holes for the mast. I trimmed off the edges with a (mostly) straight line so it tapers to the end. At the ends, I made little notches to hold the wire, and drilled a hole for some unwaxed dental floss.
Once I made a simple knot with the floss, I hit it with some superglue an called it good. I don’t have any pictures from forming the loops. This is an often ignored part of building one of these antennas. I understand why so many people leave that part out, though. It’s amazingly stressful, and easy to forget about documentation. To do it, I started with long pieces of the copper wire. Use more than you think you need. Once your wire is cut, measure to the center. Measure out from the center one loop radius on each side and mark it with sharpie. Bend one side of the wire as sharply as you can, and slide the longer side of the loop through your lower pair of holes. With what will be the center of the loop in the pipe, bend the other side of the loop. Carefully twist the helix, meeting the spacers on the way. It’s probably not a bad idea to temporarily (make sure the copper can be adjusted) attach the wires to the spacer. Measure from the mast one loop radius and bend the wire toward the mast square with the upper holes. Cut the extra wire so that it fits inside the pipe with a little room to spare. Once you’re sure it’s the right length, bend the wire 45 degrees toward the other loop. Do this for each side of each loop.
The picture above is skipping ahead a little, but this way you can see what I mean about bending the wires inside the pipe. As you can tell, I had a very hard time soldering the feed point. The copper ground wire is a hard to solder with an iron because it conducts heat readily, and has a lot of thermal mass. I usually use a torch to solder it. In this case the solder point is inside the tube almost an inch. The first time I tried to solder it, I set the end of the tube on fire. Later, I insulated the coax and tube with aluminum foil. Though the foil provided sufficient protection, I still charred the sides a bit. The cable was adequately protected.
Though the feed for this antenna is technically 50 ohms impedance, it’s a balanced design. Fundamentally, coax is unbalanced. By coiling the cable around the mast a few times you’re able to implement a simple balun. I’m not sure how many times to do it, I’ve seen different numbers different places, so I chose 6; because, why not? It seems to work fine.
This is the antenna all finished up. I’ve received several quality satellite images with it so far, but this is certainly not where I’m going to keep it. First of all, my house is to the north-west of it, and seriously degrades the signal, secondly my wife wouldn’t like that very much, I don’t think.
I decided to keep it outside (some people put them in their attic), so I had to take some measures to spider and wasp proof the mast. I epoxied all the openings for wire and cable at the top of the antenna, including capping the pipe, then I stuffed the bottom with some pipe-wrap foam.
I ended up settling on the barn roof as a home for the antenna. There was a small gap in the plywood below the metal roof cap that I think I can exploit for the feedline. I unscrewed a section of capping, found an existing hole in the tar-paper backing and went to work affixing the tripod.
It’s really important to think about water infiltration whenever you do anything to a roof, especially in Oregon. For a while, I did some contract work installing antennas for a cell phone company, and we installed outdoor antennas on almost any kind of roof you can think of. For metal roofs like these, the easiest way to make it water-tight is to clean the metal surface off, then cover the place where you’re going to screw the mast to with silicon sealant and screw through it.
This way, water doesn’t have a chance to wick through the roof through the screw threads (or that’s the idea… I explicitly disclaim any responsibility for roof damage if you follow these instructions).
Here, you can see the ultimate goal: Antenna on the roof of the barn and feed line inside!
Finally, I screwed the receiver to a joist and routed some wires. I don’t know what I would do without my insulated staples! 🙂
The National Oceanic and Atmospheric Administration (NOAA) manages a few satellites in low earth orbit. There are three actively transmitting APT signals at the moment, NOAA15, 17, and 18. Each of these satellites passes overhead a few times a day. I’ve been interested in learning how to receive their signals for a while now, and I’ve finally succeeded!
A bit ago, I bought a “SoftRock” SDR (Software Defined Radio, read the excellent 3-part article by Bob Larkin at the ARRL site.) receiver kit from Tony Parks. (A note about his site, he puts a few kits up for sale a few times a month, so he’s almost always sold out.) I think SDR is really, really interesting. I don’t want to get too bogged down in the details of it, because it’s not the point of this post, but I’m going to briefly discuss it. Basically, the idea is that you want to have some minimum amount of electronics to deal with the antenna; letting your computer handle the rest. This can take a variety of forms, but the simplest is the QSD, or Quadrature Sampling Detector.
It sounds complex, but it’s quite like using a strobe light to look at a spinning wheel. The bright light of the strobe “samples” the world at a given interval. If it strobe rate matches the speed of the wheel, the wheel appears still. Stretching the analogy a bit further, imagine that information is written on the wheel. Using the strobe you can read it, even if it’s spinning. While that is an awful analogy, but the idea is that we can sample the radio signal at (nearly) the same frequency as the carrier of our desired signal. When we do this, the signal we want magically appears at the output. If we’re using AM (or its derivatives such as LSB or USB) we can even listen to it directly. It only gets a bit more complex when we consider the quadrature part. Quadrature just means “90 degrees out of phase.” Using another copy of the radio signal, and a sampling clock in quadrature, we can cancel out some noise and interfering signals. Sorry for the tangent, if you read this far (without falling asleep), I recommend you read the linked articles at the top of this paragraph. The math isn’t too hard, and it’s sooo powerful!
This isn’t an image of my SoftRock, it’s a slightly different version, but I don’t have an image handy. It’s a really easy kit to build, and it’s fairly inexpensive. More than that, it’s really easy to use. Once it’s all setup, you just attach it to your computer, power it, and install the antenna!
Once you’ve attached the receiver hardware to your computer, you need some software to use it. This is an image I took of a very well written SDR program on the Mac called DSP Radio. On the left side of the spectrum window, there is the live radio spectrum coming from the satellite. The green box around it represents the bandwidth of my software radio receiver. In a traditional radio, this bandwidth would be set by a filter circuit. Most communication radios only have about 15 kHz of bandwidth. This makes them unable to properly receive satellite weather images. Traditionally, you would have to build or buy a specially-built satellite receiver. With SDR, I can move a slider to scale the bandwidth way, way up! In this case, I’m using about 37 kHz of bandwidth! Notice that there’s all this empty space on the right, this is radio spectrum that I’m receiving, but there’s nothing there. Maybe you can notice the shadow of the satellite data on the right; this is an “image.” These images are the bane of all radio designers. The true test of a receiver (other than sensitivity) is how well these images are suppressed. In this case, they’re suppressed rather well, notice how bright the lines are on the left compared to the right.
The DSP Radio program takes the signals from the Softrock through the audio input of the computer. When you have something tuned in through its interface the demodulated signal appears at the audio output. I’ve been recording these signals as well as passing them to another program called WXtoIMG. This program is not great, I’ll be honest. It’s barely maintained, and you can tell it’s an ugly cross-platform mess. To even get it to work is tricky. But, what it does, it does well. The image at the head of this post was generated using it. When I made that image, I literally had to connect the audio out of one computer to the audio in of another. I’m not sure how I’m going to get around that issue. It can accept data in the form of wav files, provided that they’re linear PCM sampled at 11,025 Hz with at least 16 bit samples. The problem is that the nice political boundaries, lat/lon lines and ground image comes from the program. It does this by computing the location of the satellite and where on the earth its photographing. It has to decode the audio in real time for this to work, which means that I can’t use an audio file. For you to play around with, if you wish, I’ve included a sample wav file. It starts before the satellite pass and ends after, so if begins and ends with static.
NOAA15-baseband.wav (large file warning: 28 MB)
The included audio can be used to create the image below. I used WXtoImg to generate it, though the open source WXAPT could be used under Linux. This image was taken when the satellite was traveling south-north, so I had to flip it vertically and horizontally. On the right side of the image is the A channel, which is visible light, and the left side is the B channel, infrared. Normally, these channels are reversed left-to-right. The stripes and color bars help the decoder line up the image and adjust brightness, contrast, and gamma. (Right-click here to download full-size image: 2080 x 1260 pixels)
My next step is to write some shell scripts on a Linux box to automate this whole process. My goal is to have a page that has the latest satellite image and an archive available at all time. But first, I have to write a post about the antenna I built to receive these signals. Stay tuned!
I’ve finally gotten my home weather station online! I’m using a Oregon Scientific WMR918 system with anemometer (wind speed), wind direction, temperature, humidity, and barometric pressure sensors. Click here to access the current and recent readings, or the beta version of an integrated page (also available in the navigation bar below the blog title) and “read more” to find out how it works.
The WMR 918 has a serial port that sends-out all the data that it collects (and lightly processes). I have this signal going into my home server, which is running the open source wview. Because my weather setup is a bit different than what is typical, I had to make some changes to the source code. (probably the best part about open source!) I guess most weather stations come with a “mushroom” sensor which is a thermohygrometer (temperature and humidity) that is encased in a radiation shield (so direct sunlight doesn’t affect the readings). I don’t have one of those. My outdoor temperature sensor is reported differently on the serial output. The wview software was expecting a certain kind of data packet, and having never gotten it, it never initialized. Once I changed the source code so that it ignored the difference I was all set. B.T.W: The thread about this process is on the wview google groups page.
I’ve gotten the data up in several personal weather aggregation services. I’ve decided that I don’t really like weather underground (way too cluttered), but I’m up on PWS (Personal Weather Stations), and APRS (Automatic Position Reporting System). APRS is a weird amateur radio thing. Basically, what it was designed to be is a way for small bits of digital data to be passed around so that, especially during search-and-rescue, operators can know where one another are. Over the years it has been expanded to include many interesting uses. One of which is the sharing of personal weather station observations. Eventually, NOAA even started using the data in their models. One of the many things I want to do, one of these days, is install a APRS node in my car. I know it is geeky to the max, but that is what I am.
Anyway, back to the point, everything works! I hope you enjoy the fascinating (not really) weather trends from Philomath Oregon! Oh, and hopefully soon I’ll have a handy widget that displays a digest of the current conditions in this sidebar. But for now, feel free to check the current conditions page.