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! 🙂