Receiving weather satellite pictures in New Zealand – Part 2

June 28th, 2010

You may want to read part 1 of this story about receiving weather satellite pictures if you haven’t already.

Now I will describe how I made a much neater QFH antenna to receive the satellite signals. My wooden one worked fine but it was not waterproof or particularly pretty or accurately made. So I decided to make a waterproof, prettier and more accurate one.

Again I followed the directions given here:

IMG_8320_2 I started off with the raw materials which I got from one of my most frequented places – Bunnings Warehouse. There are probably cheaper places to buy this stuff but Bunnings is convenient. I bought a 1m length of 32mm diamater PVC pipe and three lengths of black 15mm diameter Enduroflex pipe. I have no idea what that stuff is used for. Water piping I think. The PVC pipe cost NZ$15.51 and the black pipe was $11.16 for the three lengths although I only ended up using two of them. That black pipe incidentally is the perfect size for the barrel for a home made Nerf gun firing suction darts (a project for another day)!

I also bought some RG-58 (50 ohm) co-axial cable from Jaycar rather than the RG-6U I used on the wooden antenna. The The problem I had with the RG-6U is that the braiding isn’t copper but rather aluminium so it was impossible for me to solder the connection easily. The RG-58 is somewhat thinner diameter than the RG-6U which does mean the antenna is more easily bent out of shape but as my antenna will only be used occasionally it should be fine for now. The 15 metres of co-ax was $28.50.I cut the PVC down to about 800mm long and marked it up ready to drill the holes for the cross arms. I was very careful with the dimensions on this antenna. An easy way to mark around the pipe is to wrap a piece of printer paper around it ensuring the paper is square to the pipe by making it overlap itself perfectly as it wraps over itself. You can then mark off the place the sheet overlaps. You unroll the paper and the distance form the edge of the paper to the mark you a made is exactly the circumference of the pipe. It is then easy to mark off the quarter and half way points on the flat paper. Then you wrap the paper around the pipe once more to transfer those marks to it.

IMG_8322_1 I then put the pipe on a nice flat surface and use the ended of a long, straight piece of wood to transfer the lines down the entire length of the pipe.

IMG_8324_1 After carefully marking the pipe and measuring it several times I drilled the holes to hold the cross members. I first drilled a pilot hole with a 3mm bit then I used the spade bit to drill out a larger hole (sorry for the badly focused photo).

IMG_8326_1 To make the holes a nice, tight fit I drilled slightly undersized then used a tapered reamer to enlarge the holes to the exact right diameter.

IMG_8327_1 Once done you end up with mast finished. The positioning of the middle set of holes holes isn’t very critical as those arms just hold the ends of the cables in the correct place. Obviously you want them close to the mid point though.

IMG_8335_1 I next carefully cut the six cross arms from the black pipe. Note the two different sizes for the two different sized loops in the antenna. I trimmed the ends square in the lathe (because I could, not for any technical reason) then carefully marked and drilled the holes in the ends. Looking at the plans you can see these holes need to be a fixed width apart to ensure the loops of the antenna are the correct size. They also need to be drilled at 45 degree angles to the mast so the cable has the correct spiral to it.

IMG_8331_1 To drill the holes so they are at 45 degrees to the mast when installed you actually need to drill the holes in each cross member 90 degrees apart, one at either end of the support. Since the drill press drills down vertically all I did was drill one hole then push a piece of aluminium tube through it temporarily. Then, turning the tube around I simply adjusted it in the vice until the aluminium tube was parallel to my drill table. With it parallel when I drilled the vertical hole I could be sure the two were 90 degrees apart. When you push the crossmember through the mast you simply twist it so that one of the holes is at 45 degrees to the mast and the other will be aligned at 45 degrees too. You do have to twist it the right way though so that the cable is spiraling down anti-clockwise when viewed from above.

IMG_8339_1 Because I might want to experiment with this antenna a little I didn’t want to glue the crossmembers fully in place yet so I am temporarily using some pieces of split ribbed tubing to stop the crossmembers moving in the mast.

IMG_8336_1 In the top and bottom sets of holes I needed to file slots so that the cable could pass through into the mast. The holes for the arms at the ends of the mast I drilled so that the centre of the co-ax cable would be sitting at the exact right measurement given in the plans with the cable running along the outside of the crossmember , not inside it. Running the cable inside the crossmembers would of course be neater. But there is then no way to make sure the cable lies flat in the correct place. Does this make much difference? I have no idea! But the dimensions are given down to fractions of a mm accuracy so I decided to get things as close as possible.

IMG_8340_1 I started putting the crossmembers in place and also soldering the connections. Shown here is the bottom most connection. That is the feed wire soldered to the shield of the bottom of one of the loops. I insulated the connection with waterproof tape then carefully pushed it into the mast and then pushed the crossmember fully home.

IMG_8347_1_1 Similarly at the top I carefully soldered the connection inside the top of the mast once the loops and the crossmembers were all in place.  Notice the anti-clockwise direction of the spirals. I used lots of cable ties to hold the co-ax down flat across the crossmembers. Eventually I will remove the split tube and just glue the crossmembers in place once I know everything is correctly positioned.

IMG_8348_1 To finish off the antenna I put a PVC cap over the top of the mast and I also made a quick and dirty stand from a scrap piece of wood and another off-cut of pipe that the mast happens to slide into perfectly. Eventually my plan is to hang it from a line suspended above my garage roof where it will have a clear view of the sky. The antenna might be connected to a line going around a pulley so I can hook the antenna onto the line and pull it up into position but still bring it inside if the weather is too bad. Either that or I will make a third, more robust version to permanently mount on my house roof.

IMG_8357_1 With the antenna on it’s temporary stand I was able to move it about outside to see how well it would work. It works well when it has a clear line to the satellite down to very low on the horizon. The signal is very easily blocked though so building and even trees will cause you to lose the signal.

Now I just need my receiver kit to arrive so I can build that and really see how this works!

Update: The kit arrived! Read about it in Part 3.

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3 Responses to “Receiving weather satellite pictures in New Zealand – Part 2”

  1. Asciimation » Blog Archives » Receiving weather satellite pictures in New Zealand – Part 1 Says:

    […] describe that in part 2 here. Posted in Projects | Trackback | | Top Of […]

  2. Asciimation » Blog Archives » Receiving weather satellite pictures in New Zealand – Part 3 Says:

    […] The other two parts of this are available here:  Part1 and here: Part2 […]

  3. Asciimation » Blog Archives » Using RTL-SDR to automatically receive weather satellite images. Says:

    […] One, Part Two, Part […]