The Quadrifilar helix antenna for 137Mhz - Weather Satellite Image Receiving
This was our very first club project the hit the internet. The project is fairly straight forward, and will suit beginners well.
The Quadrifilar helix antenna is all over the internet as it is, but we thought we would document our efforts for all to see "warts and all"
This first phase covers the preparation of the support mast, choke balun and the coaxial connection.
Hope you enjoy it.
Stage 1: Hole template.
We used the great information from this website CLICK HERE for our project
These guys have set the standard for home building this antenna, and we are using the resources from this site to complete our project
They have a hole template that can be used to ensure the correct position of the holes for the loops, these positions are very important if you want the antenna to work properly. Click the template image, and print as normal, it should come out the right size for use with this project.
Stage 2: Drilling the holes.
As mentioned earlier, the hole position is critical for this project, and if you have access to a pillar drill, or drill press USE IT. You can drill by hand, but you really need to ensure that you drill though perpendicular to the tube, otherwise threading the elements through becomes more difficult.
DON'T DO IT LIKE THIS
Phase 2 of the project is the choke balun, for those of you who don't know what it does....
The balun basically electrically disconnects the antenna from the coax feeder, that way the feeder does not interfere with the way the antenna is supposed to operate, well thats the theory anyway.
Stage 1: Winding the Balun
This is just about the simplest balun you can get, just a matter of winding the coax around the mast section. 4 turns are required for this antenna. Begin by feeding the end of the coax which will attach to the elements through the top most hole drilled for the coax, NOT the one drilled for the elements.
Ensure you give yourself enough spare at the open end to strip back for the connection to the element connection disk.
The rest of the coax now runs down the inside of the support tube, again allow enough coax for the connection to a PL259 plug.
THIS IS WHAT HAPPENS WHEN YOU MAKE IT TOO SHORT.... WHOOPS!
Phase 3: The feeder/element disk
To make the connection between the feeder coax and the elements of the antenna is always a tricky one, too shoddy and the antenna won't work properly. We stuck with the same design as the website we took the information from, and it is so much easier than alternatives we have seen and tried, it also makes a professional job of it.
Phase 3: The feeder/element disk
The disk is made from a piece of double sided copper clad PCB board, tis is available from most places, but we got ours from Maplins for quickness. The board makes it much easier to mke the connection between the feedline and the elements of the antenna. If we had tried the altrnative method of directly soldering the coax to the copper elements, it would have looked messy, and the connection would probably have failed in service.
Below is a diagram of the actual disk, the diameter is such that a snug fit inside the plastic pipe is maintained.
Here is a larger view of the actual disk
The penultimate phase of the project, and probably the most fiddly of them al is the construction of the elements for the antenna. The material used is easy to work, but its shape is difficult to maintain.
Straightening the copper pipe
The copper pipe we used for the project is supplied in 15m coils, it is easy to work with, but does require some thought when you come to try and straighten it out to cut the elements the length. We found the best method is to roll the tube on a hard surface, this removes as much of the curves and kinks as possible before measuring.
Assembly of the cross elements
This is the part where you find out if you have drilled the holes in the right place, if you haven't the elements do not meet in the correct place for the feed point, and the whole thing will look an odd shape. You can either make the feed point connections at this stage, or wait until the main elements are fixed in place. It is important at this stage that you check your measurements too, remember! measure twice cut once, and if possible get someone to check it for you too. In our case, the head Quality Control Inspector is Harry the Guide Dog, see below.
The WRAA Quality Control Inspector "Harry"
Bending the elements
There are several ways to get the correct shape and curve in the main elements. You can use a jig, or a line drawing, but we decided upon the good old faithful method of doing it by eye, we found this the quickest and simplest method to be honest. If you do use this method, make sure that you dont try and bend it over your thumbs too much, otherwise you will end up with a kink that you can;t get rid of, "softly softly catchy monkey"
Soldering the elements in place
This part of the job is definately for two people, I guarantee as soon as you warm up the elbow fitting the pipe will pop out, you have been warned. Make sure you use flux paste on all the joints to get a good flow of solder around the joint, and it pays to use a good gas torch too. You can just about get away with one of those small pocket torches, but you will be there a while trying to warm the joint.
Last but by no means least, make sure you clean all of the soldered joints well. Just a little bit of emery or scouring pad will do the job, this is to remove any excess flux as it will go green once outside. Some of our chaps have sprayed a clear laquer onto the copper to maintain its shine.
Well, it's all over but the kicking and screaming, the guys seemed to get a buzz out of building this strange looking antenna and have all been looking forward to using them.
The completed antenna
Here are a few images of the completed antennas, they all turned out fine and looked just as they should have. It paid dividends to double check at every stage after all.