Building a QFH-Antenna and How I Did It

To receive images from NOAA satellites, I've built myself a QFH-antenna out of copper tubing. Whilst building I had trouble finding useful information on how to do it exactly...so here is my way of doing it :D

I used:

- For the frame:

1x 1m length, 110mm diameter plastic rainwater pipe

(Optional1x lid for the same kind of tubing)

- For the actual antenna:

about 4m of 15mm diameter copper tubing (rolled --> see Step 4 for what I mean)

about 2m of 15mm diameter copper tubing (straight)

8x 90° copper tube elbows

some soldering tin

a blowtorch

- For the bending template:

about 10x 1m Woodplanks about 2cm thick (2x4s)

2x 29cm wooden circles (about 5cm thick)

2x 28cm wooden circles (about 5cm thick)

- For the wiring:

4m RG58 C/U coax

1x BNC-Adapter

(optional: Crimping wrench)

- For the stand:

1x sunshade stand

1x 50mm rainwater pipe

1x rainwater pipe adapter 110mm -> 50mm

- Misc:

hotglue

screws

You can use this (http://www.jcoppens.com/ant/qfh/calc.en.php#total1c) online calculator to see what length you need for you preferred frequency. I wanted to optimise my antenna for 137,5 MHz. So I put in the frequency and the conductor diameter of 15mm and had all the measurements I needed.

Don't be overwhelmed by all the numbers just take a look at the website and try to imagine what each length is for. You've got this!

For the antenna it is important to be somewhat precise, so measure three or four times before drilling or cutting.

First choose the "up" side of the frame and mark a point about 10cm from the top. This will be the upper part of your antenna. The horizontal rods for the upper part need to be 90° from another, so take a piece of paper (or string etc.) and wrap it around your frame until it overlaps. Mark where it overlaps. Now fold the paper in half where you marked it (one corner onto the mark) and fold it in half again. You should now have a piece of paper with four evenly seperated kinks.

Congratulations! You've got a 90° template for your frame.

Wrap the template around your frame and align one of the kinks with your marking on your frame. Hold it in place with what you have on hand and mark the frame where the other kinks are. Be careful to be on the same level as the first marking.

Next, take the antenna height H1 from the calculations. This will be the distance of the horizontal rods for the larger loop (you know what I mean when we wire it). The antenna won't get bigger than this. Measure the distance from one of the previous points straight downwards and mark it. Then do the same on the opposite side of the frame.

Repeat this step with antenna height H2 on the remaining two sides. It should look like picture 1, just without the pipes

Now we need to cut the horizontal tubes in length, so take the straight copper pipes and take a look in the calculations for:

Dc1 and Dc2

These will be your lengths for the horizontal pipes. Dc1 for the larger loop and Dc2 for the shorter.

Cut two tubes for each length and label them as such.

Because the upper tubes of the antenna will be on the same plane, you have to cut Dc1(top) and Dc2(top) into two smaller pieces each, so that everything finds room in the frame. I cut both of them in half and took one side of each of them about 2cm off. That gave me more room to wire later, but look what you seem fits the best.

If you follow what I did you should now have:

1x Dc1 length

2x a bit smaller than 1/2 Dc1 length --> Dc1(top)

1x Dc2 length

2x a bit smaller than 1/2 Dc2 length --> Dc2(top)

To make the wiring easier, drill a small hole about 1cm from one end of every Dc1(top) and Dc2(top). You'll get what I mean in step 5 :)

Getting the pipes into the frame you will need to drill 15mm holes into every marking that you did on the frame beforehand. That will be eight drillings: four 90° apart up top and two times two 180° drilling at the bottom. See picture 2+3 as reference for the bottom drilling.

I didn't have a drill big enough, so i took some scrap cutting of the 15mm tube, heated it up with a blowtorch and pushed it through the plastic. It worked okay, but some of the holes came out too big. I'd recommend getting a proper sized drill though, because I guess melting the plastic isn't too great for your health.

Anyways, now that everything is the right size you can assemble the antenna as seen in picture 1. The Dc1 Pipes have to be 97.5 mm out of the frame and the Dc2 90mm. I used hotglue to hold everything in place.

I had a hard time bending and found the easiest way was to build a template.

Because a helix has a steady circumference, I built something like a barrel where I attached the pipe and bent it by hand. You will need two templates. One for each Loop, but you can recycle the materials, as you'll soon see.

For the first barrel I cut out two wooden circles each 29cm in diameter (in picture 1 I wrote the wrong measurement on there, don't get confused). I then screwed 1m wood laths (about 2cm thick) around the whole circle like in picture 2. Put them tightly next to each other to make it sturdy.

Next, measure and mark the distance H1 from step 2 on one of the laths. Do the same on the exact opposite one (180° from the first). You can now stretch a small rope from the top mark on one to the bottom mark on the other side and use it as a reference for the upcoming bending. Along the rope put a few screws in the wood to make it easier for yourself.

Now to the bending take around 1m of the rolled pipe and roll it out as straight as possible. See here (https://www.youtube.com/watch?v=C4Y5YDYbFEE) how to do it right. Unfortunately it's in german, but you'll see what I mean.

Screw it down onto the top part of the rope on the barrel-template and bend it slowly along the rope. Hold it in place with screws in regular intervals. Optionally use bungee cord etc to secure it more tightly. See picture 2 as reference. It's like a steady rhythm: Bend a bit, secure it, bend a little more. Remember, little steps are better than overshooting.

Anyhow, Bend two pipes with this template.

When you're done take the template contraption apart and rebuild it with 28cm wooden circles for the smaller loop. Repeat everything as before until you are left with 4 bent pipes:

2x for the larger loop

2x for the smaller loop

Left is just to cut them in length. The tubes for the longer loop should be about 90cm long and for the smaller loop about 86cm. For example, use a rope of the respective length and attach it to the tubing as a ruler to cut from. I just eyeballed it in the following step. You will still receive stuff, if it's not exact, so don't worry too much.

I did a bad job at step 2 and the small pipes at the top were quite loose, so I improvise a holding mechanism consisting of two wooden circles pushing the four pipes together. As seen in picture 1 it's just one piece of wood above the pipes and one under them screwed together by three bolts. I additionally put some styrofoam between them to act as a cushion. You will most likey do a better job than me :D

Anyways, assemble everything like in picture 2. Take the elbows and put them on the horizontal tubes and then the vertical pipes on the elbows. Make sure the longer bent pipes go on the Dc1 and the shorter on the Dc2. I believe in you!

If everything fits nicely, solder it in place like you would normal plumbing. If it doesn't quite fit, bend it a bit by hand or revisit step 3.

Btw, don't mind by bad soldering in picture 3...

The wiring is somewhat straight forward.

The idea is, that the braid and he center are connected to both larger and smaller loop respectively as seen in picture 1.

So, first drill a hole for the coax into the frame in between one Dc1(top) and one Dc2 (top). Then strip the insulation off the braid at a length of 20 mm and the center at a length of 10mm. Push the coax into the frame until it is between the four pipes and split the wires of braid and center each in half. Now comes the fiddly part: Put one half of the braid into one Dc1(top) hole and the other into one Dc2(top) hole. Do the same for the center. See picture 2 for reference.

To hold the wires in place I put small screws into the holes, but you can use hotglue as well.

Because we're wiring a balanced antenna to an unbalanced cabel, the coax, you'll need a Balun (meaning balanced to unbalanced) to negate signalloss. To build it, just wrap the coax around the frame 4-5 times and hold it in place with zipties. In picture 1 of step 6 you can see how I looks like, when it's done. I don't quite know how it works, but I believe each coax-loop acts like a transformer upon itself and therefore negates out of phase waves in the braid. But this is just a guess.

Still, Congratulations :D you just have to put on the BNC-adapter and the antenna is ready to go!

Because the rainwater pipe I used for the frame is easily compatible with other rainwater pipes the stand is quickly finished. Just get a sunshade stand, a 50mm pipe with adapter from 110mm to 50mm and screw everything together with nuts and bolts.

I was lucky to find a collapsible sunshade stand, but any will do.

I hope everything was easily understandable and good to reproduce on your own. If you found any mistakes or have anything to add in general feel free to hit me up :D

Reddit: https://www.reddit.com/user/Al-Dorado

Thanks to my girlfriend for the nice drawings <3