# Stopping/reducing wobbliness in threaded rod

I am working on a 1.5-meter satellite dish with a custom receiver cantenna-like feed. The dish didn't come with feed arms, so I need a way to hold the feed into place. What I tried was using a 1-meter threaded rod. My threaded rod was 1m (length) x D=14mm (thickness). I used nuts to hold the feed in place and held the threaded rod from the little hole in the center of the dish (again, using nuts). This has the advantage of allowing me to adjust the position of the feed (focal length) so I can play around and experiment to see where my signal strength is at its peak.

Another advantage is that I don't need to use metallic arms (like many large dishes do) to keep it in place, as they can block a (small) portion of the radio waves from hitting the reflector and reduce its gain.

But there's a problem which leads me to come here and ask for help: even slightly touching the feed (or the edge of the rod) will have it wobble around for quite a while. This means that if there's a small wind, the feed will be wobbling around, which is not good.

My question is: what can I do about this? Can I use another type of rod, or make any adjustments to my current threaded rod in order to stop it from wobbling back and forth by the slightest touch? Could using a thicker threaded rod (the hole is 1 inch = 2.54 cm, but I could always drill a larger hole if needed), or would that not fix the problem?

A few pictures of the dish:

Any ideas are appreciated!

1. Increasing the rod thickness would be very effective. If I've visualizing the configuration correctly, the rod is acting as a cantilevered beam (i.e., a beam rigidly attached at one end):

The lateral stiffness (which governs the vibration amplitude of such a cantilevered beam) scales up with the area moment of inertia, which remarkably has a fourth-power dependence on the diameter. This means that you could double the stiffness with only a 19% increase in rod diameter, and doubling the diameter would increase the stiffness (and suppress the vibration amplitude) by 16×.

2. What about stabilizing the end of the rod by attaching nonconductive tensioned cord or string stretching back to a support?

This is how tall and narrow towers (which would otherwise be unacceptably laterally compliant) are stabilized. The limit of the tensioning load would be the buckling threshold of the rod (if the tension points back to the base; therefore, try to orient the tension loads so that they tend to elongate the rod).

3. (Whimsical) If you have substantially excess time and money, you might add a set of strain gauges and actuators to the length of the rod to detect vibrations and suppress them, respectively. One example of such a "smart structure" is a cannon barrel that can stabilize itself against vibration as the shell moves through it, thus increasing accuracy, for example. Overkill?

• Wow, I didn't know a thicker rod could make that big of a difference! I'll definitely consider the first two options, even though I'm guessing doubling the diameter of the rod (= stiffness x16) won't require me to also use tension cords (point 3 is indeed an overkill haha). <s>P.S.: I'm looking at thicker threaded rods to buy, and they all have an "M" sign in front of them (i.e. M8, M12, M14, ...). Any idea what "M" refers to? Can't find an explanation online...</s> EDIT: Found that it stands for diameter thickness in mm. – Coto TheArcher Feb 15 '19 at 20:54
• Exactly. You'd tap the hole using the corresponding metric tap. – Chemomechanics Feb 15 '19 at 20:59
• So I tried with an M22 threaded rod and it really seemed to help. Now, here's the problem. After installing a new feed, the feed is having a similar wobbliness problem. I think it's easier to show it than describe it with words, so let me know if you can think of a way to solve this: youtu.be/rAFGbEhhDZ8 If I take a guess I'd say it's probably because the base (the back-plate that holds the feed) isn't very thick, and as a result doesn't hold the rest of the structure stiffly. What do you think? – Coto TheArcher Mar 6 '19 at 21:14
• You could try adding nonconductive guy wires, as described above. – Chemomechanics Mar 8 '19 at 17:39
• This isn't an easy option because I'll have to find the write cord etc. and I'll need to somehow attach it to the wobbly feed. Won't increasing the thickness of the feed back-plate make it stiffer? – Coto TheArcher Mar 9 '19 at 13:39