Singing Isn't Just About the Element

Many owners of HF yagis are familiar with the "singing" phenomenon. When the wind blows the elements vibrate. Not always, since it depends on wind speed and direction and element design. It is an interesting sight (or sound) but often not healthy for the antenna. That is, we want to stop it from happening.

Aluminum can fatigue and fail at well below its yield strength when stress cycled enough times. There are many examples of yagi elements breaking and the tips falling to the ground. This is dangerous and can be expensive to repair. Rebuild the yagi with the identical replacement parts and failure is likely to recur. Some brands and antennas are notorious for singing-induced fatigue failures. Here is one example:

The tower and antennas have been derelict for several years. The ham is a silent key and before that was unable to deal with the problem. Elements broke off one by one, leaving the denuded antenna seen above. Someone gathered the broken elements and leaned them against the tower. We found another sticking in the ground like a forgotten spear. Luckily the tower is located in a rarely used field so no one was injured. The tower is tentatively on my list for removal this fall.

I know many hams dealing with these issues. Large yagis with widely spaced elements may have to be lowered or carefully manipulated on the tower to access and repair broken elements. Some hams take the trouble to redesign the elements, with varying degrees of success. Haphazard changes might change the singing but not eliminate it.

Yagis that are prone to element breakage are typically the ones that "sing" in the wind. You can often hear the thrumming from the ground. The sound is louder when you're on the tower near the antenna. When the pitch is too low to hear you can still see the vibration. It may not happen at all in a very strong wind, but in those cases you are more likely to be concerned with survival of the antenna and tower.

Why do antenna sing? When they do, how serious a problem is it and what can be done about it? These are interesting and important questions whether you buy or build yagis. I've found too many hams that simply shrug their shoulders and say that's just how it is. Others are convinced that there is one true method to defeat singing and they apply that to every yagi they own. 

Fatalism and certainty are nothing more than excuses to avoid understanding the issue. This is an opportunity to learn, and if you read this blog with any regularity you will know that, to me, learning is one of the greatest benefits of our hobby. We don't have to be structural engineers to gain an insight into the why of singing and what, if anything, we ought to do about it. I like to keep yagi elements up in the air and I believe that you want the same thing.

Singing is not an arcane branch of engineering. It is very well understood in the profession. Despite owning my first yagi in 1975, I had little understanding of the phenomenon until relatively recently. I would venture to say that my ignorance is typical of most hams. With the wonders of the internet at our fingertips there is no reason to remain ignorant. It is easy to learn the fundamentals with only a modest effort. Readers who are structural engineers may cringe a little at what follows, but please humour those of us who lack the background.

[Diagram credit unknown] When a fluid like air encounters a solid object it will flow around it. The crowding of fluid at the front and sides of the object increases the pressure, and in the lee of the object there is a low pressure region. Purposely shaped objects like wings use this effect to generate lift, while the high and low pressure regions due to hurricane force winds can lift the roof off a building.

There is a critical fluid velocity where the flow changes from laminar to vortex generation, and finally to turbulence. Vortexes form on the lee side of the object, with the low pressure regions cycling between the upper and lower boundaries of the object. This is know as vortex shedding. Objects with freedom of movement will move toward the low pressure regions. As the low pressure region moves back and forth the object moves back and forth. If that frequency is close to the structure's resonant frequency it can oscillate quite vigourously.

For cylindrical objects like antenna elements there is a formula to calculate the frequency. Of course it's never this simple, but for our purposes it is perfectly adequate. One complication among many is the incident angle of the wind. I won't discuss this and other complications in any depth.

F = 0.22 (V / D)

V is the fluid velocity and D is the cylinder's diameter in the same units. For example, if V is expressed as m/s (meters per second) then D must be expressed in meters.

Let's try an example. Please note that in the following discussion I will liberally round quantities. One significant figure is enough to understand the basics.

For a 3 m/s light breeze (about 11 kph or 7 mph) impinging on a ½" (1.3 cm) diameter element tube, F is 55 Hz. The frequency is proportional to wind velocity and inversely proportional to element diameter. Thus for a 1" diameter tube in a 6 m/s wind the frequency is identical. 

It is not a given that the tube will vibrate at this frequency since only select vortex frequency ranges will excite the element. The most common is the (mechanical) fundamental resonant frequency. At other frequencies the element will move but not oscillate; yagi elements wiggling when the wind blows is normal and is not to be confused with singing. 

If any tube or tube combination in the tapered element is resonant it can shake the entire element. The tip may vibrate most vigourously because it is smallest tube and unrestrained at one end. A stronger wind on a thin element tube may be at an audible frequency that you hear on the ground. This is what we know (and fear) as singing.

It should be evident that singing depends on wind speed and direction. Too slow or too fast and the oscillation will be mild because the element's mechanical resonance is not excited. Also, a wind direction well off from normal to the elements is less able to excite resonances.

There are several approaches to mitigate singing:

• Mass: A heavier tube with more inertia is more difficult to excite into oscillation. Greater wall thickness or tube overlap is often all it takes to prevent singing. Very thin wall tubes used in some antennas are more prone to singing.
• Damping: Per the ARRL Antenna Book (22nd ed., 25.2.3): "Metal antenna elements have high mechanical Q, resulting in a tendency to vibrate in the wind." Just like in an RF network, resistance lowers the Q and therefore the oscillation potential by dissipating energy. The most common method is to insert rope into the element or just the element tip. Another is to run a piping helix around the tube surface. This is uncommon on antenna elements but I've seen it used to good effect to damp oscillations on Phyllistran synthetic guys.
• Texture: Although impractical for hams antennas, the surface of the tubes can be sculpted with patterns that reduce vortex formation and their amplitude.
• Turn the antenna: A yagi is typically pointed in the direction of stations you want to work. But when you are not in the shack the singing can be reduced by parking the yagi normal to the prevailing or current wind direction. For example, turn the antenna north or south for prevailing westerlies.

I won't delve into the finer points of mechanical mitigation measures except to note a few items that I've learned from experience and listening to those with greater knowledge. First, don't use excessively thin wall tubes. It is used in many antennas because aluminum is expensive and manufacturers know that keeping prices low increases sales. Stay at or above the typical 0.058" wall of aerospace high strength tubes, and overlap tubes more than the 3" minimum to further increase mass and strength. Second, beware product instructions that tell you to rope the elements. This is very close to an admission that there is a deficiency in the mechanical design.

I have watched hams thread rope through under-engineered full size 40 meter yagi elements and I've seen roped element tips fall to the ground. Damping from an internal rope may be insufficient to keep singing in check. Vibrating yagis can also cause fasteners to loosen or fail, and that can prove disastrous even without tube breakage. My TH6 elements are not roped and the non-trapped 15 meter elements sing; those of the TH7 were roped yet they also sang.

Singing isn't always straight forward to resolve, whether by treating the cause (design) or the symptom  with rope. Without a comprehensive mechanical model and simulation software we are too often left guessing. When one mitigation method fails should we blindly substitute, say, a different or longer rope, or should we try something completely different?

We may be going about it completely wrong. For example, mistaking the location of the symptom as the cause of the singing. Consider the following two yagis in my station. These are the upper and lower 5-element yagis of the 20 meter stack. We are looking toward the back of both antennas.

The construction, size and placement of the elements of the stacked yagis are identical. It may surprise you to learn that one yagi sings and the other does not. I know that I was surprised. Clearly there must be a difference. However we can eliminate a difference in the directions they are pointing since the disparate behaviour remains when the direction is the same (northeast, since the lower yagi is fixed).

The lower side-mounted yagi occasionally sings at moderate wind speeds. It isn't always noticable from the ground and I haven't carefully noted the wind speed and direction when it occurs. Typical for this latitude the prevailing winds are west to southwest, and that is approximately normal to the elements when they sing.

If not the elements themselves, what is the cause of the singing. I believe the answer lies with the frequency of the singing and how the entire antenna behaves. The audio frequency is low, below what a human can hear. The singing is not a sine wave so it may be the weaker harmonics that are heard. But when the singing is bad, all the elements shake and so do the boom and boom truss. That's unusual in my experience.

It is instructive to return to the above equation for vortex induced vibration. A low frequency implies a larger cylinder than is present in the elements, the largest being the 1" × 0.120" centre tube. The boom is the only large cylinder in the antenna and it is approximately the same for both antennas: 3". The equation for the same 3 m/s breeze gives a frequency of 8 Hz. That is in the range of what I observe. I didn't note whether the wind was from the northwest or southeast, which are normal to the boom.

Why only the lower yagi? I think the answer is that the booms are constructed differently despite having the same diameter. The tubes comprising the boom of the lower yagi have a wall thickness of 1/16" other than the thicker 10' long pipe at the centre of the boom. The weight difference of the two booms is substantial. As for elements, the mass of the boom seems to make all the difference.

Compared to the elements, yagi booms oscillate at lower frequencies and boom truss cables at higher frequencies. Unlike a singing element, when the boom sings (oscillates) all the elements shake. That appears to be what is happening in this case.

The solution is simple: replace the boom. I probably have what's needed in my stock of pipes to do that. If not, I can probably find more surplus. It isn't urgent. Watching the entire yagi vibrate is worrisome but it is not an emergency. It happens only occasionally when the wind direction and speed are just right. Maybe next year.

Singing is an example of structural resonance where the oscillation frequency is usually in the audible range. The 20 meter yagi is singing but the fundamental frequency is not audible. I can't hear it from the ground. A low resonant frequency is common to large structures like houses, bridges, skyscrapers and towers. 

Perhaps the best known example of undamped large structure oscillation is the Tacoma Narrows Bridge. Large structure oscillations are more difficult to analyze and defeat because there are many structural elements whose behaviours can be both independent and synergistic. Even so, as with yagis, there are a variety of effective mitigation methods in the engineer's toolkit.

Years ago I made use of structural oscillations excited by footsteps, wind and various mechanical impulses to sense and measure activity in buildings. It was interesting work. The oscillations were damped by the structure but lasted long enough to glean useful data. Although only minimally relevant to singing yagi, or suspension bridges, the physical principles are similar.