In most cases, radio amateurs love it when their antenna resonates, because a resonating antenna is usually an efficiently radiating antenna. At least, when we talk about electrical resonance and not acoustic resonance. The latter can cause considerable nuisance and even dangerous situations. Fortunately, there is an effective and inexpensive way to keep an antenna out of annoying and potential dangerous acoustic resonance; simple rope.
Humming, booming or whistling
Many radio amateurs are familiar with the phenomenon of an acoustic resonating installation. Like a buzzing, humming booming or whistling antenna or antenna mast. The sound that causes that resonance can be quite loud and propagate throughout the entire house. Quite annoying if your antenna (mast) sits on the roof of your house, or is mounted to a side wall. The same resonance can also cause improperly secured bolts and nuts to vibrate loose. Resonance of your antenna installation is therefore not only a nuisance but can also be dangerous.
Tackle the source
In practice, all kinds of means are used to reduce vibrations. A mast can be attached to a base with vibration dampers, placed on rubber garden tiles or clamped in wall brackets with rubber blocks. But that is only symptom relief, while tackling the source is much more effective, easier and a lot cheaper. That is why it is good to know what actually causes this resonance.
Kármán Vortex
When air flows past an object, underpressure is created on the back of that object. Turbulence arises in that underpressure. Air vortices that you also see in the wake of an airplane’s wings or ship’s tail. These swirls are also referred to as the “Von Kármán Street”, also known as the “ Kármán Vortex Street ”.
A notorious noise maker
Air vortices produce vibrations with a certain frequency. When that frequency reaches the mechanical resonance frequency of the object, the object starts to vibrate, which further amplifies the sound. In principle, this can happen to any object, but round objects (such as the cylindrical shape of a tube) are notorious noise makers. Steel and aluminum tubes are popular for constructing antennas and masts. That makes the average antenna installation susceptible to acoustic resonance.
Formula 1-style spoilers
If we want to tackle the problem at source, we must prevent air vortices from arising. Now you can use all kinds of streamlined shapes for your antenna installation, but of course that is anything but practical. So we have to do something about the shape of the construction to avoid air vortices. I do not mean fancy Formula 1-style spoilers or diffusers. Combating acoustic antenna noise is very simple.
The spiral stair case
A very effective method to disturb the formation of air vortices behind a cylindrical object is by adding a spiral around it. That spiral prevents the air from swirling like in the Kármán Vortex Street and resonate. Large structures are protected this way from producing noise and structural damage due to vibrations. When you see a spiral staircase around a large chimney or tall silo, there is a good chance that it is not only made to serve as stairs, but also to prevent these air vortices.
Cheap rope
I had quite a few resonance problems myself. Typically when air reaches wind speed of about 6 Bft, the my vertical antenna (which stands on the roof of a bedroom) start sto hum. I tested the method of applying a spiral around the antenna. For that I used cheap 4mm (5/32″) rope bought at a local shop. This rope is suitable for outdoor use and is also available in fashionable colors. I wound that rope around the vertical like a spiral and secured it every 1m (3ft) with a cable tie. You can basically use any kind of rope or cord, if it is thick enough. It is better if it is resistant to UV, otherwise the rope will eventually decay.
Silent ever since
The result is impressive, since I applied the rope, the antenna has been as quiet as a mouse. That for less than € 2.00 for a 20 meter spool of rope. Although the cord retains some water after a rain shower, I have not measured any significant differences in the performance of the antenna, let alone a change in SWR.
Vertical with traps
When you have a vertical with traps, the rope might conduct electricity when it’s wet, and current will bypass the the traps. But there is a simple solution. Just stop below the traps when spiraling the rope and secure it with a cable tie. Then secure the rope above the trap with a cable tie and resume until the next trap.
This article was also published in Dutch on www.veron.nl.