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how to eliminate mechanical resonance?
Hello all,
I build one year ago an 8 element 50 MHz yagi. Boom is made with 50 mm square aluminum tubing with a second square tube on the center of the antenna to strenghten the boom. Elements are made with 20 mm round aluminum tube in electrical contact with the boom. The antenna is a good performer but it has one problem: it sounds even in light wind and vibrations come down the mast and are very annoying in the whole house. The worst problem is that resonance succeeded in unscrewing the rotor bolts. I'm going to use a special for bolts and screws but I'd like to find a method to avoid mechanical resonance of the antenna. Does anyone know a good method? Would it be better to add two insulated pulling wires at both ends of the boom (even if probably also elements produce udible resonance)? Thanks in advance Francesco IS0FKQ/I8 |
how to eliminate mechanical resonance?
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how to eliminate mechanical resonance?
Take a sheet of rubber (say 3-5mm thick) maybe 150mm long and the
circumference of the element wide. Make 3 equally spaced cuts 70% of the distance of the long dimension. It will now look like a 4 fingered hand shape! Using a hose clamp tie this rubber sheet around the uncut end around the elements at some convenient place. The idea being to damp mech oscillation. Maybe 1/3 the way down each element but I haven't exactly thought about where the best place would be. It may slightly detune the antenna but I haven't thought that through either! Cheers Bob VK2YQA wrote: Hello all, I build one year ago an 8 element 50 MHz yagi. Boom is made with 50 mm square aluminum tubing with |
how to eliminate mechanical resonance?
Fill it with expanding foam used for heat insulation. Art Bob wrote: Lots of commercial antennas come with rope inside the elements for mechanical damping. Just find some dense and preferably synthetic rope that just fits inside the elements. If it's a vertical antenna, cap the top elements to keep rain from entering, otherwise leave the elements open so any moisture will bake out in the sun. Bob wrote: Hello all, I build one year ago an 8 element 50 MHz yagi. Boom is made with 50 mm square aluminum tubing with a second square tube on the center of the antenna to strenghten the boom. Elements are made with 20 mm round aluminum tube in electrical contact with the boom. The antenna is a good performer but it has one problem: it sounds even in light wind and vibrations come down the mast and are very annoying in the whole house. The worst problem is that resonance succeeded in unscrewing the rotor bolts. I'm going to use a special for bolts and screws but I'd like to find a method to avoid mechanical resonance of the antenna. Does anyone know a good method? Would it be better to add two insulated pulling wires at both ends of the boom (even if probably also elements produce udible resonance)? Thanks in advance Francesco IS0FKQ/I8 |
how to eliminate mechanical resonance?
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how to eliminate mechanical resonance?
Also, be aware that you MUST eliminate that
resonance ! Many years ago, myself (and another ham) obtained the 6 meter moon bounce array from a well known ham (he built it, got echos, than abandoned it- reason, at that time, he was the ONLY op capible of 6 meter moonbounce! helped the owner of the first half put it up on 5 inch irrigation pipe! The array worked well- but the wind started oscillations in the elements. The elements began to shear off from metal fatigue, at the point where they went thru the boom. Same happened to mine, even as stand alone yagis! These had elements of 6061-t , 5/16 inch dia. tubeing, if memory serves. Translation: IF allowed to continue, look for the elements to start breaking off, at some point in time! Would agree that the insulating foam might be enough to stop the oscillations in the elements. Good luck Jim NN7K Gene Fuller wrote: Hi Francesco, You did not describe the vibrations, so this may be off base. If it is a "sound", then perhaps the tubing is acting like a flute. Plugging the ends of the open tubes would stop the sound. It may be necessary to leave a small hole for drainage. 73, Gene W4SZ |
how to eliminate mechanical resonance?
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how to eliminate mechanical resonance?
Hi Gene, wrote: Hello all, I build one year ago an 8 element 50 MHz yagi. Boom is made with 50 mm square aluminum tubing with a second square tube on the center of the antenna to strenghten the boom. Elements are made with 20 mm round aluminum tube in electrical contact with the boom. The antenna is a good performer but it has one problem: it sounds even in light wind and vibrations come down the mast and are very annoying in the whole house. The worst problem is that resonance succeeded in unscrewing the rotor bolts. I'm going to use a special for bolts and screws but I'd like to find a method to avoid mechanical resonance of the antenna. Does anyone know a good method? Would it be better to add two insulated pulling wires at both ends of the boom (even if probably also elements produce udible resonance)? Thanks in advance Francesco IS0FKQ/I8 Hi Francesco, You did not describe the vibrations, so this may be off base. If it is a "sound", then perhaps the tubing is acting like a flute. Plugging the ends of the open tubes would stop the sound. It may be necessary to leave a small hole for drainage. The sound is low frequency, I'd say less than 100 Hz with amplitude modulation of 1-2 Hz probably due to boom vibration. I thought about flute effect but I think it would be an higher pitch in that case. I read many interesting suggestions though, I hope to be able to try some of them in the next week end. 73 Francesco IS0FKQ/I8 |
how to eliminate mechanical resonance?
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