Humming Aerials

[Panrock]

When the wind is from a certain direction and a certain speed, many has been the night when I've been soothed to sleep by a deep, powerful, throbbing hum. It's not so much me I'm worried about... but the neigbours have yet to complain.

This hum is caused by a vertical Band II three-element array on my chimney, pointing at Sutton. Previously, when I had the Band I array pointing at Crystal Palace, the hum was still more serious. Then, I stuffed bamboo poles inside the elements to try to damp it, with limited success. Hum seems to be a feature of long-element aerials.

A question for the old hands (and maybe radio amateurs too). Are there any old and trusted remedies to deal with this problem? Maybe screw-on weights would lower the resonant frequency to sub-audio...

Steve

[Sean Williams]

Normally a length of sisal cord down the middle of the element, sealed with a dollop of bitumous compound will sort it - Its what I did with my 50Mhz Beam antenna - howled like a banshee!

Cheers
Sean

[arjoll]

We get vibrations on our 9 element Lincrad (code 113A here) in high winds. The band I elements have wood inside them to dampen things a bit, and while its better than the cheaper aerial at our old place its still quite noisy in storms (our community - just out of Invercargill - is a high wind zone and 100km/h+ winds are not unusual).

Talking to aerial installers there's very little that can be done about it, on domestic systems anyway. That might just be because they can't be bothered putting as much effort in as Sean and also the folded dipole part has no wood in it as far as I'm aware - would be a manufacturing nightmare to curve the wood like that

[Aub]

Hi,

You may find that it's not so bad outside as it sounds inside, due to the sound being transmitted through the brickwork.

regards

Aub

[Station X]

Such "humming" can lead to the elements snapping off due to metal fatigue. This happened to my FM antena which had round elements. One after the other they snapped off at the bolt hole holding the element to the beam. The replacement has square elements and these have survived so far.

You could try injecting silicone sealer into the elements.

A solution I've seen recommended in Amateur Radio Books it to push a 6" length of small diameter hose over the element and hold it in place with a hole clip. The hose is then split lengthways in about four places. I don't know it it's effective, but it's certainly unsightly.

[HG MICKE]

In the past we have injected the elements with polystyrene foam, The kind you get from builders merchants for filling cavities in walls or around window frames, This dampens the resonant oscillations of the elements and has the advantage of being light weight.

Colin

[Sam]

The hum is, I think, caused by a Vortex Street. This is the same effect as causes a Telegraph Line to hum in the wind. We did a Lab Experiment on this in my 3rd year at Uni (too long ago to remember exactly now, but I can look it up if people are really interested). If the resonant frequency of the element is similar to the Shedding frequency then it can be excited and resonate. Altering the stiffness of mass of the element will then tune this resonant frequency up or down from the Shedding frequency.

Assuming the element to be cylindrical and slender then flows around an infinite cylinder will give a reasonable indication of these frequencies. Isn't Fluid Dynamics great!

Sam

[alynG1TBL]

Hi All,I have used foam filler in my amateur radio aerials befor now and it worked quite well.PS used to transmit amateur television in the period from 1986 to 1994 on 70cms with my callsign G1TBL (still keep it may soon go back on air with some TV)Best wishes Alyn

[russell_w_b]

Originally Posted by Panrock

A question for the old hands (and maybe radio amateurs too). Are there any old and trusted remedies to deal with this problem? Maybe screw-on weights would lower the resonant frequency to sub-audio...

Steve

Some of our transmitter mast stay-wires are fitted with 'Stockbridge Dampers' (the dumbell-thingies seen on electrical power lines near the pylon) to dampen aeolean vibrations. I appreciate that yours is a solid rod rather than a taut wire, but it might be worth adding weights to see if your vibration damps successfully.

I've seen stay-wires (again, not really applicable in your case) with chains hanging off them to keep them damped.