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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?
|
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?
|
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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how to eliminate mechanical resonance?
On 18 Oct 2006 01:46:45 -0700, wrote:
Francesco Do you receive my email? Wes N7WS |
how to eliminate mechanical resonance?
Hello Wes,
didn't receive anything, I sent an email to you. 73 Francesco IS0FKQ Wes Stewart ha scritto: On 18 Oct 2006 01:46:45 -0700, wrote: Francesco Do you receive my email? Wes N7WS |
how to eliminate mechanical resonance?
Dan Andersson wrote: wrote: snip Don't make it complicated! Complicated equals heavy! You most probably suffer from the effects of turbulence combined with harmonics from the aerial design itself. The immediate solution is to do what you do with high chimneys and car radio aerials - you wind a thin spiral round the full ( most? ) length of the aerial rods. This will break most of the turbulence. You choose lightweight and thin material for this. Try a couple of materials until you are happy. Also, bear in mind that it might only be one part of the aerial that generates the problem! By adding ropes etc, you just contain the result of the turbulence and the harmonics! But David isn't that the object of the exercise? The object is to dampen the occillation such that metal fatigue does not take place You can remove or redirect the causes of occillations but energy cannot be created or destroyed. Recognising this some manufactures place rope inside the elements and it works even tho it costs the manufacturer. To change the surface of the object receiving these stress reversals only hurts what you are trying to achieve unless what you are adding delays the effect of stress. With foam insulation I only followed the path that is generally used except I also wanted to isolate the inside from environmental effects such winds from other directions as well as providing stiffness. The bottom line is to make the elements as aerodynamic as possible and dampen the occillation that are left. Making things less aerodynamic only serves to worsen things. Are you sure that these additions to chimneys is to dampen occillations as opposed to strengthening the structure? Cheers Art By adding the spiral around the antenna rods, you have a good chance of eliminating the source of the problem which is the rods being susceptive to air turbulence, created by the environment as well as the aerial construction itself. The reasons for adding spirals on high rise chimneys are twofold, first to create a controlled turbulence as an added up draft as well as eliminating the possibility of creating a self oscillation due to the air pressure/turbulence and the harmonics generated in the design. By adding these spirals, your aerial will be susceptive to icing at cold weather! Check so that you don't get a crashing aerial because of heavy ice coating! Cheers Dan / M0DFI |
how to eliminate mechanical resonance?
art wrote: Dan Andersson wrote: But David isn't that the object of the exercise? The object is to dampen the occillation such that metal fatigue does not take place You can remove or redirect the causes of occillations but energy cannot be created or destroyed. Recognising this some manufactures place rope inside the elements and it works even tho it costs the manufacturer. To change the surface of the object receiving these stress reversals only hurts what you are trying to achieve unless what you are adding delays the effect of stress. With foam insulation I only followed the path that is generally used except I also wanted to isolate the inside from environmental effects such winds from other directions as well as providing stiffness. The bottom line is to make the elements as aerodynamic as possible and dampen the occillation that are left. Making things less aerodynamic only serves to worsen things. Are you sure that these additions to chimneys is to dampen occillations as opposed to strengthening the structure? Looks to me like you're overconcerned about fatique problems. Given a 2M antenna the elements would have to deflect +/- some large amount, an inch or so, probably more before the bending stresses would exceed the endurance limit of the types of aluminum alloys normally used to build yagis. If the stresses are below the endurance limit the element can deflect "forever" without suffering a fatigue failure. Further the clue in the description of the original problem indicates that the deflections are generating oscillations with frequencies in the audio range. If you do a freebody diagram of the "system" you'll find it's a rather classic spring-mass with excitation vibration problem. If you crunch the numbers and calculate the resulting time period on the basis of the mass of a half-element, the spring constant of the element and the observed frequency of vibration I suspect you'll find there's no way the element can deflect +/- fast enough to generate stresses on the element as high as the endurance limit. Stuff rope through the element and plug the ends with some heavy goop to add a bit of inertia out where it'll do some good and call it a day. Cheers Art Brian w3rv |
how to eliminate mechanical resonance?
wrote in message ups.com... 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. those vibrations are quite common in 10m and 6m yagis where the elements are made of aluminum tubing. they are from the elements fluttering in the wind (usually just in light wind). to put a few of the misconceptions to rest... First this is not a 'wind over the bottle' or flute effect. those are much higher frequency and are easily stopped with caps on the ends of booms and elements. so forget the solutions about plugging the ends of elements and booms for this one. This problem is caused by 2 properties of antennas about that size coming into phase with each other. the first is the vortex shedding around round objects like tubing or wire. this is an aerodynamics/fluid flow problem that is caused by air flowing around the tubing creating a low pressure area behind the tubing, this sets up two vortices behind the element. at a critical wind speed these vortices start to slip away from the element, and when they do they create an unbalanced force... i.e. if the top vortex sheds first the element gets pushed down, this often disrupts the flow causing the bottom one to shed which then pushes the element back up, which causes the top one to shed again, etc, etc, etc,... the effect alternately pushes the element up and down at a rate mostly determined by the wind speed. the cure for this is the spiral wrappings that you often see on chimneys and power lines. the spiral wrap is a way of breaking up the smooth flow around the object so that the vortices don't have a chance to form in the first place. they have nothing to do with strengthening the object, in fact they add to the wind load a bit, but the breaking up of the vortices is their main purpose. This vortex shedding problem is there all the time on all types and sizes of antennas, but on most antennas it does not cause the damaging type of vibration you see on some of them. why is this??? as long as the shedding frequency is not close to the element's or boom's mechanical resonance there is no problem, the forces occur but they are out of step with the natural response of the element so the vibration can't build. however in 6m and 10m yagis with commonly used tubing the combination of tubing weight and stiffness happens to have a mechanical resonance about the same frequency as the vortex shedding frequency at low wind speeds around that size tubing... everything is wrong. to make it worse, the added motion from the mechanical resonance makes the vortex shedding worse since the element moves more, causing it to move even more, and eventually fatigue and break in extreme cases. note, that the combination of very stiff larger diameter tubing used in later design telrex yagis down to 15m and 20m also had this problem, i found this when i started picking up pieces of elements and eventually whole elements from the ground under the towers from this effect. if you don't break up the vortices the other option is to change the resonant frequency. there are 2 ways to go, you can increase the frequency, which is very hard... to do that you have to make the element stiffer or lighter. you can't get much stiffer than the tubing being used without extreme measures, and likewise lighter is generally not an option. so the answer is heavier or less stiff. its easier to go heavier, just fill the element up with something heavy. This can also effect the 'stiffness' of the element, if the weight you put in the tubing is free to move it acts like a floppy weight and reduces the effective stiffness of the element. this is the combination that led to putting rope into elements. a good heavy hunk of rope (this is not the time for polypropelene) smaller than the inside diameter of the tubing adds weight and flops around effectively absorbing the vibration before it gets going. this gets harder on small diameter tubing usually seen on 6m yagis... on those try either aircraft cable or the plastic coated wire clothes line, or even heavy guage insulated wire. there are also other ways to stop vibrations like this. i have some 20m yagis now that have a long gap between the 1st and 2nd director, this gap was fluttering in certain winds so i snubbed it by adding a second boom brace rope in the middle of the section with just a little bit of tension to keep the boom from vibrating. electric utilities also use dumbell like spring devices that change the resonant frequency of large cables, these are often seen in pairs on either side of the insulators on towers in flat wind prone areas. |
how to eliminate mechanical resonance?
Very interesting , why can't you post like this on other threads?
Now back to this post. I have never seen spiral windings on a chimney and certainly would not put it on an antenna. Ice formation is the worst enemy for any antenna and spiral windings increases the adhesion of ice to an element. I will say it again that the best solution is to make the surface as aero dynamic as possible and to reduce the adhesion of ice which means no clamps on the element. One must also design connections of element tubing such that added weight to the element increases tension that is used to hold the element sections together such as a telescopic type taper connection which helps regarding air flow. Making the element lighter also helps because it allows for the willow tree effect like bending in the wind instead of stiffness that provide for occillation and fatigue failure. Putting all these facts together and since I live in the Midwest known for its wind and ice storms I decided a long while ago to change the method of manufacture of elements which especially helps today as aluminum costs go sky high. First I chose a foundation design which was inexpensive, light, and telescopic The 20 feet telescopic fishing pole available for less than $10 on ebay was the final choice ( I did not use the thin portion at the end) I then placed glass reinforced tape along on one side to counteract distributed weight sagging and then drilled small hole along that side so that urathane foam could be squirted inside while at the same time exuding air to provide a long homogenous section of the closed cell type. Now we get to the conductive surface addition. Hardware stores sell sticky back aluminum tape for air sealing purposes which bearing in mind that only material to allow for twice the skin depth penetration is required is then placed length wise along the element length thus overcoming joint resistance change and with a rub down of the surface to remove air bubbles you have a radiating element that is superior to the present state of the art. One more point before closing, imagine a 20 pound block of ice hanging on the end of the element, would the ice hang on to the clamp or aluminum without breaking the element? If the same ice load was on a non clamped form of element I just described would it slide off, bend like it was holding a fish or...... immediately break? One other point, these telescopic poles made in China appear to be standardised and thus intechangeable also when home brewing you just add or remove the added foil at the ends without messing with aluminum section joints. I changed over to this design years ago when I homebrewed a long boom yagi with 13 elements so my fold over tower could handle lifting it and to this day I use this method for making elements. I have mentioned this before but I got a lot of hoots and cries from old timers who object to change and fully expect a repeat of the cat calls. When hiking just take the sections as is and assemble at site as for short term closed cell foam is not really needed. Hopefully some of the younger hams will be attracted to this approach as the cost is way less than the $1000 required for a manufactured antenna. Cheers and beers Art and nothing li Brian Kelly wrote: art wrote: Dan Andersson wrote: But David isn't that the object of the exercise? The object is to dampen the occillation such that metal fatigue does not take place You can remove or redirect the causes of occillations but energy cannot be created or destroyed. Recognising this some manufactures place rope inside the elements and it works even tho it costs the manufacturer. To change the surface of the object receiving these stress reversals only hurts what you are trying to achieve unless what you are adding delays the effect of stress. With foam insulation I only followed the path that is generally used except I also wanted to isolate the inside from environmental effects such winds from other directions as well as providing stiffness. The bottom line is to make the elements as aerodynamic as possible and dampen the occillation that are left. Making things less aerodynamic only serves to worsen things. Are you sure that these additions to chimneys is to dampen occillations as opposed to strengthening the structure? Looks to me like you're overconcerned about fatique problems. Given a 2M antenna the elements would have to deflect +/- some large amount, an inch or so, probably more before the bending stresses would exceed the endurance limit of the types of aluminum alloys normally used to build yagis. If the stresses are below the endurance limit the element can deflect "forever" without suffering a fatigue failure. Further the clue in the description of the original problem indicates that the deflections are generating oscillations with frequencies in the audio range. If you do a freebody diagram of the "system" you'll find it's a rather classic spring-mass with excitation vibration problem. If you crunch the numbers and calculate the resulting time period on the basis of the mass of a half-element, the spring constant of the element and the observed frequency of vibration I suspect you'll find there's no way the element can deflect +/- fast enough to generate stresses on the element as high as the endurance limit. Stuff rope through the element and plug the ends with some heavy goop to add a bit of inertia out where it'll do some good and call it a day. Cheers Art Brian w3rv |
how to eliminate mechanical resonance?
Very interesting , why can't you post like this on other threads?
Now back to this post. I have never seen spiral windings on a chimney and certainly would not put it on an antenna. Ice formation is the worst enemy for any antenna and spiral windings increases the adhesion of ice to an element. I will say it again that the best solution is to make the surface as aero dynamic as possible and to reduce the adhesion of ice which means no clamps on the element. One must also design connections of element tubing such that added weight to the element increases tension that is used to hold the element sections together such as a telescopic type taper connection which helps regarding air flow. Making the element lighter also helps because it allows for the willow tree effect like bending in the wind instead of stiffness that provide for occillation and fatigue failure. Putting all these facts together and since I live in the Midwest known for its wind and ice storms I decided a long while ago to change the method of manufacture of elements which especially helps today as aluminum costs go sky high. First I chose a foundation design which was inexpensive, light, and telescopic The 20 feet telescopic fishing pole available for less than $10 on ebay was the final choice ( I did not use the thin portion at the end) I then placed glass reinforced tape along on one side to counteract distributed weight sagging and then drilled small hole along that side so that urathane foam could be squirted inside while at the same time exuding air to provide a long homogenous section of the closed cell type. Now we get to the conductive surface addition. Hardware stores sell sticky back aluminum tape for air sealing purposes which bearing in mind that only material to allow for twice the skin depth penetration is required is then placed length wise along the element length thus overcoming joint resistance change and with a rub down of the surface to remove air bubbles you have a radiating element that is superior to the present state of the art. One more point before closing, imagine a 20 pound block of ice hanging on the end of the element, would the ice hang on to the clamp or aluminum without breaking the element? If the same ice load was on a non clamped form of element I just described would it slide off, bend like it was holding a fish or...... immediately break? One other point, these telescopic poles made in China appear to be standardised and thus intechangeable also when home brewing you just add or remove the added foil at the ends without messing with aluminum section joints. I changed over to this design years ago when I homebrewed a long boom yagi with 13 elements so my fold over tower could handle lifting it and to this day I use this method for making elements. I have mentioned this before but I got a lot of hoots and cries from old timers who object to change and fully expect a repeat of the cat calls. When hiking just take the sections as is and assemble at site as for short term closed cell foam is not really needed. Hopefully some of the younger hams will be attracted to this approach as the cost is way less than the $1000 required for a manufactured antenna. Cheers and beers Art and nothing li Brian Kelly wrote: art wrote: Dan Andersson wrote: But David isn't that the object of the exercise? The object is to dampen the occillation such that metal fatigue does not take place You can remove or redirect the causes of occillations but energy cannot be created or destroyed. Recognising this some manufactures place rope inside the elements and it works even tho it costs the manufacturer. To change the surface of the object receiving these stress reversals only hurts what you are trying to achieve unless what you are adding delays the effect of stress. With foam insulation I only followed the path that is generally used except I also wanted to isolate the inside from environmental effects such winds from other directions as well as providing stiffness. The bottom line is to make the elements as aerodynamic as possible and dampen the occillation that are left. Making things less aerodynamic only serves to worsen things. Are you sure that these additions to chimneys is to dampen occillations as opposed to strengthening the structure? Looks to me like you're overconcerned about fatique problems. Given a 2M antenna the elements would have to deflect +/- some large amount, an inch or so, probably more before the bending stresses would exceed the endurance limit of the types of aluminum alloys normally used to build yagis. If the stresses are below the endurance limit the element can deflect "forever" without suffering a fatigue failure. Further the clue in the description of the original problem indicates that the deflections are generating oscillations with frequencies in the audio range. If you do a freebody diagram of the "system" you'll find it's a rather classic spring-mass with excitation vibration problem. If you crunch the numbers and calculate the resulting time period on the basis of the mass of a half-element, the spring constant of the element and the observed frequency of vibration I suspect you'll find there's no way the element can deflect +/- fast enough to generate stresses on the element as high as the endurance limit. Stuff rope through the element and plug the ends with some heavy goop to add a bit of inertia out where it'll do some good and call it a day. Cheers Art Brian w3rv |
how to eliminate mechanical resonance?
art wrote:
Very interesting , why can't you post like this on other threads? What?? You're new around here aren't you? Run a search in this group on "w3rv"for the years 1995 thru 2005 inclusive then kindly reconsider your comment. Any of my online food-fight posts which have shown up in this group recently were cross-posted from Rec.Radio.Amateur.Policy without my intent or knowledge. RRAP was founded to specifically debate the contentious code test issue and I've posted to it many times over the years often in a bit of a combative mode. Which is normal, expected practice in that particular group. Food-fights are *not* normal, expected practice in this group and as your search results will illustrate I *don't* indulge in them in this group. The problem all the ham radio USENET groups is suffering involves the behavior of a few members of the "ham community" who have no respect at all for propriety and civilized behavior and they're destroying valuable resourses like this group. They obviously come from levels of society which the average ham would not like to have living anywhere near their neighborhoods. For lack of a better way to put it . . It's the same bunch which are turning the HF phone bands into embarrassments. It's a shame but that's the way it is these days. The only ways out seem to be moderated reflectors and discussion groups. Sad. Brian w3rv |
how to eliminate mechanical resonance?
I don,t think that is all encompasing. During the last couple of days I
have written two new postings regarding the academic side of antennas which is what I thought members wanted but that is not the case, read a few answers that I got which are very few. On the other side of the coin there are more members responding to non the antenna issues with slander so as with any group the majority rules. Throwing mud is always more enjoyable in the absence of applicable logic, but then it is safer to say what Roy resorts to "I don't understand " The days that ham radio pursued the sciences are long gone and this antenna newsgroup is a perfect example of that We have to recognise that simple fact and find a way to turn it around to our own advantage, or alternatively get a gun and play fire with fire. As far as being a short time poster that is not true, Over the years I have seen more slanderous postings from the suedo experts of which there is still a few around At one time such comments were thick and heavy even racial with respect to jews so many knowledgable hams left and were replaced by posters who enjoyed that sort of exchange. Go to the archives and pick out a name that applies to a self percieved antenna expert bearing in mind that I am a learner and you will quickly come across the worst that you will ever see. As I said earlier some of those early combantants are still having a hayday.and more silly logic is coming in from all sides to join them. Again count where the majority postings have headed in the last few days to gauge what antenna types really want. I must exclude Walt from all others since no matter what politeness rules in all of his postings as well as applicable logic. Sometimes I have been driven to a lessor ind of response based on slander given. Cheers Art Brian Kelly wrote: art wrote: Very interesting , why can't you post like this on other threads? What?? You're new around here aren't you? Run a search in this group on "w3rv"for the years 1995 thru 2005 inclusive then kindly reconsider your comment. Any of my online food-fight posts which have shown up in this group recently were cross-posted from Rec.Radio.Amateur.Policy without my intent or knowledge. RRAP was founded to specifically debate the contentious code test issue and I've posted to it many times over the years often in a bit of a combative mode. Which is normal, expected practice in that particular group. Food-fights are *not* normal, expected practice in this group and as your search results will illustrate I *don't* indulge in them in this group. The problem all the ham radio USENET groups is suffering involves the behavior of a few members of the "ham community" who have no respect at all for propriety and civilized behavior and they're destroying valuable resourses like this group. They obviously come from levels of society which the average ham would not like to have living anywhere near their neighborhoods. For lack of a better way to put it . . It's the same bunch which are turning the HF phone bands into embarrassments. It's a shame but that's the way it is these days. The only ways out seem to be moderated reflectors and discussion groups. Sad. Brian w3rv |
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