![]() |
The dipole and the violin
Yeah the folded dipole that is. We see there 4 x ¼ = 2 lambda. Following the antennabook one can earth the middle of that folded dipole. And feed it with a coax. So from there we can bridge the mantle from the coax to the middle of the straight dipole piece. See it ? Now my problem in visualising that; the current from hot goes to the ½ wave dipole....hits ground there.....appears out of the ground(!).... hits the second ½ wave....and dives back in the coax mantle. If this is true i can't imagine it ! Now the violin string. Take one string attached between two walls or whatever. Exite it. Now ground the string in the middle.....exite the first half......and the second halve will vibrate also ?? Thru ground ?? Are the two statements right? The mind boggles. |
The dipole and the violin
Calltrex wrote:
Yeah the folded dipole that is. We see there 4 x ¼ = 2 lambda. Following the antennabook one can earth the middle of that folded dipole. And feed it with a coax. So from there we can bridge the mantle from the coax to the middle of the straight dipole piece. See it ? Now my problem in visualising that; the current from hot goes to the ½ wave dipole....hits ground there.....appears out of the ground(!).... hits the second ½ wave....and dives back in the coax mantle. If this is true i can't imagine it ! Now the violin string. Take one string attached between two walls or whatever. Exite it. Now ground the string in the middle.....exite the first half......and the second halve will vibrate also ?? Thru ground ?? Are the two statements right? The mind boggles. It's even cooler than that. You can put two antennas miles apart with nothing but space between. Put current through one, and current will appear in the other -- THROUGH SPACE! Put two violins close together and pluck the string of one. The corresponding string of the other will vibrate. THROUGH AIR! You mind has just begun to boggle. Roy Lewallen, W7EL |
The dipole and the violin
Calltrex wrote:
Yeah the folded dipole that is. We see there 4 x ¼ = 2 lambda. Following the antennabook one can earth the middle of that folded dipole. And feed it with a coax. So from there we can bridge the mantle from the coax to the middle of the straight dipole piece. See it ? Now my problem in visualising that; the current from hot goes to the ½ wave dipole....hits ground there.....appears out of the ground(!).... hits the second ½ wave....and dives back in the coax mantle. If this is true i can't imagine it ! Now the violin string. Take one string attached between two walls or whatever. Exite it. Now ground the string in the middle.....exite the first half......and the second halve will vibrate also ?? Thru ground ?? Are the two statements right? The statement in the antenna book is probably correct. The statement 4 x 1/4 = 2 is not correct. With regard to violin strings, they only vibrate if they are mechanically stimulated. Otherwise they behave in a manner similar to other inanimate objects. :-) 73, ac6xg |
The dipole and the violin
Roy Lewallen wrote in
treetonline: Put two violins close together and pluck the string of one. The corresponding string of the other will vibrate. THROUGH AIR! Then consider the (say) three strings of a piano note, and that if their resonant frequencies are close enough, they vibrate in phase (that implies at the same frequency of course), well they do until the amplitude of vibration dies down sufficiently and they 'unlock' and vibrate independently. You mind has just begun to boggle. Now, somehow I can see this being used to explain an antenna. I hear the behaviour of a string in a violin being used to explain why resonant antennas are just better, and how they "fairly suck the power out of the transmitter, like a string sucks the power out of the bow". Owen |
The dipole and the violin
On Mon, 16 Mar 2009 12:24:55 -0800, Jim Kelley
wrote: Calltrex wrote: Yeah the folded dipole that is. We see there 4 x ¼ = 2 lambda. Following the antennabook one can earth the middle of that folded dipole. And feed it with a coax. So from there we can bridge the mantle from the coax to the middle of the straight dipole piece. See it ? Now my problem in visualising that; the current from hot goes to the ½ wave dipole....hits ground there.....appears out of the ground(!).... hits the second ½ wave....and dives back in the coax mantle. If this is true i can't imagine it ! Now the violin string. Take one string attached between two walls or whatever. Exite it. Now ground the string in the middle.....exite the first half......and the second halve will vibrate also ?? Thru ground ?? Are the two statements right? The statement in the antenna book is probably correct. The statement 4 x 1/4 = 2 is not correct. With regard to violin strings, they only vibrate if they are mechanically stimulated. Otherwise they behave in a manner similar to other inanimate objects. :-) 73, ac6xg Well as both a ham and a better than competent violinst (I play in symphonies and also do solo recitals.) I have been playing violin for 57 years vice only 53 years as a ham, so I am a bit of a rookie on the ham side of things. You are sort of right and sort of confusing the laws of acoustics with the laws of electronics. When we bisect the violin string (which gives you an octave of the open string) the played (excited) part will give you a frequency double that of the open string. We violinists prefer to think of that as an octave. In practice, the unexcited string is damped to inaudibility. Aside from our standard tuning note of A= 440 Hz, violinists don't normally think of frequencies. In practice though violin strings don't give out pure sine waves, they have all sorts of somewhat random overtones, something which, along with the resonant box we call a violin, give each instrument its character. Without them we'd sound like a MIDI keyboard. The only time both sides of a stopped string vibrate is when we play what we call a "harmonic" and there are several of them on violin strings. A harmonic is when the violinist lightly touches the string so that the unexcited side (e.g. not the bowed side) can also vibrate and these have their own set of rules and playing implications. Fortunately the composers usually indicated this stuff for us, so we really don't have to think very much about the physics of all of this. If we lightly touch the violin string at the octave point rather than pressing all the way down as we usually do, we get something of a whistleing sound an octave higher than the normal stopped note (both sides ot the sting are vibrating.) If we touch lightly at the interval of a fourth above an open string (on an E string, this would be A above the E string, and the resultant sound is two octaves above the fundamental because you are dividing the string at a one-fourth point and are creating what in effect is a standing wave (though that concept is unknown among violinsts unless they happen to be EE's or hams.) Of course I wave length at our frequencies doesn't quite come out 300,000,000 / frequency = Lambda. We'd have some pretty damn long strings at A above middle C. I think that comes out to a wavelength of 68,181 meters. Just wouldn't fit on a violin. If you touch one note above that (B natural on the E string) the result is a not an octave and a fifth (or high B) above the fundamental frequency, because lightly stopping the B and causing it to vibrate on both side of the stop divides the string into thirds. In practice though violin strings are all over the map as to what is vibrating where and when, though if violins would produce pure sine wave instead of a rich blend of harmonics and overtones. If the FCC monitored violin harmonics, I doubt we could meet the 40 db spectral puriity requirement. Mercifully the Gov't stays out of the violin world. At a lab level, we can sort of come up with a demo of sympathetic vibrations. In practice, of course we don't. Few violinist are actually that close to in tune (close, but not exact...otherwise and orchestra would sound like a signal generator.) In string instrument history, going back to about the early 15th century, The string insturments known a viols in some cases actually did have a set of strings under the bridge which did vibrate sympathetically giving an effect somewhat similar to the wow-wow effect of a cheap organ. Most of these instruments are know played by historical music specialists and are generally out of the mainstream of the orchestral world in which I dwell, but there are a few compositions which call for the Viola d'Amore which does have such strings. (there is a short solo in the Opera "Manon" by Massenet which calls for this instrument and its sympathetically vibrating strings. Those string has some technical name, but I don't remember what they are called.) Somehow, despite knowing both RF theory pretty well and violin playing damn well, I never really ever considered the positioning of my fingers in playing as analogous to "grounding." I must work on that one. I don't think I have ever also considered my violins strings to have impedance, or reactance, and I haven't quite considered the feedline problem. Is my bow a 52 ohm feed point or 72 Ohms? :-) My bows don't have any PL-259s or BNCs on the, just horse hair. Resonance, of course is provided by the violin body, but I've never considered this in terms of RF resonance. Interesting concept. Now if we could do something like a Yagi and get sympathetic strings to provide director and reflector gain and front to back ratios, we might solves a lot of auditorium problems and not be blasted to inaudibilty by the brass instruments which of course do have rather considerable front to back ratios (though the French Horns have theirs the wrong way.) Now I have to reconcile my fiddle knowledge with all the stuff I had to learn to pass my FCC tests. I'm old enough to have passed the General and Extra tests before they were multiple choice format. The music certainly helps with the code. Jon Teske, W3JT and concert violinist. |
The dipole and the violin
I'm also a ham and a one-time violist, as well as an amateur
instrument-maker. Jon Teske wrote: The only time both sides of a stopped string vibrate is when we play what we call a "harmonic" and there are several of them on violin strings. A harmonic is when the violinist lightly touches the string so that the unexcited side (e.g. not the bowed side) can also vibrate In particular, you have to touch lightly enough that some energy can couple into the other half of the string. Some of the energy is transferred through up-down motion and some through the bending- stiffness of the string itself. You could see the finger as a series capacitor to ground, attached to each half of the string by a pair of resisters bypassed by another capacitor. You can tweak the values by the way you touch. I don't think I have ever also considered my violins strings to have impedance, or reactance, and I haven't quite considered the feedline problem. Mechanical impedance/reactance is a widely used idea in mechanical and civil engineering. The dynamic motion of bridges and buildings is sometimes even modelled using electronic circuit modelling tools amongst other methods. Clifford Heath. |
The dipole and the violin
The early radio pioneers were schooled in acoustics, mechanics en chemistry. So they understood quite soon how to resonate the electric version of a wave. That takes a bit of the mystery away. Yes, 4 x ¼ = 1 lambda. So a folded dipole is 1 lambda and an open dipole is a half lambda. But a folded dipole can thus be halved. A sort of trombone shape. BUT the ends resonate with tension right ? So we have to feed one end with high impedance also. Next detail and the big question for me; may the other end be grounded ? In case of a situation where no counterpoise can be installed. Because of the folded dipole which is earthted in the straight middle, i think the half wave end fed can be earthed. Agreed ? The idea is to topfeed a mast which stands in conductive grounds. So ¼ wire up and the tower is ¼ wave down, which to my paper insight should work as an ½ wave antenna ? pls say yes. :-) ? |
The dipole and the violin
On Tue, 17 Mar 2009 15:18:35 +1100, Clifford Heath
wrote: I'm also a ham and a one-time violist, as well as an amateur instrument-maker. Yeah! I am not alone. BTW I also play viola at a symphonic level. Never tried making any life is too short to do both. Jon Teske wrote: The only time both sides of a stopped string vibrate is when we play what we call a "harmonic" and there are several of them on violin strings. A harmonic is when the violinist lightly touches the string so that the unexcited side (e.g. not the bowed side) can also vibrate In particular, you have to touch lightly enough that some energy can couple into the other half of the string. Some of the energy is transferred through up-down motion and some through the bending- stiffness of the string itself. You could see the finger as a series capacitor to ground, attached to each half of the string by a pair of resisters bypassed by another capacitor. You can tweak the values by the way you touch. Yeah I forgot to mention the energy transfer side of that. Since harmonics are hard enough to produce in any event, a tunable capacitor might be welcome. Unfortunately a fiddler has only two hand, both heavily employed. I don't think I have ever also considered my violins strings to have impedance, or reactance, and I haven't quite considered the feedline problem. Mechanical impedance/reactance is a widely used idea in mechanical and civil engineering. The dynamic motion of bridges and buildings is sometimes even modelled using electronic circuit modelling tools amongst other methods. I guess that is the thing they forgot to calculate, or didn't know how, on the infamous Takoma Narrows Bridge in Washington State which collapsed when cross winds cause wild vibrations. Jon Teske W3JT Clifford Heath. |
The dipole and the violin
Jon Teske wrote:
If you touch one note above that (B natural on the E string) the result is a not an octave and a fifth (or high B) above the fundamental frequency, because lightly stopping the B and causing it to vibrate on both side of the stop divides the string into thirds. Jon Teske, W3JT and concert violinist. And this is why pianos are arranged to strike the string at a point which suppresses a harmonic which is dissonant. (I think it's the 7th harmonic which is suppressed) The choice of where the hole you blow over in a flute has similar things going on. As do the locations of the holes on any wind instrument. That musical instrument design thing is not as simple as it might seem in first year physics class. |
The dipole and the violin
Mechanical impedance/reactance is a widely used idea in mechanical and civil engineering. The dynamic motion of bridges and buildings is sometimes even modelled using electronic circuit modelling tools amongst other methods. I guess that is the thing they forgot to calculate, or didn't know how, on the infamous Takoma Narrows Bridge in Washington State which collapsed when cross winds cause wild vibrations. That was an unexpected coupling between the force from the wind and torsional vibration of the roadbed. As the roadbed tilted, it "caught" more of the wind and had more force applied, moving it further. The torsional resonance was such that it oscillated with ever greater amplitude (not much different than a flag flapping, or a blade of grass in the wind.. not quite like a wind instrument reed, though) As for whether it could have been anticipated? I don't know that modeling was that advanced back then (1930s). The bridge was an architectural feat, with a very delicate looking thin roadbed and much longer than most other bridges (3rd longest when it was built, some 1500 feet longer than the Golden Gate, for instance). It was much longer and thinner as compared to other suspension bridges of the time which were double decked, (SF Oakland Bay Bridge) for instance.. making them torsionally much stiffer). Interestingly, the designer of Tacoma Narrows (Moisseiff) was also involved in the Golden Gate. |
The dipole and the violin
Richard Clark wrote:
On Wed, 18 Mar 2009 11:11:42 -0700, Jim Lux wrote: I guess that is the thing they forgot to calculate, or didn't know how, on the infamous Takoma Narrows Bridge in Washington State which collapsed when cross winds cause wild vibrations. That was an unexpected coupling between the force from the wind and torsional vibration of the roadbed. As the roadbed tilted, it "caught" more of the wind and had more force applied, moving it further. The torsional resonance was such that it oscillated with ever greater amplitude (not much different than a flag flapping, or a blade of grass in the wind.. not quite like a wind instrument reed, though) In fact, it was exactly like a reed. The Tacoma Narrows Bridge exhibited the highest roadbed length to roadbed width ratio of the designs of that era, and this was a contributing factor. Reeds don't oscillate torsionally (at least not as the dominant mode) They're more of a fixed/free beam that oscillates in longitudinal bending. The bridge was an architectural feat, with a very delicate looking thin roadbed and much longer than most other bridges (3rd longest when it was built, some 1500 feet longer than the Golden Gate, for instance). You can't be third in the list to the GGB and longer both unless you are speaking of the insignificance of approaches. The GGB is shorter than the TNB. It was third on the list on July 1, 1940 according to Wa DoT. (Verrazano narrows was built in 1964) George Washington was built in 31 and was 3500 ft, and was longest until 37, when GGB was built in 37. TNB was 5939 ft long (per Washington state DOT). GGB is 4200 ft (wikipedia gives 5000 ft for the length of the TNB) T Moisseiff was also the weak link for both the Narrows bridge and the GGB closure due to high winds in 1950. He underestimated the dynamic wind load. Ellis was the inventor of the math, but not a chief project engineer. In the field of bridge engineering, and especially for the GGB, there were a lot of Prima Donnas - Strauss the first of firsts. Moisseiff, by some accounts, appears to have been used as a resource rather than a principle engineer in the Narrows bridge construction. The bridge owners conspired to a lot of monkey shines in cost-cutting choices which turned out to be fatal. They eliminated the cross bracing from the bridge towers, above and below the roadbed; and they dispensed with the roadbed stiffening truss. Moisseiff, along with GGB designers Ammann and Cone, was appointed to the review board to study why the bridge failed - that was doomed to failure, too, by the bridge owners (who had their own insurance problems because they declined to find an outside insurer and decided to carry the risk themselves). The story of the back room feuding and remarkable Reaganomic theories are case lessons in planned disaster. Interesting. |
The dipole and the violin
On Wed, 18 Mar 2009 11:11:42 -0700, Jim Lux
wrote: I guess that is the thing they forgot to calculate, or didn't know how, on the infamous Takoma Narrows Bridge in Washington State which collapsed when cross winds cause wild vibrations. That was an unexpected coupling between the force from the wind and torsional vibration of the roadbed. As the roadbed tilted, it "caught" more of the wind and had more force applied, moving it further. The torsional resonance was such that it oscillated with ever greater amplitude (not much different than a flag flapping, or a blade of grass in the wind.. not quite like a wind instrument reed, though) In fact, it was exactly like a reed. The Tacoma Narrows Bridge exhibited the highest roadbed length to roadbed width ratio of the designs of that era, and this was a contributing factor. As for whether it could have been anticipated? I don't know that modeling was that advanced back then (1930s). Charles Ellis (an engineer for the GGB) is the inventor of the math behind the modern suspension bridge. He developed 33 equations embracing from 6 to 30 variable to account for shape, structure, temperature, winds, and stress that were due to both dead and live loads. The GGB was designed for a wind load of 30 pounds per square foot at the roadbed and 50 pounds per square foot on the towers. The thirty pound spec is equivalent to a hurricane, the GGB typically sees only 10 pounds per square foot for 50MPH winds. Under the wind load designed to, the towers would bend five inches (they swayed free, unstressed, 12 feet during construction and an earthquake). Basically, Ellis designed the GGB to the sum of all probable stresses, not their average, not their RMS. The only thing missing was harmonic amplification. The GGB was closed due to wind in 1950 and later retrofitted with 5,000 tons of cross bracing (as was the replacement Narrows bridge). The bridge was an architectural feat, with a very delicate looking thin roadbed and much longer than most other bridges (3rd longest when it was built, some 1500 feet longer than the Golden Gate, for instance). You can't be third in the list to the GGB and longer both unless you are speaking of the insignificance of approaches. The Verezzano span (designed by GGB engineer Ammann) is only 60 feet longer but carrying much more weight. It was much longer and thinner as compared to other suspension bridges of the time which were double decked, (SF Oakland Bay Bridge) for instance.. making them torsionally much stiffer). Interestingly, the designer of Tacoma More the legacy of (GGB engineer) Russell Cone's assistants. Narrows (Moisseiff) was also involved in the Golden Gate. Moisseiff was also the weak link for both the Narrows bridge and the GGB closure due to high winds in 1950. He underestimated the dynamic wind load. Ellis was the inventor of the math, but not a chief project engineer. In the field of bridge engineering, and especially for the GGB, there were a lot of Prima Donnas - Strauss the first of firsts. Moisseiff, by some accounts, appears to have been used as a resource rather than a principle engineer in the Narrows bridge construction. The bridge owners conspired to a lot of monkey shines in cost-cutting choices which turned out to be fatal. They eliminated the cross bracing from the bridge towers, above and below the roadbed; and they dispensed with the roadbed stiffening truss. Moisseiff, along with GGB designers Ammann and Cone, was appointed to the review board to study why the bridge failed - that was doomed to failure, too, by the bridge owners (who had their own insurance problems because they declined to find an outside insurer and decided to carry the risk themselves). The story of the back room feuding and remarkable Reaganomic theories are case lessons in planned disaster. 73's Richard Clark, KB7QHC |
The dipole and the violin
On Wed, 18 Mar 2009 17:17:14 -0700, Jim Lux
wrote: Reeds don't oscillate torsionally (at least not as the dominant mode) They're more of a fixed/free beam that oscillates in longitudinal bending. And yet the failure mode I've most seen with them is splitting. Perhaps this only reveals a limited experience of their failure. The bridge was an architectural feat, with a very delicate looking thin roadbed and much longer than most other bridges (3rd longest when it was built, some 1500 feet longer than the Golden Gate, for instance). You can't be third in the list to the GGB and longer both unless you are speaking of the insignificance of approaches. The GGB is shorter than the TNB. It was third on the list on July 1, 1940 according to Wa DoT. (Verrazano narrows was built in 1964) George Washington was built in 31 and was 3500 ft, and was longest until 37, when GGB was built in 37. TNB was 5939 ft long (per Washington state DOT). GGB is 4200 ft (wikipedia gives 5000 ft for the length of the TNB) You have inadvertently summed in ordinary approaches. We have freeway interchanges with more complexity. Bridge span is the significant indicator of interest. The GGB comes in at 4200 feet suspension span, yes, the Narrows is much shorter at 2800 suspension span. If push came to shove about overall length over water, we, here in Seattle, have vastly larger bridges that float. The I-90 bridge logs in at 6620 feet, and the 520 bridge pushes that to 7578 feet. Nearby, we have the Hood Canal bridge that is longer at 7869 feet. I've been across all five many, many, many times, and I used to live halfway across the bay on the way to Oakland, by way of the Bay Bridge in Frisco. The one suspension bridge I refused to drive across is SW of Colorado Springs, over the Royal Gorge - the highest suspension bridge (1053 above the river below). Just standing on the approach as a car goes over gives you the shakes (talk about harmonic coupling). 73's Richard Clark, KB7QHC |
The dipole and the violin
In article , Jim Lux
wrote: Jon Teske wrote: If you touch one note above that (B natural on the E string) the result is a not an octave and a fifth (or high B) above the fundamental frequency, because lightly stopping the B and causing it to vibrate on both side of the stop divides the string into thirds. Jon Teske, W3JT and concert violinist. And this is why pianos are arranged to strike the string at a point which suppresses a harmonic which is dissonant. (I think it's the 7th harmonic which is suppressed) Hello, and acoustic dissonance is defined by the production of "unacceptable" beats between the partials (overtones (harmonics)) that can, but are not generally, exact multiples of the fundamental) generated by two or more fundamentals. Dissonance can also be defined when two fundamentals are in close proximity as to produce a kind of "roughness". Dissonance has no relevance for one fundamental (and its partials). It is the partials that give a pitch on a particular instrument its quality or timbre. There is also a "contextual" dissonance associated with particular intervals/ chord structures in Western classical music that, due to accepted practice in a particular era, in many cases bears no relation to the acoustic dissonance (sounding the chord in isolation (out of context)). If you want more enlightenment in this area pop on over to rec.music.theory or rec.music.makers.piano. Sincerely, and 73s from N4GGO, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
The dipole and the violin
On Mar 19, 5:44*am, (J. B. Wood) wrote:
In article , Jim Lux wrote: Jon Teske wrote: If you touch one note above that (B natural on the E string) the result is a not an octave and a fifth (or high B) above the fundamental frequency, because lightly stopping the B and causing it to vibrate on both side of the stop divides the string into thirds.. Jon Teske, W3JT *and concert violinist. And this is why pianos are arranged to strike the string at a point which suppresses a harmonic which is dissonant. (I think it's the 7th harmonic which is suppressed) Hello, and acoustic dissonance is defined by the production of "unacceptable" beats between the partials (overtones (harmonics)) that can, but are not generally, exact multiples of the fundamental) generated by two or more fundamentals. *Dissonance can also be defined when two fundamentals are in close proximity as to produce a kind of "roughness". Dissonance has no relevance for one fundamental (and its partials). *It is the partials that give a pitch on a particular instrument its quality or timbre. There is also a "contextual" dissonance associated with particular intervals/ chord structures in Western classical music that, due to accepted practice in a particular era, in many cases bears no relation to the acoustic dissonance (sounding the chord in isolation (out of context)). *If you want more enlightenment in this area pop on over to rec.music.theory or rec.music.makers.piano. *Sincerely, and 73s from N4GGO, John Wood (Code 5550) * * * *e-mail: * * * * * * * * * * Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 Hmm, I seem to differ tho I am not a player of violins With respect to the movement of violin wires and physics ALL vibrations of a lever or wire is three dimensional unless the ends are secured encastre where it is damped to a two dimensional swing. An example is a pendulum where the lever or "bob" is of a short distance as with a clock where the "hinge" restricts oscillation to two dimensions. In the case of a pendulum such as seen in certain museums that are pivoted some 100 feet or more high the oscillations take up a three dimensional pattern such that it takes many many oscillations before it can arrive at its starting point. The same analogy can be applied to a radiator or antenna or any other oscillation as it follows the same action of a tank circuit which is universal in the sciences of nature ie the standard model and the conservation of energy format. |
All times are GMT +1. The time now is 05:31 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com