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.