Not sure there's much skin effect problems at AM band frequencies ;)
I want to know everything about the antenna
Posted: 2/27/2008 8:37:54 PM
Has anyone tried using steel brake lines? These are fairly easy to bend with a tubing bender and they already have nice built in fittings.
Don
Don
Posted: 2/29/2008 6:39:54 PM
hmmm... well, personally I changed my Theremax design because:
a) I wanted it in an Atari 2600 case (which actually happens to work out pretty perfectly, as far as how well everything fits)
b) I did not like the malleable wire antennae that PAiA provides... they are unwieldy enough to be difficult and get in the way, yet so easily bendable that if they bump anything they get bent out of whack.
So... I went with telescoping aluminum antennae, against the venerable advice of other members on Theremin World. Mostly I wanted to make sure my Theremin is easily portable; I have 2 screw-mounts which stick out of the case, and I can attach or disassemble the antennae with a quick twist onto the screw.
Personally, the telescoping antenna seems to work fine, at least for pitch (I can't get my volume working, so maybe I am speaking too soon...); as long as it is extended the same distance each time, it holds the same tuning whenever I dismantle and re-mount it. Sensitivity and reaction seem to be perfect. Plus, it's easier to deal with than the PAiA ones...
the only drawbacks I see, so far, are that they are a bit flimsy... it's possible they could bend or snap if they were hit too hard. But with the screw mount, it would be very easy to just buy a replacement one if it broke.
My only fear is that somehow my antennae and/or their proximity (the Atari case is slightly narrower than the wooden lectern case) is preventing me from getting the volume antenna/circuit working.
a) I wanted it in an Atari 2600 case (which actually happens to work out pretty perfectly, as far as how well everything fits)
b) I did not like the malleable wire antennae that PAiA provides... they are unwieldy enough to be difficult and get in the way, yet so easily bendable that if they bump anything they get bent out of whack.
So... I went with telescoping aluminum antennae, against the venerable advice of other members on Theremin World. Mostly I wanted to make sure my Theremin is easily portable; I have 2 screw-mounts which stick out of the case, and I can attach or disassemble the antennae with a quick twist onto the screw.
Personally, the telescoping antenna seems to work fine, at least for pitch (I can't get my volume working, so maybe I am speaking too soon...); as long as it is extended the same distance each time, it holds the same tuning whenever I dismantle and re-mount it. Sensitivity and reaction seem to be perfect. Plus, it's easier to deal with than the PAiA ones...
the only drawbacks I see, so far, are that they are a bit flimsy... it's possible they could bend or snap if they were hit too hard. But with the screw mount, it would be very easy to just buy a replacement one if it broke.
My only fear is that somehow my antennae and/or their proximity (the Atari case is slightly narrower than the wooden lectern case) is preventing me from getting the volume antenna/circuit working.
Posted: 3/18/2008 10:44:12 PM
Has anybody ever tried using brass instrument parts for antennas? The bell of an old horn might make an interesting volume control. Does this seem feasible? Also, somebody said something about silver tubing making a good antenna. Could part of an old flute perhaps work? Might look very interesting...
Posted: 3/19/2008 2:05:28 AM
I don't see why those wouldn't work. Pretty much any conducive metal, I presume, would work (but others on here will know more about that than I would)
Those items would, indeed, make for an interesting-looking, funky theremin. My only concern (with things like the bell of a horn) would be for playability; might have weird pickup/induction patterns and radius based on the shape, and thus might be a little harder (or just weirder) in how you interact with it.
But again, someone else who knows more than I can chime in.
PS. Still don't have the Theremax working. But haven't really worked on it much since my last update, in which I soldered some loose joints and suddenly everything was magically broken.
Those items would, indeed, make for an interesting-looking, funky theremin. My only concern (with things like the bell of a horn) would be for playability; might have weird pickup/induction patterns and radius based on the shape, and thus might be a little harder (or just weirder) in how you interact with it.
But again, someone else who knows more than I can chime in.
PS. Still don't have the Theremax working. But haven't really worked on it much since my last update, in which I soldered some loose joints and suddenly everything was magically broken.
Posted: 3/20/2008 2:10:53 AM
From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................
Joined: 12/7/2007
>> I want to know everything about the antenna
.. I share this wish! Alas, have a long way to go, and know "everything" is not obtainable.. but hope this helps you a bit...
As far as I can tell from the theory, and from what I have seen with my prototypes, the "RF" factor makes no difference at all..
What one is doing with the "antenna" is forming one plate of a capacitor - The "ONLY" thing which matters is the area of the "plate", the area of the other "plate" (the players hand) and the distance between "plates".
The simplest formula for determining aproximate capacitance C(in pico Farads) is:
C = 0.0885 (A/t) where A is Area of plates in cm, t is distance between plates in cm.
Now, whilst the above looks simple, in the reality of Theremin design, it gets horribly complicated! If one extends ones hand towards a narrow antenna, the major contribution to capacitance will be the surface area of the antenna closest to the hand - and as the proximal area directly in line will be small for a narrow antenna, the change in capacitance will be small - but sections of the antenna above or below this will also contribute, however this relationship is based on reletive distances / areas, and is inverse square.
It can be seen that a short, fat antenna (or a flat plate) lined up directly with hand movement, having an area equal to the hand, is the best 'sensor'... BUT.. (as I have recently discovered in re-designing the TherAsynth) longer, thinner antennas, due to the contribution of the "not so close" capacitance from the hand, CAN improve the linearity.. This is because our perception of pitch is exponential, straight "fat antenna" capacitance -> distance relationship is inverse square, and the conversion of capacitance -> frequency -> beat frequency is complex and determined by the type of circuitry (R*C or 1/(2pi LC) for example).
It must also be remembered that the player is a 'plate' of the capacitor which is capacitively coupled to "ground" - And this coupling effectively looks like 2 capacitors in series... So it is important that there is GOOD capacitive coupling between the player and the THEREMIN'S "GROUND" - Battery operated theremins, or those with un-earthed supply, need to have a large ground 'radiator' (With the TherAsynth, the entire inside of the housing is coated in Aluminium which is connected to the oscillator / system ground, and the total capacitive coupling is continuously monitored to adjust the sensitivity and linearity dynamically)..
Ground is the 'unseen' antenna - and it is just as important (perhaps even the most important) an "antenna" as the others.. If the players coupling to the theremin is only via the instrument itself, then the players position reletive to the instrument is as important as their position reletive to the other antennas - and any change in this position will have the same effect. This problem is less likely with 'earthed' Theremins.
I have found long (70-100cm) fat (22mm copper tube) antennas seem to be the best compromise for sensitivity and 'natural' linearity ( with LC oscillator based Theremins at about 250kHz ).. wheras lower sensitivity and better linearity can be obtained by reducing to 15mm or less.
What you make the antenna from will not affect things one iota.. Aluminium foil wrapped round a plastic tube will work just as well as a solid gold rod (and be a lot lighter :)!
//-> EDIT (added)
The reason why the conductive material one uses is unimportant is this.. the impedence of the circuitry connected to the antenna must be extremely high (if it is low, the capacitor is shunted, and nothing works.. the charge on a few pF dissipates rapidly, so, for example, at 250kHz = 4us and 10pf into 400k = 4us, so the impedence must be far greater than 400k ohms for things to work.. (more like 10M ohms!)... Even a few k ohms in series with the antenna makes no difference to performance, and therefore the d
.. I share this wish! Alas, have a long way to go, and know "everything" is not obtainable.. but hope this helps you a bit...
As far as I can tell from the theory, and from what I have seen with my prototypes, the "RF" factor makes no difference at all..
What one is doing with the "antenna" is forming one plate of a capacitor - The "ONLY" thing which matters is the area of the "plate", the area of the other "plate" (the players hand) and the distance between "plates".
The simplest formula for determining aproximate capacitance C(in pico Farads) is:
C = 0.0885 (A/t) where A is Area of plates in cm, t is distance between plates in cm.
Now, whilst the above looks simple, in the reality of Theremin design, it gets horribly complicated! If one extends ones hand towards a narrow antenna, the major contribution to capacitance will be the surface area of the antenna closest to the hand - and as the proximal area directly in line will be small for a narrow antenna, the change in capacitance will be small - but sections of the antenna above or below this will also contribute, however this relationship is based on reletive distances / areas, and is inverse square.
It can be seen that a short, fat antenna (or a flat plate) lined up directly with hand movement, having an area equal to the hand, is the best 'sensor'... BUT.. (as I have recently discovered in re-designing the TherAsynth) longer, thinner antennas, due to the contribution of the "not so close" capacitance from the hand, CAN improve the linearity.. This is because our perception of pitch is exponential, straight "fat antenna" capacitance -> distance relationship is inverse square, and the conversion of capacitance -> frequency -> beat frequency is complex and determined by the type of circuitry (R*C or 1/(2pi LC) for example).
It must also be remembered that the player is a 'plate' of the capacitor which is capacitively coupled to "ground" - And this coupling effectively looks like 2 capacitors in series... So it is important that there is GOOD capacitive coupling between the player and the THEREMIN'S "GROUND" - Battery operated theremins, or those with un-earthed supply, need to have a large ground 'radiator' (With the TherAsynth, the entire inside of the housing is coated in Aluminium which is connected to the oscillator / system ground, and the total capacitive coupling is continuously monitored to adjust the sensitivity and linearity dynamically)..
Ground is the 'unseen' antenna - and it is just as important (perhaps even the most important) an "antenna" as the others.. If the players coupling to the theremin is only via the instrument itself, then the players position reletive to the instrument is as important as their position reletive to the other antennas - and any change in this position will have the same effect. This problem is less likely with 'earthed' Theremins.
I have found long (70-100cm) fat (22mm copper tube) antennas seem to be the best compromise for sensitivity and 'natural' linearity ( with LC oscillator based Theremins at about 250kHz ).. wheras lower sensitivity and better linearity can be obtained by reducing to 15mm or less.
What you make the antenna from will not affect things one iota.. Aluminium foil wrapped round a plastic tube will work just as well as a solid gold rod (and be a lot lighter :)!
//-> EDIT (added)
The reason why the conductive material one uses is unimportant is this.. the impedence of the circuitry connected to the antenna must be extremely high (if it is low, the capacitor is shunted, and nothing works.. the charge on a few pF dissipates rapidly, so, for example, at 250kHz = 4us and 10pf into 400k = 4us, so the impedence must be far greater than 400k ohms for things to work.. (more like 10M ohms!)... Even a few k ohms in series with the antenna makes no difference to performance, and therefore the d
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