Antenna in a box ?

This is probably a weird, near blasphemous idea..

It came as I was thinking about antenna geometry and its impact on linearity - I have often noticed that long, thin antennas seemed to improve linearity - and although I have never verified this, I have a hypothesis which works - I believe it is the angular capacitive coupling to the arm which offsets some of the rapid change in capacitance as the hand approaches the antenna.. I could write a mini thesis on this probably, but I wont.. ;-)

The diagrams below show my crazy idea.. Have a floor standing box or enclosure or whatever, into which is fitted a shaped "antenna" (this could be a tube, metal strip, wire, conductive tape - whatever.. With correct shaping, I think one could create a perfectly linear response.

The hand / arm could be slid on the top of the enclosure - and pitch should then be a linear(ized) function of the total arm and hand capacitance overlapping the shaped antenna.

If the hand / arm are raised above the top of the enclosure, total capacitance will decrease and pitch raise. Linearity would only be truly maintained (I think) if the arm / hand are resting / moving on the top of the enclosure..

No antenna equalization would be required - this idea should work fine with simple RC oscillators, and as one could make the "antenna" a thin metal strip of whatever width (as long as it is thinner than the arm) one likes, could be well suited to low cost implementation.

The above diagrams show a system where the arms capacitance is included - but I see no reason why the antenna could not be shaped such that one could slide the hand over the top - as in, playing it somewhat like a ribon controller.

Fred.

Are you seriously thinking about flying to Alpha Centauri?

Interesting idea, and beautiful figures!

Forgive me, but I still think digital / numbers are the easiest way to crack the linearity nut.  Run the numbers through a function and it's all over but the shouting.  Gets you completely away from all this geometry / interacting resonances stuff.

"Interesting idea, and beautiful figures!"

The beauty of being able to grab images from the www and hack them - I cannot even draw a decent stick man - tried drawing a hand + arm and realised I might reveal that my "inner artist" was about 3 years old! LOL.. A few minutes of browsing, and a few paint manipulations and its done..

A bit like how most people seem to "design" circuits or "develop" software these days!

Fred.

I think I see what you're chasing here.

I remember how older radios, at least the good ones, had their tuning capacitor plates shaped a lot like your diagram to improve linearity with regards to dial tracking.

Your curved pitch antenna might be working along the same lines.

Or I could be full of shiitake mushrooms...

"Or I could be full of shiitake mushrooms"

LOL - No, I think you are right - I think plate shaping was done to make the law non-linear.

One of my reasons at the moment "I think I see what you're chasing here" has perhaps to do with my health - For some weird reason, extending my arm for any length of time makes my heart go into irregular rythm (far more erratic than for other, more strenuous activities) - so, unless this improves with time, theremin playing is probably something I wont be able to do in any serious way.

What im "chasing" may well be a means to overcome this difficulty - Perhaps if my arm is supported, I will be able to play. I dont seem to have any problems with my left arm / hand playing volume if the volume antenna is really low, so that I only move the lower half (below the elbow) and keep the upper half by the side of my body.

Fred.

There have been basically two types of variable capacitors in the time. This had to do with the scale displays of ancient radios. Either they were calibrated by frequency or by wavelength. Thus there have been the frequency linear capacitors with an almost circle segment plate shape and the wavelength linear capacitors where the plates had a rather kidney-like shape.

"Thus there have been the frequency linear capacitors with an almost circle segment plate shape and the wavelength linear capacitors where the plates had a rather kidney-like shape." - Thierry

Facinating! - Amazing what one learns here at TW - Thanks Thierry.

Back in the 70's I was pulling old radios to bits, fixed an old tube communications reciever, did a C+G course in wireless technology - and never noticed or learned the function of these different variable capacitors! LOL!

Fred.

Fred said: "Back in the 70's I was pulling old radios to bits, fixed an old tube communications receiver, did a C+G course in wireless technology - and never noticed or learned the function of these different variable capacitors!"

This reminds me of WWII era radio receivers I gutted for the joy of in 1960 collecting parts. Most beautiful were the designs of large pivoting coils within a bigger coil tuning methods. Not quite sure why this approach over variable capacitors.

We had electronic surplus stores with tons of war era electronics and parts, so my journey would begin. I was so fascinated at age 12, but poor, I would steal all kinds of stuff from the surplus bends just to organize the parts at home.

I'm bad!

Fred said..."The diagrams below show my crazy idea.. Have a floor standing box or enclosure or whatever, into which is fitted a shaped "antenna" (this could be a tube, metal strip, wire, conductive tape - whatever.. With correct shaping, I think one could create a perfectly linear response."

Invisiblejelly says..."Fred that's a clever idea!..you could paint a keyboard on top of the box and then you would have a theremin version of the Ondes Martenot well nearly but you couldn't play it like the Ondes Martenot ie sitting at a keyboard...or could you?That is if the antenna is a thin metal strip the capacitive sensing would be directional therefore you may be able to sit next to your box and slide you hand side to side over the keyboard...perhaps though the keyboard would have to be at an angle..a great idea to experiment with.