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Loop antenna?

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Posted: 5/9/2012 6:12:05 PM
luc234
Joined: 5/9/2012

threads posts

Hello all,

 

I'm considering building a theremin, and for the most part I think I have an idea on how it functions, but I haven't found anything yet that states why the volume antenna is a loop antenna. I've seen various examples of theremins that do not use a loop antenna, so I was just wondering is it for aesthetics only , or does it have some added functionality.

 

Thanks

Posted: 5/9/2012 6:21:32 PM
RS Theremin
From: 60 mi. N of San Diego CA
Joined: 2/15/2005

threads posts

Thereminist live in a zombie like state, you have seen that stare?  The loop makes it easier for them to tell which antenna is which. I guess you could give the loop more explanation about sensitivity but really they both can be straight. That is what I use and my volume is an upside down metal L with an adjustable elbow so it can be higher on the open end. There is an advantage to this when you have broad volume shading. Here is a volume demo of shading without distortion Radio Sample

Pictures and sound add so much more character to a thread.

Posted: 5/9/2012 6:30:28 PM
luc234
Joined: 5/9/2012

threads posts

So for a first run, two straight antennas would work (monopole?) ? What type of material would be superior for the antennas? Since, as I understand, they're not transmitting a signal but working as a plate on a capacitor, would any shape work?

 

Oh, also sorry meant to post this is construction but it appears I accidently made it in general.

 

thanks

Posted: 5/9/2012 6:45:53 PM
RS Theremin
From: 60 mi. N of San Diego CA
Joined: 2/15/2005

threads posts

Yes two straight metal tubes will work until you know what you want on your finale design. You don't want to ask me if the theremin transmits a signal, put an AM radio next to it. If your frequency is too low for AM then tune the radio to twice the theremin frequency. In some countries theremins would be illegal.

I use dangling alligator clip leads when experimenting. Just keep your setup clear of metal surroundings and table top frames.

luc234=> Where on planet earth are you located?

Posted: 5/9/2012 8:02:46 PM
luc234
Joined: 5/9/2012

threads posts

Yes two straight metal tubes will work until you know what you want on your finale design. You don't want to ask me if the theremin transmits a signal, put an AM radio next to it. If your frequency is too low for AM then tune the radio to twice the theremin frequency. In some countries theremins would be illegal.

I use dangling alligator clip leads when experimenting. Just keep your setup clear of metal surroundings and table top frames.

luc234=> Where on planet earth are you located?

 

Thanks for the replies. I'm located in central oklahoma.

 

Another seemingly unrelated question, any idea why the difference from the fixed and the varying frequency is used?  Why would it not just create a frequency with the oscillator from the antenna so it falls within an audible range and have that amplified?

Posted: 5/9/2012 8:11:56 PM
RS Theremin
From: 60 mi. N of San Diego CA
Joined: 2/15/2005

threads posts

I will leave that excellent unrelated question to an engineer. They may not respond to you because I have. I try and keep myself Black Listed in this group so they won't talk to me. I would not listen anyway. (-'

You are about to embark on a wonderful journey.

Always remember. . . "Theremins Have Attitude!"

Christopher

Posted: 5/9/2012 8:39:46 PM
GordonC
From: Croxley Green, Hertfordshire, UK
Joined: 10/5/2005

threads posts

For me, the loop shape has a distinct advantage, that the size of the volume field varies according to the angle from which you approach it. Specifically the field is smaller on the side, where I can get rapid changes in volume, whereas playing above the centre of the loop gives me better control over slow changes in volume in the larger field.

 

(With regard to the "why heterodyne an two RF signals" question, I think (I'm not an expert) it's that the effect of the pitch rod on that frequency of the oscillator is quite small - but a small change in a very high frequency can be heterodyned into a large change in the audible range.)

Posted: 5/9/2012 8:40:31 PM
dewster
From: Northern NJ, USA
Joined: 2/17/2012

threads posts

"In some countries theremins would be illegal."

From the standpoint of radiated emissions they are probably a nightmare.  I wonder if any manufacturers like MOOG have run into problems with this?


"Another seemingly unrelated question, any idea why the difference from the fixed and the varying frequency is used?  Why would it not just create a frequency with the oscillator from the antenna so it falls within an audible range and have that amplified?"

On the pitch side you are sensing capacitance on the order of femto Farads.  And you want it to operate over many octaves.  And you want it to be somewhat stable.  The easiest way to do this is with the difference of two high frequency oscillators.  But then you start playing the crying game of trying to make it linear (or more properly: logarithmic pitch change with linear hand position change).

Posted: 5/10/2012 1:48:36 AM
luc234
Joined: 5/9/2012

threads posts

Ahhh ok thanks that does make more sense. I was playing with a hartley oscillator circuit today and tried to vary the output with a long wire as an antenna and saw very very little change on the oscope and frequency counter. Not sure if I built it wrong or if it was just cause it is such a small change to begin with.  Looking forward to playing around with this build a lot, I imagine I'll learn a great deal in the process and have a new instrument to learn!

Posted: 5/10/2012 3:09:55 AM
RS Theremin
From: 60 mi. N of San Diego CA
Joined: 2/15/2005

threads posts

luc234 said: Thanks for the replies. I'm located in central oklahoma.

I like to ask location to see if there is a metaphysical connection, what ever that means! lol

I think there is one as I had spent many a summers 50 years ago near Wichita Falls swimming in the Red River. You know this special place.

My theremin journey is slowing and yours is just beginning. You must find clarity in where you want to go with this project. Everyone is an expert, so navigate with common sense. What may seem to be the ideal path may only be an illusion.

Christopher

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