Antenna question

Posted: 11/13/2020 6:11:20 PM

Joined: 10/24/2020

I wanted to ask about this before I started making parts...

If I use an anodized aluminum antenna mounted in an anodized aluminum clamp (with a proper conductive connection to the antenna independent of the clamp) is there enough conductivity between the antenna and the clamp to create undesirable noise or interference? The clamp would be mounted to a plate that the circuitry will also be grounded to. 

I can isolate the clamp from the plate, but it requires a bit more machining (including a new reamer) to isolate the bolts from the plate using phenolic spacers. I'm hoping not to isolate the antenna from the clamp, as any interface material makes the antenna a bit "floppy." 
I can machine the clamps out of a plastic, such as Nylon 6 or Delrin, but I'd prefer to use 6063. 

Standard analog type theremin. 



Posted: 11/13/2020 7:08:08 PM

From: Minnesota USA

Joined: 11/27/2015

Even if you have the intentional electrical connection separate from the mechanical mount of mating anodized surfaces, you may want to either insulate the two anodized parts with a separate material as you suggest or go the other way and use something to cut through the anodizing to force contact. Multiple  conduction paths caused by intermittent contact between the anodized parts on top of the separate electrical connection could cause your pitch to jump, and more.  It's hard to guarantee that two anodized parts will stay insulated from each other.

Could you just spot machine the anodized parts to force contact so that you maintain the structural rigidity that you want?

Posted: 11/13/2020 7:19:12 PM

Joined: 10/24/2020

No issue...I discovered I had some other reamers in another toolkit (in another building, wtf) that will work. I'll just ensure that everything is isolated, JIC. 

I'm making some nice quick-release antenna mounts for my rig. The current method (using the ubiquitous plumbing bits) is a bit clunky and non-portable. I'm putting RF antenna connectors on the end of the new antenna, which will slide through the clamp and plug into the coax coming from the board. A nifty cam-lock QR (similar to the ones on a bike) will lock the antenna at any angle. 

The toughest bit is making the RF connector on the antenna flush so that it will slide through the clamp...I have installed it with a couple of isolating washers between the ground and the antenna body, and I'm going to turn it down on the lathe BEFORE bending the antenna. This requires me to somehow clamp the antenna through the spindle of the lathe without marring the anodized finish, and get it centered. I'm thinking maybe heatshrink tubing and a 4 jaw chuck...although I might have a collet that will work...hmmm. 

Since the RF connector has a good electrical connection to the metal of the antenna, next time I will make the antenna out of raw aluminum, and anodize it afterwards, which will avoid all this jiggery-pokery!

I still have to make a volume antenna, so I will use that method. I'm thinking I might bend it in a spiral (since I can heat it for bending PRE anodizing)

Any thoughts on a spiral shaped vol antenna?

Posted: 11/13/2020 7:36:36 PM

Joined: 10/24/2020

Here's a shot of the antenna mounts on my first DIY build. Circuit was loosely based on a Theremax, with elements from an old Model 142.

Antennas are made out of tent poles. Clamps are the type used for lab glass holders. Isolated from metal body using heat shrink and phenolic tube. Connected to circuit with those loops of coax, just held to contact with a sheet metal screw. I want something more solid for the new build, and I need it to be portable. 

Posted: 11/13/2020 9:18:47 PM

From: Northern NJ, USA

Joined: 2/17/2012

Wow, that's a lot of connectivity!  Theremin antenna currents are quite low, so as long as the connection is sound just about anything will conduct them.  Rule of thumb might be to stay at or above the wire diameter of the tank or EQ coil winding (on my D-Lev prototypes the coils connect to the plates via a single strand of bare wire wrap wire).  The main reason to use thicker wire would be to minimize case vibration modulation of the fields, and/or to use the wire itself as a C trimmer, ala the EW.  Thinner wire will interact with your hand less, and will therefore concentrate more of the C field at the antenna (if that indeed is what you want).

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