While you guys were busy speculating and debating, I ordered a Nano theremin. It arrived today. I am neither an electronics expert nor a an expert thereminist, so I am not sure how well I can answer some of your more technical questions. Here, though, are my initial impressions:
This thing really is nano! Not counting the volume pressure pad, the floor unit is smaller than a deck of cards. All of the hardware seems to be sturdy. Red and green LEDs on the floor unit and the volume antenna indicate when each component is powered on and they help with the tuning process. Everything comes packed into a couple of plastic boxes similar to VHS cassette cases. The boxes are pretty flimsy. I don't think I will use them.
Assembly was a bit of a puzzle. There was no instruction manual included. The pitch antenna has threads compatible with European standard microphone stands. So, I combined a couple of odd adapters I had on hand to mount the pitch antenna to a US microphone stand. The volume antenna is held to the side of the mic stand with Velcro straps. A rubber gasket prevents the volume antenna from grounding to the mic stand. This mounting method doesn't feel as sturdy as I would like it to, and it takes some care and effort to get the straps secure. The power supply has European-style pins. An adapter was not included for use with North American standard mains outlets. So, I used a universal power supply that I already had on hand, set to 12VDC, positive tip. The floor unit has a jack for each antenna. They are not labeled, but the jacks are different sizes, so it is impossible to connect them incorrectly. The pitch preview earphone took a bit of trial-and-error to install. The cable ends with a very small two-prong plug, but the floor unit has a three-prong jack that is not labeled. So, that makes four possible ways to plug in the cable. Two ways work and the other two ways don't work. The mono earplug is the type that is supposed to wedge into the cartilage of your outer ear. I find that type of earphone very uncomfortable, so I may make my own adapter to use other types of earphones. The pitch preview option doesn't seem to have any active electronics in the cable or earphone, so you could probably get by without this component if you wire in your own earphones. This is a cheap option, though, so I didn't mind paying that bit extra for it.
Both antennae are very long, compared to most other theremins I've played. The pitch antenna is more thick and sturdy that a typical, telescoping portable radio antenna. The volume antenna has four segments. The first two segments are black and are telescopic. They are not responsive to volume gesture. The next segment is chrome plated and it houses the tuning knob and related electronics. If you touch this segment, the pitch is slightly affected, but not the volume. The outermost chrome portion is isolated by a rubber gasket. It is the playable segment of the volume antenna. I found the volume antenna more comfortable to play if I tilt it toward my body, so I don't have to reach so far as I do with the antenna parallel to my shoulders. Those sharp teeth on a couple of the antenna segments are positioned in such a way that you would have to make a deliberate effort to hurt yourself on them. I am not concerned about any potential for accidental injury.
Tuning is easier than I expected. For volume tuning, I touched the outer segment of the antenna with my left hand, then rotated the tuning knob with my right hand until just the point where an LED on the antenna turned from red to green. This is the position for complete silence. Similarly, on the pitch antenna, I held my right arm to about where I wanted the zero beat zone to end, then rotated the tuning knob until an LED turned from red to green. It is a bit awkward to have to hold the floor unit while tuning. It may take several attempts to get the tuning just right. In stocking feet, though, I've found that I can gently adjust the tuning with my toes. Although I've only played my Nano theremin for several minutes, so far, the tuning seems to be quite stable.
The foot pressure volume control pad works okay. The volume goes up as you apply more pressure and goes down with less pressure. If you step off the pad you get complete silence. The dynamic range is not great and it takes a light touch to get subtle variations in volume. Playing from a sitting position may make it easier to vary the pressure. The foot control is probably okay if you are trying to play another instrument at the same time, but I recommend the hand-controlled volume antenna for most other situations. The volume antenna is more responsive and works just as you would expect it to.
Lastly, a couple of recessed switches on the floor unit affect the timbre and pitch range. The timbre switch has already been discussed in this thread. One setting is more sweet, with the qualities of a violin or female operatic vocalist. The other setting is more buzzy/brassy sounding and is more reminiscent of a vintage RCA theremin in the low register. I like both timbre options. It's too bad that you cannot mix them or select a waveform somewhere in the range between them like you can on an Etherwave, but both sounds are interesting and I'm glad to have the choice. It isn't quite so obvious what the pitch switch does. It changes the playable octave range, serving a roughly similar function to Thierry Frenkel's Etherwave pitch mod. With the switch in one position, you get a sound all the way down to the lowest audible pitches before it reaches zero beat. With the switch in the other position, the bass frequency range is truncated, with sound cutting out while it is still at a conventionally musical pitch.
I'm a rank amateur player, so I can't really tell you much about the linearity of the pitch antenna. I can say that, for my purposes, I find the Nano's playability to be similar to the Etherwave Plus and the Burns B3 Pro. I'm satisfied with it.
Overall, I'm glad I made the purchase. The Nano theremin has a unique look. It seems to work well and sound good. It's relatively inexpensive. And its compact design will make it very easy to store and to transport. I recommend it.
