Newbie here: I really need help, I'm not sure what I'm doing wrong.

Hello!

I just got my Moog Etherwave Plus today, and I was really excited!

However, I'm at a loss. I watched several videos, read several forum posts, but for some reason, I can't get the tuning right.

The pitch field is really small, even when the pitch knob is turned all the way to the left - it's roughly 25-30cm wide, and it only gets smaller as I move the knob around. 

I tried moving the theremin around, putting it in a spot where there was nothing within at least 2 meters around it (or above and below it), but it didn't change anything.

Firstly: I'm a complete newbie. I've read words such as "amplifier" and all sorts of stuff, and I simply don't know what it means in this context. I'm sorry if this is tedious to adepts, but I don't know where to turn or who to ask  

Now, I have some ideas as to what could be wrong, but I don't know how to test it or what to do:

Maybe it's my phones? I plugged in a pair of headphones. The headphones have a 3,5mm audio jack and I plugged them into an adapter (do you call it that?) that fit into the input for the phones on the theremin. Is this wrong? Should I use something else? 

Maybe it has something to do with power? I'm from Europe so I plugged it into my 230V power socket using the power-adapter provided with the theremin. The plug has two legs, but the manual says something about the connection having to be grounded - should it have three legs? Am I doing something wrong with the power?

I'm at a loss as I said.

For diagnostic purpose try a small conductive body attached to pitch antenna like from this video:

https://m.vk.com/video-98653861_456240470

How would I go about doing that? After a quick google search I couldn't really figure out what "small conductive body" really is. Sorry for being difficult :I 

A piece of wire w/o insulaton, a metal spoon, fork, etc.

Ah okay, and I'd just tape it to the pitch antenna? 

Ah, turns out I had to tune the theremin using the little red plastic tool that came with it! And it worked!

Now I have a second problem though

The volume antenna acts kinda strange. It seems that the volume antenna is altering the pitch ever so slightly as I move my hand away from it (by about a semitone), and I'm not sure how to fix that. Any ideas?

From your description of how you have connected things it becomes clear that your theremin does not have a connection to (safety) earth/ground. The 230V power supply the Etherwave Std and Plus comes with in Europe does not connect the theremin to earth, even when it's plugged into a socket with safety earth side contacts. That's the reason why even when using the (bad sounding) phones out, it still needs to be plugged in via the audio out into a keyboard/instrument combo amplifier or a powered speaker that has and is connected to the electricity outlet's safety ground. You might have adjusted the pitch antenna range to kind of work in an ungrounded state, but as you noticed, it still doesn't function as it should. I hope I didn't make this too technical but it is a fundamental part of practical knowledge about the 'electrical needs and required conditions' of this quirky beloved instrument. 

Ah I see, that makes sense to me in a very limited way.

I'll look around on the web, but I'm getting the feeling that I have to plug it into an amplifier that is itself grounded - is that true? And if so, is there any way I can use phones so as not to drive my girlfriend insane? 

Yes, that's the case with this theremin (I have an Etherwave Plus too). There's a smallish Behringer keyboard amplifier that has a good reputation. Electrical guitar amplifiers are less suited, as the theremin's output is more powerful than an e-guitar (and of a different impedance, but that gets very technical, it means that the signal is mismatched with what a guitar amplifier is designed for) the result being is that guitar amplifiers tend to distort and colour the sound in an unfavourable way. 

It has also been reported that when a long instrument cable is used to plug into an amplifier that is not grounded, and one drapes the cable on the floor, it can provide enough capacitive coupling to earth/ground to make the theremin function well.

Several instrument/keyboard amplifiers actually do have a heaphones output that cancels the speaker sound, and that will definately sound better than the Etherwave Plus' rather crude headphones output, especially if the amplifier has some built-in reverb to make it sound like your playing in a room with nice reverbrant acoustical properties.

Ah okay. 

I'm going to my local music store in a bit. I'll be on the lookout for a small keyboard amplifier. Is there anything else that is a "must have" in the beginning? I'm considering getting the pitch extension module, but first I need to find someone with soldering experience - I'm not daring enough to try it myself haha.