Field of Big Briar is heavily compressed in the lower octaves but normal at the higher ones

I'm struggling to tune my theremin. I thought I had an issue in my technique but my instructor verified it was in fact a problem with how my instrument is set up.  

I can successfully set an appropriate position for zero beat. 

I can make slight adjustments and find octaves throughout the upper registry. 

The zero beat remains more or less where it should be. 

But at this point the lower notes are incredibly close together compared the the higher ones & I don't know how to fix this without undoing the rest of the field. 

Anybody else have this problems or any solutions? 

Jon

jon, what theremin do you have? sounds like you need to adjust the pitch oscillators a bit. a bit a tricky thing and you can worsen up things more if you do it wrong. did this happen all of a sudden or was it always like that? and you noticed because your skill improved?

I have the Big Briar Etherwave. 

I think it was always like this but as you say my technique improved to the point where I was trying to hit exact notes instead of making noise but I couldn't get it right & my instructor realized that it wasn't my fault but an inconsistency of the instrument to the low to high range. 

As I said the higher range is correct but the lower notes are heavily compressed.