Help with distillation on for backpacking still.

You should be getting 2 cups of water for every 2 cups you put in, if you run it dry. My guess is a leak, or you are losing vapor due to not enough cooling of the copper coil, so steam is leaking out the end of the condenser.

A better option would be to run that coil through a bucket of water, and have the line exit at the bottom. It is called a worm bucket. Something like this. You could use sea water, no problem.

To test for leaks, hold up a small mirror to all joints. A leak will fog it up.

If you really need to do air cooling, the coil might need to be longer, or run with less flame. Once you get it boiling, turn down the flame until it is just barely bubbling. The higher the heat, the larger volume of steam you will need to condense. This requires a better condenser. A worm bucket will solve that easily.

In distillation you need a good condensing technique, or you will only get steam escaping and evaporating.

Glassware for condensing the steam often has a cold stream of water keeping the glass cold.

not very useful in your case except as a demonstration.

copper tubing through ice works for small amounts.

reverse osmosis with a pressure pump may be the most practical (but high cost) tool you can buy.

Use a copper worm in a bucket to cool the steam. Then distill that (already hot) cooling water, and save yourself the extra heating time.

[deleted]

Here is a completely different take on desalination, a solar still.