Vertical separation for two antennas mounted on a single mast?

Question for the antenna boffins out there:-

If I want to mount two antennas on one mast, how much vertical separation do I need between them if I want to minimise/avoid interaction between the two antennas?

I know that some people mount several antennas on one mast and phase them to get more gain or improve the pattern (usually for large contest or moon bounce stations), in this case I specifically don’t want them to interact with each other!

I currently have a mono-band 20m Moxon on my 12m mast. I have a MFJ 1888 magnetic loop which I have just put up on another smaller mast.

I am getting a bit of interaction between them which means that neither of the antennas are performing as they should, especially on 20m where the Moxon is resonant.

I think the pattern from both antennas is getting distorted. This is hardly surprising as the masts are roughly 4-5 meters apart (about a 1/4 wave on 20m).

Things improve on bands away from 20m, or if I tune the loop to a frequency away from 20m while using the Moxon. Again, not really anything surprising about that.

I wondered if moving the magnetic loop onto the main mast, mounted above the Moxon beam so that the Moxon is in the null of the loop (loop mounted horizontally) if this might improve things, and want to know how much vertical separation I would need between the two antennas.

This would also get the loop higher up in the air than it currently is, so that should be another benefit there.

I’m guessing that due to the large wavelengths involved (the loop covers from 15m, and goes down to 40m), it might not be practical as the vertical separation needed might be too great, but it’s a thought.

There is no rule of thumb. The best/only way is to model the situation.

Since the magloop will be above the feedpoint of the Moxon and will presumably rotate with it the effect will not vary so the best approach is just to trim the Moxon by varying the length of the spacers and elements.