Yagi antennas are very narrowbanded and it’s very doubtful they’d be able to cover the whole of a single band effectively, say 2.4GHz, let alone both 2.4 and 5.8.
If anything you’re probably increasing the amount of RF reflected back at the transmitter. Which 1) reduces the amount of RF transmitted and 2) increases the load on the transmitter, effectively wearing it out faster.
Cutting the pringles can in half now…
Anecdotal I know, but putting parabolics ( 3D printed with foil stuck on ) on the TX allowed my to fly my old Mavic Air up to 2km away, whereas without them the limit was around 700m - on the same day in the same location
Thanks for replies , lots of variables on a couple of flights , one clear day i got furthest ive been in a built up area and on a crappy overcast with the rain coming in humid day it was pants … im not convinced so im away to buy some snake oil to rub on the antennas.
They are a bit of a faff anyway and looking at the drone hacks site digesting the info
Bought a set of yagi 2.4 ghz aerials to use with smart controller.Did a test using them yesterday open countryside with few trees hedges.Verdict work very well and full signal maintained if you keep the antenna pointed directly at drone ,mavic air zoom only time you get drop of is if you turn controller away from it.I delibratly did so to test this.signal strength was maximum and video feed signal maximum whilst pointing at drone .Overall good to use in open countryside .
Moved your post to this topic about boosters 
Complex (pun intended) subject excellently explained.
Thanks