Ignacio,
Height is your friend. The ground reflection tilts the main lobe up. Even a vertical and 4 elevated radials, unless it is high, it won’t give a really low take-off angle.
So how high? 10 wavelengths? Hard to get on 20 m with a backpack pole.
But if you can find a summit shaped like a sharp cone or one with a steep fall off in your desired direction then the standard 7 m pole will do. As the ground immediately in front of the antenna is not there it can’t reflect, or if it is at a steep angle the reflection is away from the antenna so it has a minimal effect on the pattern.
In such a location a simple dipole in inverted vee (one mast) or strung between two masts will give value for effort.
Operating at harmonic frequencies is OK if you have the lobes that form in the right direction.
A simple vertical with three elevated radials will give the best all round radiation pattern and if near a cliff edge etc will be great for dx.
The best bang for buck in gain antennas is the two element beam. For multi frequency operation the W8JK design, although 80 years old, is still worth considering. It works with two wire elements in inverted vee shape.
Personally I prefer simplicity. I have used a delta loop on 20 m in the hope of some dx. On a ridge or cone it will be about as good as it gets. You do need to get the bottom wire elevated and that’s a problem with a 7 m mast. A delta loop for 20 m can be opened out to become an inverted vee on 40 m.
All my SOTA dx, 16,000 to 18,000 km, has actually been worked on a linked dipole. The 3,000 km and less stuff is local down here.
73
Ron
VK3AFW