The two elements yagi-uda is the smallest one among the yagi-uda familly.
Since gain comes from boomlength. The gain of a 2element is relative low. Some manufacturers might claim they have a “special” feed system or have some other fancy way of putting things. (Like a HB9CV/ZL-special etc.) But sadly, it just isn’t possible to produce more gain on such a boomlength with the described element length. Oke, but is it worth the effort then?
Here comes: THE SECRET PLUS OF HAVING YOUR FIRST YAGI !
Most people, forget there is a thing called “ground gain”. That ground gain only adds gain to horizontal antennas when placed above earth.
In other words: besides the say 6dBI wich can be expected from a 2 elements yagi there will be a additional approximently 6 dB due to that ground gain.It adds up to about 12 dBI.
That is completely different if you would compare that to your halve wave vertical antenna!
So if your thinking… is it worth the effort to replace my vertical for a relative small beam antenna…then

YES!

Besides that ground gain, you will have the benefit of rejecting signals you don’t want! And don’t forget about the “less noise” due to polarization etc…
Please if your thinking about stepping into the beam world for DXing, do try!
The traditional 2elements yagi exist out of a radiator and a reflector, but it would also be possible to make a radiator/director version. We could feed both elements and it becomes a so called HB9CV or ZL-special we can bend the elements and it becomes a Moxon.
All those individual different 2 elements beams have there own plus and down sides.
Described below is a “straight forward” 2elements wich has been used already by many.

 

THE HPSD “NORMAL” 2elements:

 

In this case, it consist of only one Reflector (B) and one Radiating element (A).
The Gain given for a 2 elements yagi is in the order of 6,2 dBI (260 watts on a dipole compared to 100 watt on this antenna)