Sometimes "omnidirectional" means "equally radiating in all azimuth
directions", which is the interpretation Cecil has used. In that case,
yes, of course, you can achieve gain by concentrating the power in the
vertical direction, i.e., increasing the power density at some elevation
angles at the expense of others. A collinear antenna (linear antennas
placed end-to-end as Cecil describes) is a common way of achieving that.

I inferred from the original posting that the intended meaning was truly
omnidirectional, as in isotropic, the same in all directions in three
dimensional space. If that's correct, the answer is no.

The explanation in my response was in terms of a transmitting antenna.
It's well established (the reciprocity principle) that the gain of an
antenna is the same for transmitting and receiving, so the conclusion is
equally valid for a receiving antenna.

Roy Lewallen, W7EL

W5DXP wrote:
wrote:

Is it possible to build an omnidirectional receiving antenna with
gain?


Of course, the Aug. 2003 QST has some of them on page 19 & 108. The
Comet GP-24 for 2.4GHz is twenty end-to-end 1/2WL antennas.