Absorber effect on EM RF pattern
Normally a ground plane doubles the effective radiated power by inverting polarity and adding to the potential. Thus 1/4 wavelngth resonator antennae become 1/2 wave matched.

If the absorbing has a ground plane or strong dielectric constant connected to the common potential in or under it to influence the nearby E-Field from fringe effects.  i.e. capacitive effects.

If not, then the loss from absorption would not affect the adjacent paths and would appear transparent for line of sight.  This is how RF anechoic chambers measure antenna polar plots and stealth planes do not reflect energy since they are floating.

But a 1/4 wave antenna without a ground plane would need to be 1/2 wave with an isolator to resonate and radiate effectively.

So you can say the absorber or isolator reduces 1/2 the available isotropic radiated energy.

If this doesn't clear it up,  consider light intensity is doubled in 1 direction with a reflector or white diffused background and the rear light normally used in the other direct is "effectly lost" by a black background which is half the emitting power. Or simply if no background it is just not shining in your direction.