problem with such a tight-beam is you have to be in direct line of sight for it to be seen. which means a very small number of receivers civilization can ever k

problem with such a tight-beam is you have to be in direct line of sight for it to be seen. which means a very small number of receivers civilization can ever know, that is exact opposite of the goals of a 'lighthouse'. perhaps something that sweeps around at regular intervals with some precession thrown in for coverage, that makes it closer to GP's comment about neutron star emitting prime numbers.