I always thought the dream transmission ...

A tachyon is a sub-atomic particle. Which means that, like any particle-based transmission, it can be focused into a beam and aimed at a specific point much like a laser. And, much like a laser, you can focus the beam as tight as a needle(or smaller) or wide enough to totally encompass, say, a church(or larger) depending on need. It definitely wouldn't transmit like radio waves which is omnidirectional from point of origin. Even then, anything broadcast using tachyons would likely be received much the same way radio waves are(such as anyone with a receiver can listen to the broadcast of a powerful enough radio station as long as the receiver is tuned to the same broadcasting frequency even though, technically, there is only one source).

Also, we had a discussion in a deleted thread were we came to the conclusion that Brian is the most likely person to have sent the transmission...not Anti-God. The logic being that, once Catherine makes her sacrifice, Brian is the only one who would feel the need to try and reopen a portal to Limbo(to save her) and the knowledge to do so as he's a theoretical physicist, has access to the book with all those nifty Jesus equations proving the science behind what happened and knows where and when to aim the transmission(at the location of the canister prior to Catherine's sacrifice and at his house after). Of course, this is based on the assumption that the Dream received by Brian at the end film is an authentic one only slightly changed due to Catherine altering the timeline by sacrificing herself.

So anyway, no, this isn't a plot hole as it is internally consistent with the rules/logic presented in the rest of the film.

A nozh scrap any time you say.