Limits On Interstellar Communication

"The 'instantaneous' pulses emitted by a ship in hyperdrive are detectable at an extreme range of about a light-year. They can be modulated to carry information. Unfortunately, within a few million kilometers quantum effects degrade the signal beyond recovery; even the simplest binary code becomes unintelligible."
-Poul Anderson, Flandry's Legacy (New York, 2012), p. 132.

However, the interstellar civilization in Anderson's For Love And Glory has an instantaneous hyperbeam. I remember that there was some limitation on the use of hyperbeam but not what it was.

In the Man-Kzin Wars period of Larry Niven's Known Space History:

"'Someday they'll miniaturize hyperwave equipment to the point where it'll fit into a spaceship.'"
-Poul Anderson, "Inconstant Star" IN Niven, Ed. Man-Kzin Wars III (New York, 1990), p. 211.

And, later in that history, Beowulf Shaeffer, exploring the galactic center alone in a spaceship, speaks to his puppeteer employer instantaneously by hyperphone. (Too easy.)

Anderson's Starfarers features an instantaneous and transtemporal communicator and Ursula Le Guin's future history has the instantaneous ansible. However, the true master of interstellar communication is James Blish:

his ultraphone is FTL but not instantaneous;
his CirCon radio and Dirac transmitter are instantaneous;
in one application of the Dirac transmitter, it receives messages from the past and future as well as from the present;
his Heart Stars empire and Angels also have instantaneous interstellar communication.

This is the kind of systematic treatment of a theme that we often find in Anderson's works.