There are some clues.
... The Straumers had a backup plan if the rendezvous failed. They intended to signal Relay with their ship's ultrawave.
"Now wait. Just how big is this ship?" Ravna was no physical-layer engineer, but she knew that Relay's backbone tranceivers were actually swarms of antenna elements scattered across several light-years, each element ten-thousand kilometers across.
Blueshell rolled forward and back, a quick gesture of agitation. "We don't know, but it's nothing exceptional. Unless you're looking precisely at it with a large antenna, you'd never detect it from here."
Relay devotes an entire transceiver to searching for Jefri's signal, and although I can't find a reference I got the impression it took a while to find even with those resources.
RIP seems to only be a tenth of the distance from the Tines' world as Relay is, but the OOB has a technical advantage the other fleets likely lack:
Right now the Out of Band II was in the Vrinimi yards getting bottom-lugger enhancements and a huge stock of antenna drones. In one stroke the OOB's value was increased ten-thousand-fold.
Even then, it's no picnic:
Some days, no communication was possible. Other days, when the OOB antenna swarm was tuned in exactly the right direction and when the vagaries of the zone favored it—then Ravna could hear his ship.
I also think it's worth noting that I don't see that any of the fleets have anything to gain by intercepting or even corrupting communications between Jefri and the OOB.
So, I think there are three reasons not to be concerned about the fleets intercepting Jefri's messages:
- The fleets are likely unaware that the messages even exist, and almost certainly don't have the information necessary to aim their antennas sufficiently.
- The fleets also probably don't have sufficient antennas to receive the messages even if they were looking in the right place.
- The fleets have nothing to gain by intercepting the messages anyway.
(Also, you didn't specifically ask about this, but I think it's worth mentioning: Ultrawave is described as "directional", which probably means that the fleets pursuing the OOB would be unable to receive the messages sent from the OOB to the Tines' world.)