An Airbus would be an ideal platform - stilted u/c allowing a 'boat' full of antennae slung underneath [good, unobstructed reception] + cargo space for fly-away packs. And plenty of room to carry the groundcrew for deployment.
Young, modern, fuel efficient, long endurance, room to expand internally - the list is endless.
Drones have their place on the battlefield, alongside the human. SIGINT benefits from the experienced operator who can instantly, for example, take the intonation in the voice and add it to all the other observed factors to reach a quick conclusion. That's how it works, so why throw away a top class methodology?...