What must be factored in, when using a tethered balloon to lift an AEW system aloft is the weight of the cable. Obviously the more cable, the heavier weight that must be lifted to reach a given altitude. While attractive because of their "cheapness", the weight and the cost of the cable quickly adds up and while it may be recoverable if you lose the balloon, the question has to be asked if its worth it when it means you must build a ship big enough to carry that much cable, plus service the balloon, when a helicopter or a fixed wing aircraft are smaller, cheaper and much lighter, allowing a bigger AEW set to be carried aloft. Alternatively, you could of course utilise an airship, which is a good compromise between the two sorts of aircraft...