The Balloon

[Anomalocaris]

Let’s be honest,  no balloon the size of a small car can possibly carry any substantial intelligence package.   A balloon’s carrying capacity must necessarily be less than the weight of a volume of air at the balloon’s operating altitude equal to the size of the balloon itself

A balloon the size of a small car, say 10 cubic meters, would therefore have a carrying capacity at 40k feet of 4 lbs minus the weight of balloon itself.

I'm not an electronics engineer, but it seems that you could pack a fair bit of microminiaturization into a four lb (or slightly smaller) payload.

Boru

the 4 lbs would need to include a battery that would function for the duration of balloon flight at -70 degrees temperature of 40k feet.    a add-on battery to enable an iphone to work for 4-5 days in much milder conditions already weigh a pound.  

in addition to make it worthwhile to evesdrop from a balloon rather than a satellite it would need to specialize in short wave reception that is blocked by the ionosphere from being done in space.   to do so efficiently it would need a significant antenna.    it would also need to have its own transmitter so it can send intercepted signal back to its master.    to really add value to signal intelligence it probably needs a directional antenna and a gps receiver so it can do some geolocation of the signals it intercepts. 

even if it is disposable and not meant for recovery, it would still need a beacon so it’s masters knows where to go look for it in order to receive a download of the signals it intercepted and recorded.