Helium is expensive and hard to find after nuclear war.
Hot air simply doesn’t have enough lifting capacity.
Hydrogen is something you can make even low tech environment.
The easy way is AN/TMQ-42 Hydrogen Generator!
http://www.jedsite.info/transport-ho...q42-intro.html
Hitler’s last headquarter had to use balloons to lift antenna up. After all it was air raid shelter, which had only one low capacity switchboard and no permanent antenna for long range communications.