The Portapack is an add on for the popular HackRF SDR which allows the HackRF to be used portably without a PC. Recently the cost of this hardware duo has come down to below US$150 due to low cost Chinese clones now being available on the market. Generally the clones are of good quality too.
Once you have the hardware it is possible to install third party custom firmware such as "Mayhem" on the Portapack which enables many features such as the ability to receive and transmit various different types of RF protocols. Back in 2018 we did a review of Mayhems predecessor which was known as the "Havok" firmware. More recently Tech Minds did a video overview of Mayhem.
Now over on his blog A. Petazzoni has started a new blog series which aims to introduce the basics of the Mayhem firmware, including installation and some hands on testing with RF spoofing, denial-of-service (DoS) and replay attacks. Currently only his first post is out, and in the post he show how to install Mayhem onto the Portapack, then goes on to briefly overview some applications such as RF replay attacks, replicating wireless remote controls, receiving and transmitting POCSAG, receiving and transmitting ADS-B, and creating a jammer.
Obviously a lot of what you can do with a Portapack and the Mayhem firmware is extremely illegal and very dangerous, so please do be careful with what and where you transmit especially if you are new to RF hobby. These signals should remain in your test area only, and not leak out into the wider environment.