The "software" in software defined radio means that you'll need a computer to tune the radio, easy choices are:
2. Smartphone or tablet with USB On-The-Go capability,
3. Raspberry Pi based setup for specialised uses.
Variations to the theme exist, like a Pi 3 with a touchscreen.
The best and heaviest solution, capable of running any SDR with an USB connection.
Battery life for remote operation is limited, countdown three hours for a standard 15.6" screen.
Smaller laptops with extended battery life exist, cost more than a bog-standard $300 full-size. Check that the laptop in question comes with USB ports, e.g. the $1000 dollar Acer Swift 7 doesn't have a regular USB port.
Smartphone or tablet with USB OTG
OTG means On-The-Go, charger hole accepts a connector, one end is Micro USB, other end is normal USB female port.
An OTG adapter will be required if you use Android, three types exist:
1. Straight adapter for less than $2,
2. Pigtail for $3,
3. Pigtail with external power (not shown):
Direct connection puts strain on the power/data connector, and if that goes, the device is bricked - you can't charge it, good as a doorstop.
Pigtails are better, because cable additonally compensates for small movements. I tape the OTG cable to the back of my smartphone, then use a regular USB extension lead to the RTL-SDR dongle in use. Spending $100 on a new device is much more painful and expletive-filled than using a $5 extension cord to minimise strain.
Dongles use a lot of power, so if you're visiting the cast of Life Below Zero, get a powered USB OTG hub and a mobile power bank, otherwise the tablet/smarphone will die well before you're eaten by a bear or shot by the natives.
Google your smartphone/tablet name and OTG, most devices before 2016 don't have OTG, but you might get lucky. Rooting an Android device is possible to get OTG after a long stomach-churning installation, which can brick the device. Don't.
I use a Samsung J5 in the image below, antenna is a jumper cable pushed into the MCX antenna connector.
Online marketplaces offer several tablets with fully functional USB ports or built-in OTG, and screen sizes between 8" and 10". If the tablet uses Windows 10 as an operating system, it's possible to run SDRSharp.
In essence, a fully capable and highly portable software defined radio can be had for less than $100, add an upconverter for HF and the cost still remains below $150.
Note that in the image I've placed three dongles for demonstration purposes, you'll only need one - I'd go for either a SMArt or rtl-sdr.com v.3 because of accessories. Large antenna is from the v.3 bundle, smaller telescopic comes with the SMArt.
Raspberry Pi 3
Examples for integrating a Pi 3, touchscreen and Linux-based software front-end can be found, but I've never felt the inclination to grow old trying to reinvent the wheel - SDRTouch works on Android for a few dollars.
A mobile ADS-B station is possible thought by creating a mobile hotspot on the phone and connecting to it with the Pi 3's built-in Wifi. Before Linux greybeards point it out, yes, a Pi3 can also create a hotspot, but it takes 2 taps and 10 seconds on a phone.