Here is another DIY antenna, this time for GSM/3G/4G usage. The design was calculated for 800-2100MHz and works well in this range. The antenna has a very broad bandwidth with a low gain and can be used for any application within the frequency range. Since the results are very good even up to 3GHz the antenna is also useful for WiFi. As well as ADS-B (flight radar) because 1090MHz is covered too. In general an excellent antenna for rtl-sdr operations.
Based on this calculator.