Comparing Shortwave Antennas with an RTL-SDR and FT8 Monitoring

Eric had an inverted L and T3FD antenna set up in his backyard and he wanted to test both at the same time to see which received HF better overall. Rather than relying on subjective 'by ear' measurements he decided to use the digital FT8 mode as his comparison signal. FT8 is quite useful for this purpose as the decoded data includes a calculated signal-to-noise (SNR) reading which is a non subjective measure that can be used for comparisons. It also contains information about the location of the signal which can be used for determining the DX capability of the antenna. 

To perform the comparison he used two or our RTL-SDR Blog V3 dongles running in direct sampling mode, and also added an additional low pass filter to prevent excessively strong TV and FM signals from overloading the input. Each antenna is connected to it's own RTL-SDR, and a modified version of GQRX with remote UDP control is used to switch between multiple FT8 frequencies so that multiple bands can be covered in the experiment. WSJT-X is used for decoding the FT8 packets.

After logging SNR values for several days he was able to plot and compare the number of packets received by each antenna, the maximum distance received by each antenna. His results showed that his inverted L antenna was best in both regards. He then performed a relative comparison with the SNR readings and found that the inverted L performed best apart from at 14 MHz, where the T3FD performed better.

In further tests he also compared the antennas on which signal headings they were receiving best from. The results showed that Erics inverted L was receiving best from one direction only, whereas the T3FD received signals from more headings.

Eric's post includes full instructions on the software setup and also Python code which can be used to replicate his experiments. We think that this is a great way to objectively compare two types of antennas.

Antenna directionality measurements via FT8 received headings

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