EU1KY antenna analyzer is a very cheap 600MHz VNA based on SI5351 clock generator, SA612 double balanced mixer and a STM32F7 discovery board. The original source is here(https://bitbucket.org/kuchura/eu1ky_aa_v3/wiki/Home).
KD8CEC’s website(http://www.hamskey.com/2019/02/assembling-faa-450-antenna.html) revealed much more about this analyzer, including detailed instructions on how to assemble it, and several popular modifications. Moreover, the CEC firmware has included much more functionalities than original（Other RF frontends have been included in CEC firmware such as AD chipset, but he has not released schematic）.
The original board kind of wasted lots of board space, the ferrite hangs outside the board does not look good and CEC firmware actually support external RTC. Instead of buying the board, I decided to design my own modification.
RTC, buzzer, and battery voltage detection circuits were added to the original board. A TCXO TG2016SBN was used as clock reference to SI5351, and the TCXO can be replaced directly by passive crystal with same package such as FA-128-2016.
Nonetheless, the most important modification was the balun. Instead of using a chunky external ferrite, I integrated the balun inside the PCB (here is the Altium PCB file), as shown in the following figures,
The balun consists of several ferrite core, and a coaxial cable running through it. Compared to original design which uses a bigger ferrite and wind the cable several turns around the core, the modification used several smaller but longer ferrite. This way, the balun was easier integrating into the board.
Complete Antenna Analyzer
This is the complete assembly on a STM32F4 board. Although, it was said to be working up until 1.3GHz, the performance degrades very fast when frequency exceeds 200Mhz, and barely usable over 600Mhz. Mostly due to performance limitation of the SA612 mixer presumably.