I have been trying to make a usable antenna analyzer for 868MHz. I wanted to use it without a PC/laptop so no RTLSDR as detector.
I worked with a ADF4350 generator board, a 0.5-3000MHz RF-bridge and a AD8318 as logaritmic detector. That did not work, the ADF4350 is square wave output, so a lot of uneven harmonics that masks the reflection measurements. Also the ADF4350 board did only have about zero dBm output. Reflected signals travels twice through the RF-bridge, so 6dB powerloss there, so I needed a amplifier. To eliminate the harmonics I used a 1200MHz low pass filter.
By that time I was so disappointed by the poor results of this setup that I bought a N1201SA antenna analyzer which works very well.
The N1201SA works with two ADF4350 generators. One for the signal and the second mixes the reflected signal down to a low intermediate frequency. So the detector only sees the measured frequency and is not influenced by other (f.i. GSM) signals that are received on the antenna under test.