Hi Anibal, The ATU-100 that I received also had the same diode arrangement to that shown in your picture, one was marked ¡°SA¡± which is a 1N5711 and should have a similar specification to the BAT41. I swapped both diodes for BAT41, since the compensation values used in the firmware are based on BAT41¡¯s.
I am using the ATU for QRP and wound the Stockton bridge with a 5:1 ratio and updated the constants in the EEPROM accordingly and it seems to work fine up to 30MHz.
Using the Nano-VNA, and manual selection of L and C, the measured and simulated (SIM) results for a best match with different resistive loads is shown below.
The measured and simulated results are similar. At low C values, stray capacitances is the likely reason to require the selected C to be set to zero.
73, Dave
At low frequencies, the Stockton bridge will present a low impedance to ground, producing a high SWR as expected.
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