A few months ago I was reading an article written by Owen Duffy that a compact, low-loss QRP-capable EFHW (1:49) transformer could be build using the Fair-Rite 2643625002. He used a thermal view to proof that it can handle up to 20 Watt with an acceptable amount of temperature increase.
A further theorical analysis was done by HB9EAJ showing a theoretical efficiency >90% over all relevant ham radio bands.
Enough reasons for me to build one!
I started up with buying a standard PVC box and the Ferrite.
According to the analysis of HB9EAJ a 3:21 a winding should be used. Due to the small size of the Ferrite I sticked to 0.5mm isolated copper for the windings.
For the compensation of the upper frequencies a 100pF ceramic capacitor was soldered.
For the verification of of the 1:49 transformator I attached a 2700 Ohm MOX resistor. The resistor simulates roughly a resonant wire antenna over all sweeped frequencies.
The spectrum was measured using a calibrated NanoVNA utilizing the NanoVNASaver software. The result of NanoVNASaver sweeping from 1 - 30 MHz.
The theorical results are looking great as other operators already forecasted.
It can utilize all bands from 160m up 10m with low loss and the setup is incredibly compact and lightweight.
Test the setup with a 150pF ceramic capacitor to reduce the high SWR around 27-30 MHz.