DC was power supplied by a variable regulated 0-30V 5A bench power supply. A QRP Labs 50-
ohm dummy load kit was used as the load (drastically overloaded, but only for very short periods,
enough to make the measurement). Power measurements are made by observation of the peak-
peak voltage on an oscilloscope. Efficiency calculations are power output / DC power input; where
DC power input is the product of the current and voltage displayed on the power supply.
Note that the higher turns ratio measurements had to stop at lower voltages (e.g. 20V), I could not
get the power amplifier up to the chosen 28V top of my measurement range; this is because the
higher turns ratio output transformers have much lower efficiency and larger current draw, and
caused the power supply current limiting to kick in. (The current limit evidently occurs at around
4.8A not the claimed 5A – though it does occur at 5A for lower voltage settings).
After a lot of staring and consideration at these graphs, I could eventually draw the following
conclusions.
1. The stacked Amidon BN42-202 binocular cores did indeed provide higher power output at a
given supply voltage with higher impedance ratio. However there was a high penalty (10-
15%) in the efficiency calculation.
2. The higher turns ratio did not apparently provide any power output benefit with the other
ferrite cores tested, and showed an even worse efficiency penalty.
3. 1:3 ratio was even worse than 1:2 ratio!
4. The 2:4 turns tested on the 2-ferrite-tubes produced better results than 1:2 turns, even
though the ratio is the same; I speculate that the larger amount of wire inside somehow
provides better coupling.
5. When two BN43-202 toroids were stacked end-to-end, instead of using four stacked BN43-
202, there was a noticeable deterioration in both power output and efficiency. I believe two
BN43-202 simply doesn’t provide enough ferrite and is therefore more lossy.
6. The large unbranded toroid did not give good power output nor did it give good efficiency,
compared to the binocular format ferrites; additionally it was very hard to wind the thick
diameter wire, on a toroid form. Therefore no further testing was done with this toroid.
50W QCX PA kit assembly
1.00q
49
