Evening up power output on high bands

Howard WB2VXW received his µBitx, assembled it, and started testing.  He performed a number of modifications to the µBITx in his lab before moving it to its permanent home in the shack.

In particular, he modified the transmitter to improve output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz.  His modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On his board, there was not enough range in VR1 to increase the power to anywhere near this level without modification. The other bands remain unchanged in output, at around 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve this further, but Howard was happy with the improvement for now.  He plans on adding a power amplifier later, something between 50 and 100 watts.

Howard’s fix involves adding 3 passive parts:

  • 33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.
  • Adding a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

The theory is very simple. The closed loop gain of Q911 and Q912 is set by the ratio of R86/R85, or a gain of 10. Adding an inductor into the path in the feedback loop increases the effective impedance at higher frequencies, adding “peaking” to the circuit, thus making the closed loop gain higher.

I think the poor frequency response is in the transformer as well as the transistors Hfe falling to under 25 typically at 30 MHz.  The open loop gain is also increased by bypassing the emitter resistors for higher frequencies.

The transformer is a 2 to 1 step down, (it looks like a trifilar winding) so the theoretical gain of the stage is 5.

Offer of parts

Howard also made a generous offer to  anyone wanting to try the fix and needs the parts.  He has a full reel of each of the parts (2x 270pF capacitor and 33µH inductor), so just Howard a stamped self addressed envelope, and he will send it to you by return mail. The offer is good until he runs out of parts.

Others try it on

Joel N6ALT completed the mod not long after it appeared on the BITX20 IO Groups list with spectacular results. Before the mod he had only 300mw at 13V on 28.500.  After the mod he had 3.8 watts, and after increasing the drive slightly, he now has 4.5 watts. All of the other bands also benefited from the mod.

Tim AB0WR also tried the mod, winding an FT37-43 ferrite to get 33uh. Not as neat as an smd inductor but he didn’t have any in the shack!


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