Mark Cantrell (AE7TO) has been having fun getting his uBITx to have more even power out.
His rig can be seen in the photo above. At the left hand side (with controls incorporated on the rear panel) is an Antenna Tuning Unit and in the middle is a new daughter board.
The new daughter board implements the relay-based band output power leveling mod designed by Bill (K9HZ).
This mod helped quite a bit at 10 and 14 MHz. Here is a summary of power levels, as observed by Mark, on each band after the mod:
- 7 MHz and lower frequencies = 11 watts out (no change from before the mod
- 10 MHz = 8 watts out (twice was I was seeing before the mod)
- 14 MHz = 8 watts out (also twice what I was getting before the mod)
- 18 MHz = 6 watts out (never checked that band before the mod)
- 21-28 MHz = 3-4 watts out (didn’t test that band before the mod)
Mark didn’t bother adding a 4th relay for tuning as he usually uses an antenna analyzer and manual tuner anyway.
A 3D model file is available for the plastic bridge built to hold the daughter-card above the main uBITX circuit board. Someone else may find that file useful.
Then he added a 33 uH inductor in series with R86, and 220 pF caps in parallel with R87 and R88, as suggested by Howard WB2VXW.
Here is a summary of power levels after the above changes (plus replacement of RV1 with the 3-relay/4-pot mod discussed below):
- 3.5 MHz / 28 W / 2.3 A
- 7 MHz / 28 W / 2.2 A
- 10 MHz / 16 W / 1.4 A
- 14 MHz / 20 W / 1.5 A
- 18 MHz / 11 W / 1.1 A
- 21 MHz / 7 W / 0.7 A
- 28 MHz / 7 W / 0.7 A
These numbers are good enough that Mark no longer feels the need to fiddle any more. In particular, he is not going to change the bias settings on the finals as they seem well adjusted from the factory. Mark has turned back the power output on the low bands to give no more than 20w out to protect his finals.