Bill Schmidt K9HZ has come up with a way of evening out the power output from the uBITx.
He says, “I had a couple of hours of spare time today so I started off my measuring the gain of the pre-drivers in the uBITx.. and sure enough, there is a lot of variation from 1.8-54 MHz (where I want mine to work). Substituting the RD15HVF1 (my choice of RF PA) into the circuit with no other changes gives results similar to what John saw.. but it is because of the pre-driver stages. I contemplated ways of compensating the drive with frequency but there isn’t a really good solution using reactive components because it causes some other non-desirable behavior (like a peak in drive at 21 MHz that is too high for the PA).
“I did land on a rather bruit force method that does work well… I removed RV1 (drive control) from the circuit board and replaced it with a tiny board with three small relays and four 100 ohm 10 turn pots. The relays are controlled by the KT1, KT1, and KT3 drivers… The short story is that now I have gain that is adjustable for essentially each of the bands (at least sets of bands that follow the LP filters). I’ve adjusted the drive so that the PA puts out the same power (+/- about 2 watts) across the entire frequency spectrum. I suppose this should have been an obvious answer, but I’m not fond of using relays for stuff like this even if they only draw milliwatts…”