The power output of the µBITx varies according to frequency.
Richard Pushman provided a chart showing the power output of the rig by frequency from 2MHz to 30Mhz for two different supply voltages. The shape of the peaks and troughs is, of course, influenced by the LPF filters in the output stage.
Allison KB1GMX notes that if you have a v4 board and experiencing low power output on lower bands (80m and 40m) you may want to check the emitter of Q90 to see if a .01uf or a 470pf or 220PF is there.
There was a change to lower the value from .01 to 470 pf to level the output
power so 80M wasn’t 15-20W when 20m was maybe 8 watts output.
Some have experienced that low bands work very poorly with a 470pF or 220pF capacitor. You may want to increase the capacitor back to 0.01µF or perhaps 1000pf.
Mike N6CMY has been doing some mods on his µBITx, including some of Allison’s mods by substituting 2n2222’s and a BFR106 for the 3904’s on the RF lineup (pre-driver and driver stages).
In terms of output power the results are impressive. But in terms of levelling up the output… well not so good.
Mike measured 12W on 10M and over 35W on 40M! He was a bit taken aback at this point!
He finally bit the bullet and used K9HZ’s multi relay/pot method for levelling the response.
Mike was reluctant to use such a brute-force solution for a subtle problem but he now measures 15W on 80, 40, 20 M and 12 on 10M. VICTORY!!
John VK2ETA has uploaded his latest version of a variation on Ian KD8CEC’s software with a default of “stock standard” which means it should run on units “factory” wired.
It includes the “OPTION_ALC” which use a table and interpolation to set the attenuation for 3 levels (Low, High, Max) for SSB power output.
Not a complex code at all (just search for the “option_alc” keywork). This should give up to 50dB of attenuation. This should be plenty to control the power even for WSPR low power users.
You will probably need to attenuate the signal for “Max” power on 40 and 80m otherwise you will get well past the targeted 16-17W on the lower frequencies.
Note that on 10m, you will not get 10w out. You will need to follow Allison’s work on the power chain to see if a solution can be found to the fall-off in power out on 10m.
John’s software can be found here.