The below is BOLD large print to make this very clear.
For bands below 20Mhz its NOT an issue as the low pass filters catch it.
It is only a potential problem for SSB and frequencies greater than 20mhz.
NOTE: due to the way the uBTX does CW it is never an issue on any band.
The short form is when you mix two frequencies you get a third, in a perfect world.
The diode mixers used are handy but they can present conundrums. If any of the
three ports (IF, LO, and RF) are mismatched (think SWR) The signal can be reflected
back in. Since DBMs are omnivorous in that any port can be input or output and
if mismatched both! This does not include effects of distortion in the source signals.
But in the real world this like that exist.
They also suffer from overload, to many and too strong and you get a plethora of signals.
What that means for lots of simple and complex reasons you can get "spurs" or spurious
outputs that are undesired.
Basic math, addition and subtraction:
So if you mix 45Mhz with 73mhz you get 28mhz. We want that and the radio needs that.
However if any of the 28 gets reflected back into the DBM where it originated it mixes
with 45mhz and we get 17mhz.
With those four signals you get mixtures of those like:
Those are "first order" as they do not involve harmonics. They will be the strongest
but not always equal strength.
Both inputs can have harmonics like 90mhz and 146mhz and the 34 and 56 coming
out can have harmonics too. If you add and subtract all the possibles you get an
increasing sea of signals some weak some stronger. We will not cover the possibles
as the first order ones are the most troublesome.
If the above is not clear, ask away.
The solution traditionally applied is band pass filters or if it isn't between 28 and 29.9999
the filter strongly attenuates it. But you need a band pass filter for most every band...
uBitx take the path of below a certain frequency you only need low pass filters and
fewer of them. And it generally works well especially for 80 though 17M...
But at 20mhz and up the low pass filter passes everything below 30mhz and if you
overdrive the rig slightly you get a spur for the tech window of 28.3 to 28.5 where
the spur is 16.5 to 16.7mhz and there is no filter for that. What makes this worse
is some radios are very poor at 10M putting out maybe 2W so pushing the audio
to get more invites the problem to be greatly worse. I got into this as mine was
one that would do more than 15W on 80m and with decreasing power to 1.7W on 10M.
If I forced more audio with RV1 turned all the way up I could get 4W but the 16-17mhz
spur was nearly 13 milliwatts (measured) and well out of the range of permissible.
Also there is no setting we can safely give that absolutely assures there will
be no problem that is consistent with maximum achievable power.
The above is why the discussion began.
As a licensed hams here in the USA (likely elsewhere) we are responsible for
signal quality and also not generating signals outside our assigned bands.
There are two solutions one is bandpass the other is high pass filter.
Either way the rig must be modified to allow those and there are side effects.
One side effect is you need extra switching not provided. The other is any filter
as loss though it and that would further reduce power out.
This is a case where as a new tech or even extra you trade cost for skills
and the skills needed are understanding RF electronics. There are solutions
I can suggest, one is get your General or Extra (its worth it and opens many
other bands). Another is hope one of us can crank out a complete solution you
can insert into your radio which means soldering skills and ability to do basic
electronics construction and alterations. If you have a club with technical
members to help/mentor all the better.
Short of that keep the power down and go for it...