I do have a multi z tuner kit on the way.? So can the high pass filter for 10 meters be a simple RC or LC for 27 mhz and below. Then placed in the antenna feedline?? Maybe with a toggle switch for bypass? Or am I over simplifying?

The below is BOLD large print to make this very clear.
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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.

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The short form is when you mix two frequencies you get a third, in a perfect world.

However diode mixers are handy but they can present problems.? If any of the three
ports are mismatched (think SWR) The signal can reflect back in.? Since DBMs are
omnivorous in that any power can be input or output and if mismatched both!

What that means for lots of complex reasons you can get "spurs" or spurious outputs
that are undesired.

They also suffer from overload, to many and too strong and you get a plethora of signals.

So if you mix 45 with 73 you get 28.? We want that.? However if any of the 28 gets 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:

73-17=56
28-17=11

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 two
solutions I can suggest, one is get your General or Extra (it worth it) the other
is hope one of us can crank out a solution you can insert into your radio
which means soldering skills and ability to do basic electronics construction.
If you have a club with technical members to help/mentor all the better.


Allison

Here's another shot at embedding my Mouser search, maybe this one will work:
? ??