6m with Low Band tr(u)SDX

[DD5MA]

Hi all,
just by playing araound, I put the tr(u)SDX on 6m.
I powered up with only 5 Volt USB supply.
By pressing the PTT my display starts to flicker and showed 0,00W and 0,00% eff.
By checking the transmission with a my FT-991A I saw that a transmission can be seen on the waterfall and I guess it will be in the range of 0,1watt HFout. I also saw transmission on all other bands above 20m
Does anybody have any experiences with transmitting with the tr(u)SDX low band version out of the installed bands?
vy 73 Frank

[DD5MA]

I just made my first FT8 QSO 17m 18.100 with my low band version.
5 Volt and 0,6Watt (external QRP-Power-meter) and G5RV from Germany to Italy with a report of -14.
seems to work.
Is there any risks of damaging the BS170?
Signal seems to be quite okay looking to the waterfall.
vy 73 Frank

[DL2MAN]

Relatively simple to answer: Don´t !

6m on a low Band (tr)uSDX means it´s using the 20m Band Module.
The Series resonance Filter has near 0 Ohms at f res and is calculated for added L to create the right impedance to generate the output power we want. Asking a resonant circuit to work at almost quadruple frequency means starting already at very low efficiency. Most Power is dissapated in the Fets because of Impedance mismatch.
The following Pi Filter (Low Pass with 2nd harmonic notch) has at it´s almost quadruple frequency more than 45dB Attenuation, so a lot is dissapating there.
Even if your Fets would produce half decent output, most of it is heating up the mismatched filters.

Max Power Dissapation per BS170s is around 800mW and gets less if they warm up. So in theory you can dissapate 2,4W. AT 12V this will happen within fractions of a second. At 5V not so much, but you´ll end up with a 10mW Transmitter..... I do not recommend it.

73 Manuel; DL2MAN

[John]

The output amp of the (tr)uSDX operates in Class E,or it does if it's properly tuned. But with the Q of these circuits, even properly tuned LC circuits for each band will drop in efficiency at all other points on each band than the point they were optimized on. That drop in efficiency means excess heat in the output FET's. So operating a Class E amp on any band other than the one its tuned for, (each band has an individual set of inductors and capacitors to tune it for that band that automatically gets switched in when a band is selected,) amounts to playing Russian roulette with the FET's. They are very inexpensive, true, but you probably won't want to replace them as often as you're going to blow them operating with the wrong LC components for the band. Also, the output circuitry for each band includes low-pass filtration to keep harmonic content within legal limits. For these reasons it's a very good idea to read up on how Class E output stages work before experimenting with transmissions on bands the radio is not tuned to operate on. "What happens if I do this," sorts of experimenting is one of the things ham radio is all about. But I fear in this particular case it's similar to, "What happens if I pour gasoline on this fire next to my tent?"
Here is a good 3 part explanation of Class E operation: https://www.youtube.com/watch?v=Tgrakttus3c
Enjoy.