I join this topic late and did not read all the posts above, but…there is one caveat with using any DC-DC converters to supply a transmitter, and all powerbanks use some DC-DC conversion:
All DC-DC converters create some ripple in the frequency range of the switching frequency plus its harmonics. The fundamental is typically anywhere between 300 KHz and 2.5 MHz (the datasheet will tell you). The order of magnitude of the ripple depends on the PCB design and the input and (mostly) output filtering.
Now, simply checking whether your reception is “okay” and not disturbed by the DC-DC converter is not sufficient, because you might create unwanted emissions on the frequency that is one of the two mixer products between the carrier frequency and the ripple (= switching frequency). For a 1 MHz switching frequency, this will mean a 7.031 MHz carrier will also create 6.031 MHz and 8.031 MHz. You cannot assume your final LPF to suppress that, because
a) it is a LPF, so the lower mixer product will be unaffected anyway and
b) depending on the switching frequency, the upper mixer product will be too close to the carrier frequency to be sufficiently attenuated.
Whether this is an issue (because the emissions are above FCC etc. -tolerated levels) or not depends on many factors - amplitude of the ripple, input voltage filtering at the rig side, etc.
The only way to know is to do a spectrum analysis.
This is why I designed a DC-DC converter with very strong output filtering. The design is open hardware, see here
Originally, it was designed to be fed by 4 x AA NimH batteries, but it also works with 2-cell LiPo and you could also feed it from the USB output of a commercial powerbank.
On the GitHub page, there are screenshots that show the huge difference in the amplitude of remaining ripple between my design and commercially available DC-DC converters using the same chip.
73 de Martin, DK3IT