Using rpitx, we can transmit SSB using a single GPIO port from the Raspberry Pi. This of course uses square waves so creates a lot of harmonics.

If I were to use a couple of daisy-chained 70 cm bandpass filters on the GPIO output, would there be any harm in using this output for ham radio?

My aim would be to either connect it up to a PA, or to an upconvertor to go to 2.4 GHz for Es'hail-2 usage.

I understand that it wouldn't be the "cleanest" signal, but would be be legal and usable?

  • 1
    $\begingroup$ A spectrum analyzer would be advised to ensure you're not transmitting a mess of spurious trash (despite your filtering). Also, don't forget that most USB power supplies (which you'll use to power your RPi) are very noisy. Make sure that too is clean. $\endgroup$
    – Synchros
    Commented Apr 24, 2019 at 20:08
  • $\begingroup$ 434MHz USB output from RPITX version 2 starts at 6:22 of this YouTube video. I am surprised to see that the carrier and opposite-sideband rejection are quite good. Results of a two-tone test would be very interesting. $\endgroup$
    – Brian K1LI
    Commented Apr 25, 2019 at 20:08

1 Answer 1


Two daisy-chained bandpass filters might clean up your SSB signal enough to be legal, or they might not. Much depends on the design of the bandpass filters, of course.

What you should do is to get help from someone with the equipment (a suitable dummy load and a spectrum analyzer, primarily) and expertise who can measure the important parameters of your signal to determine if it would be clean enough to be legal.

You wouldn't want to just cross your fingers and hope that such a signal is clean enough, and then amplify it and point it at a satellite. If your signal were dirty, then you would be a nuisance to everyone else using the satellite on the same band, and possibly to other people trying to use other bands as well.

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    $\begingroup$ I'd also point out that if you already have an upconverter, then quite possibly a soundcard would do a much better job at being a cheap IF frontend than rpitx, whose whole point is that it's a mixer on its own. Consider this: instead of buying two filters, you could buy another mixer to convert soundcard-produced baseband or IF to 100 MHz, for at most the same price: good filters are typically more expensive than acceptable mixers! $\endgroup$ Commented Apr 19, 2019 at 13:43

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