On 31 October 2016 at 10:37, Poul-Henning Kamp <p...@phk.freebsd.dk> wrote:
> -------- > In message <CANX10hCrGmtUuO+uX=vTuRHtiZTpzHK8Vz0kybFiq1fQ-wr6rg@mail. > gmail.com>, "Dr. David K > irkby (Kirkby Microwave Ltd)" writes: > > >> > [...] so it might be a relatively cheap way to measure humidity. > >> > >> 80m wave-guide is neither cheap, nor in most circumstances, practical > :-) > > > >But you don't need 80 m of waveguide. 100 mm or so would be sufficient. > > Then it would require a quite high frequency... > If you try to do an attenuation measurement in the normal operating wavelength of the guide, then I agree you would need a huge length unless at very high frequency (100 GHz+), where water absorption becomes high But that's not the approach I would take. I would look at trying to exploit the variation of cutoff frequency of waveguide with humidity. WR90, which has an internal width of 0.900" (that's where the 90 comes from), has a cutoff frequency around 6.5 GHz. I would expect that to be pretty dependent on the permittivity in the guide. 6.5 GHz is not a ridiculously high frequency to generate, and WR90 is not unmanageably large. Making the guide bigger would reduce the frequency you need to generate. . . _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.