Re: [time-nuts] Cavity frequency air filled vs vacuum?
On Tue, 18 Aug 2015 21:49:50 -0700 cdel...@juno.com wrote: I'm trying to figure out what change if any to a cavities resonant frequency would be when measured with ambient air pressure and then with a vacuum inside. There will not be any pressure on the cavity when under vacuum as the outside of the cavity will also be at vacuum. The cavity frequency under vacuum will be 1420Mhz + (Maser frequency). On a similar quest, i stumbled upon [1] where Hasegawa and Stokesberry used resonant microwave cavities to measure humidity of air. The paper gives a couple of formulas on how to calculate the frequency shift due to air pressure, humidity and CO2 content. HTH Attila Kinali [1] Automatic digital microwave hygrometer, by Hasegawa and Stokesberry, 1975 http://dx.doi.org/10.1063/1.1134331 -- I must not become metastable. Metastability is the mind-killer. Metastability is the little-death that brings total obliteration. I will face my metastability. I will permit it to pass over me and through me. And when it has gone past I will turn the inner eye to see its path. Where the metastability has gone there will be nothing. Only I will remain. ___ 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.
Re: [time-nuts] Cavity frequency air filled vs vacuum?
Hi And for major fun, you can get into altitude and humidity corrections on the exact number. Since your altitude does not change day to day, humidity is often the thing that drives you a bit nuts. Bob On Aug 19, 2015, at 1:30 AM, Bruce Griffiths bruce.griffi...@xtra.co.nz wrote: The ratio of the resonant frequency of the evacuated cavity to that of the air filled cavity increases by the square root of the relative permittivity of the ambient air or around 300ppm or so. Bruce On Wednesday, 19 August 2015 5:06 PM, cdel...@juno.com cdel...@juno..com wrote: Hi everyone! I'm trying to figure out what change if any to a cavities resonant frequency would be when measured with ambient air pressure and then with a vacuum inside. There will not be any pressure on the cavity when under vacuum as the outside of the cavity will also be at vacuum. The cavity frequency under vacuum will be 1420Mhz + (Maser frequency). Thanks, Corby ___ 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. ___ 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. ___ 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.
Re: [time-nuts] Cavity frequency air filled vs vacuum?
On Tue, 18 Aug 2015 21:49:50 -0700 cdel...@juno.com wrote: I'm trying to figure out what change if any to a cavities resonant frequency would be when measured with ambient air pressure and then with a vacuum inside. There will not be any pressure on the cavity when under vacuum as the outside of the cavity will also be at vacuum. The cavity frequency under vacuum will be 1420Mhz + (Maser frequency). The relative permittivity of _dry_ air is 1.0005something, so you get a 5e-4 change in frequency. But there is usually a quite substantial content of water in the air, which can lead up to a few percent % of permittivity change. (Numbers are from the top of my head, not hard numbers. Please look them up if you want to calculate anything) I am not sure whether you have to account for the absorbtion of hyperfine transition. My guess would be not, because the hydrogen atoms are bound in a molecule and thus the hyperfine transition at 1.4GHz has been shifted quite a bit. If your goal is to measure a hydrogen maser cavity, and see how far off frequency you are, then you don't need to worry about air vs vacuum, IMHO. You have to compensate for shifts due to mechanical dimension inaccuracies (production accuracy, pressure, temperature, handling) anyways. So just design a coarse tuning system with which you get the cavity to a few 1e-4 of the end frequency, then tune the rest with an varactor when you fire up the cavity. Attila Kinali Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ 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.
Re: [time-nuts] Cavity frequency air filled vs vacuum?
On 8/18/15 10:30 PM, Bruce Griffiths wrote: The ratio of the resonant frequency of the evacuated cavity to that of the air filled cavity increases by the square root of the relative permittivity of the ambient air or around 300ppm or so. Bruce I believe that there are systems that measure humidity by this means. Water content has a strong effect on permittivity. Although, since there's just not that much water in humid air, it's probably in the few ppm range, but easily measureable ___ 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.
Re: [time-nuts] Cavity frequency air filled vs vacuum?
vacuum's dielectric constant is 1, air dielectric constant is 1.00059 at 1 atm, therefore the frequency with vacuum is higher, but with air it depend on pressure and humidity also 73 KJ6UHN Alex On 8/18/2015 9:49 PM, cdel...@juno.com wrote: Hi everyone! I'm trying to figure out what change if any to a cavities resonant frequency would be when measured with ambient air pressure and then with a vacuum inside. There will not be any pressure on the cavity when under vacuum as the outside of the cavity will also be at vacuum. The cavity frequency under vacuum will be 1420Mhz + (Maser frequency). Thanks, Corby ___ 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. ___ 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.
Re: [time-nuts] Cavity frequency air filled vs vacuum?
kb...@n1k.org said: And for major fun, you can get into altitude and humidity corrections on the exact number. Since your altitude does not change day to day, humidity is often the thing that drives you a bit nuts. Is it altitude or air pressure (density)? There is a semi-joke about good enough pendulum clocks turning into garvitometers. How accurately can you measure the frequency of a cavity? Can a good setup be used as a barometer? How does humidity compare with air pressure? -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] Cavity frequency air filled vs vacuum?
The ratio of the resonant frequency of the evacuated cavity to that of the air filled cavity increases by the square root of the relative permittivity of the ambient air or around 300ppm or so. Bruce On Wednesday, 19 August 2015 5:06 PM, cdel...@juno.com cdel...@juno..com wrote: Hi everyone! I'm trying to figure out what change if any to a cavities resonant frequency would be when measured with ambient air pressure and then with a vacuum inside. There will not be any pressure on the cavity when under vacuum as the outside of the cavity will also be at vacuum. The cavity frequency under vacuum will be 1420Mhz + (Maser frequency). Thanks, Corby ___ 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. ___ 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.
[time-nuts] Cavity frequency air filled vs vacuum?
Hi everyone! I'm trying to figure out what change if any to a cavities resonant frequency would be when measured with ambient air pressure and then with a vacuum inside. There will not be any pressure on the cavity when under vacuum as the outside of the cavity will also be at vacuum. The cavity frequency under vacuum will be 1420Mhz + (Maser frequency). Thanks, Corby ___ 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.
