[time-nuts] Cavity frequency air filled vs vacuum?

[Attila Kinali]

On Tue, 18 Aug 2015 21:49:50 -0700
<cdelect at 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

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