>> -----Original Message-----
>> From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of John
>> Ponsonby
>> ... It turns out that the resonant frequency of the cavity is much more
>> critically dependent on its diameter than on its length. So it would be best
>> to be able to mount the bulb in the cavity and to measure the resonant
>> frequency with the cavity still in the lathe...
>
> This reminds me of an anecdote about the construction of the first NH3 maser
> in Charles Townes's book ("How the Laser Happened.") They were having
> trouble with the irregularities in the cavity associated with the entrance
> and exit apertures for the ammonia gas. They found it was better to get rid
> of the gas ports altogether and open the cavity completely at the ends,
> essentially replacing it with a long pipe relative to the resonant frequency
> at K band.
>
> Is that an option at 1420 MHz? Or would the cavity pipe and storage cylinder
> have to be so long that it would be even more expensive to build (and to
> shield)?
>
> -- john, KE5FX
> Miles Design LLC
>
In an H-maser the cavity works in the TE0,1,1 axially symmetric non-polarized
mode. It's length is half a waveguide wavelength long. As I'm sure you all know
the wavelength in a waveguide is longer than the free-space wavelength for the
same frequency. At a given frequency the smaller the cross-section of the guide
the longer the guide wavelength becomes until when the guide is at cut-off the
guide wavelength goes to infinity. So in principle one can make the cavity
length in an H-maser as long as one likes. When close to cut-off a small change
in cavity diameter requires a relatively large change in length to keep the
resonant frequency constant. For any uniform waveguide the guide wavelength λg
is related to the free-space wavelength λo by the expression:
λg = 1/sqrt((1/λo)^2 - (1/λc)^2)
where λc is a characteristic length which depends on the shape and size of the
guide cross-section.
For the TE0,1 mode λc = 0.820×cavity-diameter. Unfortunately these expressions
are only indicative and not exact in the case of the H-maser because the cavity
isn't empty and it isn't uniformly loaded. It is dielectrically loaded by the
storage bulb inside it and it isn't easy to compute the effect of the storage
bulb.
Harry Peters made some long-cavity H-masers when he was at Goddard SFC
but most masers are made with length ≈ diameter.
John P
_______________________________________________
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.
