[time-nuts] EFOS maser turns 34!

John Ponsonby jebponsonby at gmail.com
Sun Jan 8 12:13:43 EST 2017

>> -----Original Message-----
>> From: time-nuts [mailto:time-nuts-bounces at 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

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