Re: [time-nuts] EFOS maser turns 34!
>> -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
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Re: [time-nuts] EFOS Maser turns 34!
> -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
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[time-nuts] EFOS Maser turns 34!
"Copper has a quite high temperature expansion, so could you servo that via the cavity temperature ?" To answer, both Copper and Aluminum cavities use the temperature setting to coarse tune the cavity. Fine tuning is via Varactor diode and modern Masers do servo "Auto-tune" the cavity to reduce drift. 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.
Re: [time-nuts] EFOS Maser turns 34!
In message <8ee9c792-a2f6-402d-9d07-f8929f656...@gmail.com>, John Ponsonby writ es: > It turns out that the resonant frequency of the cavity is > much more critically dependent on its diameter than on its length. Copper has a quite high temperature expansion, so could you servo that via the cavity temperature ? -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ 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] EFOS Maser turns 34!
Re: EFOS Maser turns 34! In a hydrogen maser the hydrogen atoms have to be confined in a storage-bulb within which the oscillating RF magnetic field is all in-phase. The storage-bulb has to be made from fused quartz for mechanical stability and for very low dielectric loss. The bulb has to be very thin walled, say 1mm to keep the dielectric loading and the dielectric loss acceptably small. For an active maser the product of the loaded Q-factor of the cavity and the "filling factor" of the storage bulb has to exceed a certain minimum value. The filling factor is a function of the shape of the storage bulb. Now fused quart is very difficult stuff and it can only be worked white hot "in the flame" and very few people can do it. It is much more difficult to work than ordinary glass blowing. You can't expect a storage-bulb to be made very exactly to an engineering drawing. As a result the cavity has to be machined to suit the given storage-bulb. The guys at Oscilloquartz told me that after making a lot of EFOS m asers they eventually got their quartz bulb maker to make the bulbs interchangeable between cavities but you can't expect such accuracy if you want to make a one-off maser. The first time I had bulbs made the wall thickness was far too great. The quartz people said thay had no way of measuring the wall thickness. I said I can weigh the bulb and thus deduce the average thickness, indeed it was obvious just holding it the hand that it was too heavy. 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 to avoid having to recentre the cavity each time one needs to take off a few thou (mils to you in the US). The loaded Q should be about 35,000 and the resonance is so narrow that one has to off-set tune the cavity so that it is on-tune when it is in vacuum and when it is at the chosen working temperature (40°C is a common choice). For an aluminium cavity the resonant frequency shifts about one bandwidth per °C of temperature change. These are matters that need to be understood if one contemplates making one's own H-maser. 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.
