Cold strapping gear?
H-J Fischer mentions in his book on radar technology (Radartechnik), that CV64 magnetron strapping adjustments could be (fine) tuned in cold condition. I understand that a kind of test rig must have been employed. This was recently discussed with Rod Burman, who is an expert on vintage valves and magnetron. He provided me the photos which you find on this page. We do not know exactly how it was used, but I guess that one side of the open magnetron was attached to the cavity of the shown apparatus (thus before sealing-off and the activating procedures). Rod also mentioned, though not visible, that semi-conductor diodes were integrated in this apparatus.
I presume, that the cavity facing towards us, is attached to one side of an open magnetron by means of an adapter or interface (may be the four screw holes were fixing the magnetron adapter onto the test gear). See description of the HF source below. We have to bear in mind from Sayer's patent US2546870, that strapping had to prevent excitations at nearby frequencies (mode jumping).
Side view of the test equipment. The circular section pointing towards us, might hold a rectifier diode. Right of it, in a more dark colour, is the magnet to the "Heil tube generator" (see next photo). Richard Trim kindly add to our discussions: Rod, The permanent magnet was used to focus the transverse electron beam in the Heil Tube.
In the centre of the cavity we see the glass envelope of the HF source, which injects centrally the signal at the expected un-wanted frequency. However, I also can imagine, that it had to inject the specified (spot) frequency, and that by means of strapping adjustment correct strap_tuning could occur. Visible is the HF output of a Heil generator. Dr. Heil worked for some time in Britain in the second half of the 1930s (he returned to Germany just before the war started). He and his wife invented the klystron principle about 1935. However, this was not known in the US, and the Varian brothers invented the klystron principle for the second time (be it, that they came up with the word "klystron"). Heil's work was well appreciated in Britain, and it was for them a logical step to employ in their famous "number 10 Set" a Heil tube as local oscillator source. Richard Trim also pointed out: STC Heil Tubes were used as receiver local oscillators in Decca S-Band (10 cm) commercial marine radars during the 1950' to 1960's before the availability of solid state local oscillators
What the purpose of the grid on the left-hand side of the cavity was, is not known to me. We know, that such an arrangement is sensitive to (wave) polarisation. Is mode jumping probably changing the polarisation of the electromagnetic induced field? Or, had it simply to reduce the picked-up electromagnetic energy? I just don't know!
Rod Burman did sent me recently his additional comments:
Dear Arthur et al
Having at last dismantled with considerable difficulty the magnetron test fixture/
I can tell you that the Heil tube is a CV234. The wavelength range of this tube is 7.5 to 15 cms so it certainly is in the right range for CV64 type magnetrons.
My copy of the CV234 data is at issue 3 dated 1948 and gives the commercial protoype as DV56 which could certainly have been an STC wartime development.
The CV234 was damaged during the the dismantling process but I am replacing it with a CV2221 which has a similar spec but a reduced wavelength range of 7.5 to 11.5cms which is still OK for the CV64
As a Heil tube generator can be frequency tuned, strapping alignment might also done by tuning trough a certain wave spectrum and monitoring the un-wanted spectra.
I don't know exactly how this set worked, those who could help us solving this query, please contact me at:
Recently (30 December 2009) Henk Peek did sent me his following explication, as to how the procedure of cold strapping was accomplished:
Cold strapping magnetron gear?
Naar aanleiding van de website omtrent het afregelen van de strapping van een magnetron: Koppel het 2 zijdig open magnetron aan eenzijde aan het afregel device en stop in op een na in alle resonatorgaten van het magnetron voldoende demping en meet de resonantie frequentie van de open resonator. Schuif een ressonator op en meet zo van alle resonators de resonantie frequentie. Doe dit vanaf beide zijden. Ga daarna al metend corecties aanbregen door de straps te bij te buigen tot alle ressonantie frequenties gelijk zijn. Je kan zo ook het magnetron op frequentie zetten. Uit ervaring is de offset tussen meet en werk frequentie bekend.
My brief translation:
Mount the open magnetron at one side onto your arrangement.
Second, fill all but one resonator with a metal plunger(plug), as to keep them out of resonance. Henk Peek correcting me wrote: This plug must damp so that the cavity does not have a high Q and are resistive. Use a resistive plungers! Otherwise you could not measure a single resonator.
Next, measure its resonance frequency, rotate the plunger and the next cavity comes free.
After all was done on one side, mount the other side on the test gear and repeat the same procedure.
Start now bending the individual straps such that a uniform frequency is achieved.
I believe, that with some experience this must be possible.
Henk Peek additionally did sent me the same day the following information:
Die gleufjes in de diode kop worden zo gericht dan ze bijna haaks op het op de aanwezige veld polarisatie staan. Dit geeft een veel grotere gevoeligheid van de diode stroom voor het detecteren van de resonantie! Helemaal haaks op het aanwezige veld geeft een een dubbel gevouwen curve en dat is heel lastig te interpreteren.
Photo 1 and 3 both show on the left hand side of the cavity, a horizontally directed grid. This should be rotated such that it is rectangular towards the existing EM field. When I remember Hertz's polarisation grid experiments well; a horizontal grid is allowing vertically polarised signals to pass through, vice versa. Aligning the grid this way is improving the sensitivity of the detector signal. Additionally he noticed, that, on the other way, an exact rectangular field orientation is, however, creating a double folded curve, which is difficult to interpret.
Henk is engaged at: NIKHEF a fundamental research institute of 4 Dutch Universities
Thank you very much Henk for your contribution!
Recently added a Russian/German contribution on the theory of strapping (20 May 2010)
Please notice the final part of the intreguing chapter on aspects of understanding on German vs British strapping (LMS10 vs CV64)
Consider also: Sayer's famous strapping patent US2546870
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