FS: WW II Magnetron GL-5J29 marked Classified, NOS

I read in sci.electronics.design that Jim Adney wrote (in ) about 'FS: WW II Magnetron GL-5J29 marked Classified, NOS', on Fri, 23 Sep 2005:

That's a neat trick! What type of cutter do you use to make a cavity in a vacuum?(;-)

According to Fourier, there is an inevitable extremely wide spectrum associated with a continuous pure sine wave amplitude modulated by a sharp low-repetition-rate pulse.

The receiver can't work properly unless it has a very wide bandwith to accommodate a large proportion of it. Otherwise radar echos from different distances become blurred and merge with each other on the screen.

Its just hard luck if the transmitted spectrum interferes with not-so-adjacent frequencies.

Micowave oven magnetrons are not modulated but are are contiuous wave devices. The frequency may drift very slightly as the temperature and magnetron dimensions change. But your rice pudding couldn't care two hoots.

I read in sci.electronics.design that Reg Edwards wrote (in ) about 'FS: WW II Magnetron GL-5J29 marked Classified, NOS', on Sat, 24 Sep 2005:

Please use only SI units. The SI unit of concern is the angst. One hoot is equal to 0.5 femtoangst.

One could be flip and say that a vacuum is the absence of matter, or a cavity in the matter...

Maybe I'm missing something, but it seems to me that the only thing we're interested in here is the time between the outgoing pulse and the echo. This time delay will be the same for all frequencies, since there is virtually no dispersion (variation in velocity as a function of frequency) for microwaves in air or vacuum.

Since all frequencies would arrive back together, it really wouldn't matter whether the transmitted or returning signal was clean or "dirty." However, to avoid clutter from other signals, it would be best if the receiver was narrow-band.

OTOH, making the transmitted signal also narrow band would also make it harder for someone at the other end to detect and would keep the transmitted signal from interfering with other communications.

Also, as others have mentioned, while a CW signal can be narrow band, once you modulate/pulse it, you've introuduced sidebands that I don't think you can get rid of. These are not likely to be much of a problem, however, because they'll be at frequencies very close to the carrier since they are at the carrier freq +/- the pulse freq.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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I understand this, and I agree.

Am I missing something here? I don't see why a narrow band echo wouldn't give a nice time delay signiture. Is this a Heisenberg uncertainty principle thing, with f & t the variables?

(delta f) times (delta t) < h/2pi

Is this what we're up against here? I haven't tried to work out the numbers, but HU doesn't usually come into play until we get to really tiny quantities.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Jim Adney wrote in news: snipped-for-privacy@4ax.com:

It is not modulated, just a pulse of rf.

The reciever has a narrow bandwidth.

umm, no....

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Ahem, "it is not modulated, just a pulse of rf" is incorrect. Even *one* pulse of RF -->IS

might be an interesting read. Patent first applied for in 1940.

outside of the receiver pass band which gives a weaker return.

The ideal radar pulse, as far as resolution goes, would be a near perfect square wave which would give excellent range resolution. The sharper the pulse the more distinct the leading edge is on the return. However, the problem with a very fast rise time on the pulse is that it creates many side bands which makes for a wide signal. With a very wide signal it requires a very wide receiver pass band to reproduce it. A wide receiver pass band lowers the receiver usable sensitivity.

So a compromise is done to specifically shape the rise and fall time of the transmitter pulse that gives sufficient resolution yet reduces or limits the amount of side band energy. Then a narrower bandwidth receiver can be used and can accommodate all of the useful energy of the received pulse. The narrower bandwidth allows for a more sensitive receiver which equates to more useful range.

If the receiver pass band is too narrow it will not capture all the necessary side band energy and will "round" off the pulse and range resolution will be lost.

Regards Gary K4FMX-------------------------------------------

Yes, there is a fundamental range/velocity ambiguity, and the numbers are interesting. I'm not smart enough to explain it, at least not without a lot of work to refresh my memory.

Consider the extreme cases: 1) sharp pulse. You get back a pulse. You can clearly measure the time delay. But what is the dopler shift? 2) a continous tone. You get back a shifted tone so you can easily measure the dopler shift. But what is the time delay?

I remember something like the integral of probability of the target being at some range/velocity is 1 (after normalization). You can push it around by changing the pulse shape/pattern, but you can't make it go away. If you push the ambigiuity to someplace where you are sure the target isn't then you don't have to worry about it.

Where do the radar geeks hang out?

If one was *really* concerned aboout a "target" that was relatively fixed, one could use a pulse relatively sharp to determine range fairly accurately, and then use CW doppler shift to refine the distance within

There's a way to have a broad pulse and still get high distance resolution. You use a pulse that varies in frequency (sweeps in a controlled way during the pulse, I think it was from low to high). On the receiver the IF stages have a delay that varies with the frequency, so that the broad pulse gets 'compressed' timewise. Of course, any doppler info is lost.

Mat Nieuwenhoven

Called "chirp".

Bats invented it long time ago.

Was an interesting read, what's the story behind the story?

..and they were wise to *not* patent the idea, and put it into public domain; the other animals thought that was crazy, and so called them "bats", and the name stuck.

The real interest (for me anyway) is the sense I get when reading such documents on that site. These were development engineers (like us), at the forefront of radar, feeling their way through the problems for the first time.

The fundamental concepts, sums and circuit elements that we now take for granted were being actively explored from scratch at that time.