Re: Useless questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>



> And this is all sounding very complicated. It would be interesting
> if there was anything other than a null output from the homodyne,
> and it'd be exciting if whatever output there was, was correlated
> with either the bps or the RF.
> 
> > Would there be some statistics of the doppler that might be of
> > use?
> 
> How about its highest significant frequency component?  If the
> topload is surrounded by a 'sphere' of ionisation, then does it
> form all of a sudden, when a certain voltage is reached, or does it
> swell out and in gradually in time with the RF cycle?  My guess,
> from reading Bert's postings, is that it forms pretty quick once
> a certain V/m is reached at the surface of the topload. Presumably
> the return amplitude will be roughly proportional to the size of
> the reflecting part of the ionisation,  so amplitude modulation of
> the radar return would be nice to see. 

Casually looking through Bazelyan and Raizer, it looks like there is a very
fast ionization wave that propagates out from the electrode.  There are
then a bunch of small streamers, each of which doesn't carry much power,
and which form and disappear very quickly (nanosecond time scales).  The
total of all these streamers does carry enough current, though, to
resistively heat the air, which then can extend as a single leader step. 

The dynamics is all very complex, and, given all the handwaving in the
text, probably somewhat speculative, given the difficulty of making actual
measurements.



 But we might expect the
> highest beat frequency from the homodyne output to be related to
> the speed of the advancing ionisation, since that's the quickest
> moving thing around. Lets see, if the ionisation advances towards
> the radar at 3cm per nS, then we'd get a beat component at 1GHz?
> Anything slower than this might be visible on a 1GHz spectrum
> analyser hung on the homodyne output (directly off the mixer -
> bypassing any filtering).

Interesting.. Practically speaking, though, the mixer diodes in these
things (being optimized for doppler in the audio range) probably don't have
that kind of bandwidth (in terms of parasitic C and R, forming a low pass
filter.. obviously the junction itself is fast enough, or it wouldn't work
as a mixer).  The other problem would be that the phase noise on Gunn
oscillators is pretty high... I wonder what the scattering cross section of
the "ionization wave" is.. It's not all that highly ionized, so it's not
all that great a conductor, but, on the other hand, you don't need a huge
signal coming back from short range... 


> 
> Guess someone just needs to put a security radar and a specy-an
> in a real good faraday cage and point it at a live TC.  Just to see
> if there's any sort of meaningful output.  It sounds within reach
> when you put it like that.

Or perhaps something like a Van deGraaff generator, where the DC potential
on the electrode follows a more regular pattern... You wouldn't have to
worry about the interactions with the RF from the TC.  Set it up so that it
breaks down between two electrodes periodically.. (Maybe, put the radar
feed inside one electrode (making the electrode out of large mesh, or rods,
so it's microwave transparent)...





> 
> You never know, maybe you'd find that the breakout scatters the
> radar real well, or just the right amount of ionisation around the
> topload just absorbes it - neat stealth, eh?

I suspect that the RF scattering of expanding waves of ionized gases has
been studied very well in other applications.  As a diagnostic in nuclear
physics, for instance.