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>

Interesting idea... especially since I have been looking at just such
radars at work for another application.

One problem will be resolution, but that might not be a problem if you
consider looking at the "bulk" characteristics of a volume of air
containing some streamers. At 10 GHz, the wavelength is about 3 cm, and at
24 GHz (the other frequency for which these devices are readily available
cheap), a bit more than a cm.  While one can get resolution smaller than a
wavelength it's a pain.

The real problem will be that the inexpensive doppler radars (speed guns
and the like) are hideously sensitive to interference.  They work by mixing
a bit of the outbound signal (around 0.5 mW, typically) with whatever gets
reflected back, and then looking at the audio frequency on the mixer diode.
 This is called homodyne operation, by the way.   The problem is that the
selectivity of the receiver is pretty bad; as in non-existent.  Any RF
power that gets into the horn will be detected by the diode and its
envelope will show up.  For the speed gun and automatic door opener
applications this isn't a problem... The horn and waveguide form a very
effective low pass filter (although, with enough power, even with 100's of
dB of loss in the waveguide (f below cutoff), you can still get a few mW to
the detector..), and the likely interfering sources are low power, constant
frequency, and likely pointed somewhere else. ( I note that "jamming" a
homodyne doppler radar is trivial... !
 point a
pulse modulated source that is reasonably "in band" at it... The pulses
will get detected and the "speed counter" will count the pulse rate,
convert it to speed, and so on.)  Pointing a simple radar at a TC will
probably result in a strong output at the break rate, just because of the
modulation of the backscatter due to the sparks.   The motion detector
doppler radars have two outputs (I and Q) which might be useful... I
suspect that the doppler aspect will be a washout, but, the backscatter
measurement might work.  

What you really want in the spark application is probably more of a network
analyzer configuration, where you can look at the phase and magnitude of
the reflected or transmitted power through the volume.  The ionization
changes the EM properties, and that should be detectable.  The nuclear
engineering types use RF diagnostics all the time. 

There are also mm wave devices coming on the market (for auto collision
prevention, and similar applications) running at 66 GHz and higher, which
offer the nice combination of short wavelength (high potential resolution)
and consumer product (cheap!).

Finally, there might be some resonance effects that you might be able to
exploit to look for particular molecular species... 

This is all pretty exotic though...

Tesla list wrote:
> 
> Original poster: "Paul Nicholson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> 
> I wonder whether you could use a security or traffic radar to
> determine how quickly ionisation forms around the topload, and to
> see how much of it is maintained between RF half-cycles and
> between bangs.
> 
> Operating circa 10GHz, with the usual doppler output, it might be
> possible to witness streamer formation down to 1nS resolution.  The
> demodulated radar output could be displayed on a scope trace
> alongside the topvolts waveform, in order to determine where things
> are happening in relation to the RF cycle.
> 
> Radar has been used since the late 1940s to investigate the ionised
> trails left by meteors, perhaps it could be applied to the study of
> TC breakout?
> --
> Paul Nicholson
> --