those folks at MIT (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 12 Jun 2007 11:02:41 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: those folks at MIT

Now having read the entire paper, I have some generalized comments:

1) For such a huge crowd of folks working on it, they didn't actually 
do much.  Was this, perhaps, something like a senior project with a 
team of students?

There are a remarkable number of simple quantitative aspects that 
have been left out.  Lighting the bulb to *nominal* 
brightness?  Incandescent bulbs are notoriously non-linear. Couldn't 
they have used a ammeter and voltmeter?


2) They sort of went to a lot of trouble to rederive some pretty 
standard electromagnetics.  After all, they came up with a different 
way to estimate the inductance and self C of their coil, when they 
could have looked in any standard handbook (or used Wikipedia) to get 
something like Wheeler and Medhurst.  They cited some guy's thesis 
from 1951 as an example of no closed form equation for inductance of 
a finite solenoid. And gosh, the coupling of two inductors is 
something that has been known for decades, if not a century.   How 
could they not do a first order analysis with something like Ampere's 
law and Biot-Savart?

Yes, they used a different conceptual model and different 
notation.  Seems almost like they didn't ever look at the RF 
literature, at all.  Their reference for the oscillator is a 50s book 
on vacuum tube oscillators?

Why did they not give the estimated inductance and capacitance (since 
they obviously needed them)?
Why did they not give the Is and Id (since they said they measured them)?

Their calculation of "efficiency" is a bit hard to follow.. They 
calculate power as Gamma*L*I^2, which is sort of working backwards..

How did they measure Q? by measuring the bandwidth with a coupling 
loop? Did they allow for the coupling of the coupling loop?


3) They sort of don't really understand skin effect, much less the 
effects of turn to turn interaction in a solenoid.  This is sort of 
basic NBS Circular 74, Grover, etc.  stuff.

4) They sort of handwave on the effects of lossy and/or dielectric 
materials in the vicinity.  Uhh. if it changes the resonant frequency 
of the transmitter coil, you could change the transmitter frequency 
with a feedback system (as they mention), but somehow, you'd also 
have to "remotely" adjust the receiver's resonant frequency to match, 
or the coupling goes away (as they note).  Likewise, the receiver 
would have to adjust itself to maintain a constant resonant frequency 
that matches the transmitter.  This is non trivial with high powers.

5) RF exposure safety..  they cited the ANSI standard, but I don't 
think they read it, or understood it, because they make a spurious 
comparison between cell phones and this system.  They are comparing 
radiated power (for the cell phone) against field intensity (for the 
coil system).  For that matter, at 10 MHz, the limit for general 
population uncontrolled exposure (which this would be) is 0.219 A/m 
and 82.4 V/m.  They calculate *20cm from the receiving coil* 1.4kV/m 
and 8A/m.  (let's put this in context.. we are talking about  the 
field 8" from a 2 foot diameter, 8" long coil)

Being over by factor of 37 for magnetic field and 17 in E field 
(contrary to their analysis which cites the E field as the problem) 
does not sound like something that is a minor matter for those 
engineers to fix up.

6) They assert that the coils don't have to be the same size, and 
that as long as the product of the sizes is constant it works.  This 
is one of those "oops, practical real world losses bite you" 
problems.  Sure, you can make the receiver coil much smaller, but in 
order to get the same power out, the losses will need to be 
reduced.  But hey, I'm sure they're going to consider room 
temperature superconductors <grin>... That will make the adaptive 
tuning thing a bit more important, because the Q will be much higher.