Re: those folks at MIT (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 12 Jun 2007 13:20:37 -0800
From: Greg Leyh <lod@xxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: Re: those folks at MIT (fwd)

Hi Jim,

Your points are well taken.  The original idea of reviewing published 
research to prevent duplicate effort seems to have been lost.  It has 
failed though, largely due to the overwhelming amount of material to 
search.  Nowadays, each generation tends to 'rediscover' most fields of 
science and engineering, and electromagnetics is no exception.

I expect the reality to set in when they begin to consider EM 
compatibility standards, the actual power needed by digital devices, 
acceptable pickup loop sizes, and the horde of single-ideology groups 
out there who consider EM fields to be bad voodoo.   GL


>---------- 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.
>  
>