Re: "Background Impedance of Universe"

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Paul B. Brodie" <pbbrodie@xxxxxxxxxxxxx>

Jim,
Someone made a comment about this being dull. I find your explanation 
interesting, intriguing, and very well explained. I really like your usage 
of analogy to make something complex easy to, as you put so well, "get your 
mind around!" Do you teach? Maybe you missed your calling. :-)
Paul
Think Positive

----- Original Message -----
From: "Tesla list" <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
To: <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
Sent: Friday, May 20, 2005 9:24 PM
Subject: Re: "Background Impedance of Universe"

> Original poster: Jim Lux <<mailto:jimlux@xxxxxxxxxxxxx>jimlux@xxxxxxxxxxxxx>
>
> At 11:16 AM 5/20/2005, you wrote:
>>Original poster: <mailto:Davetracer@xxxxxxx>Davetracer@xxxxxxx
>>
>>     There's talk about impedance and I have one question I've always
>> wanted an answer to, and I bet someone here knows it ...
>>
>>     I was at a convention and ended up in a lunchtime discussion about
>> the B-2 bomber. One person said, "The impedance across the wingspan is
>> 133 ohms". Another nodded and said, "That's because that is the
>> background impedance of the universe."
>>
>>     To this day I don't understand this. Help?
>
>
> The wave impedance of free space is 120*pi (or,377 ohms).  This is a bit
> tricky to wrap your mind around at first.  Here's a short conceptual
> discussion:
>
> impedance, in a circuit, is the ratio of voltage to current: Z=E/I  (just
> that old Ohm's law stuff, right?)
>
> But it's really also the ratio of the Electric Field to the Magnetic Field
> (taken in the correct units (V/m for E field and A/m for the magnetic 
field).
>
> So, if you have a propagating wave in free space, the ratio of the E field
> to the H field is always 377 Volts/Amp or 377 Ohms.
>
> Here's an interesting thing.  If you were to take a piece of "space cloth"
> with a surface resistance of 377 ohms/square, and cut out some copper
> electrodes with the same dimensions as a transmission line (for instance,
> the same ID and OD as 50 ohm coaxial cable (that is, a dot and a ring of
> copper), and plonk them down on the space cloth, then measure the
> resistance, you'll get 50 ohms.
>
> Now..say you've got a thing that you want to be invisible to radar.  The
> way to make something invisible is to make sure that it doesn't reflect
> anything back.
>
> The only thing that will reflect an incident EM wave is a change in the
> impedance (a mismatch in transmission line terms).  So, if the wave is
> propagating in free space (impedance 377 ohms) and you hang a piece of
> space cloth up, nothing reflects back. (It might get absorbed, because
> space cloth is lossy, being a resistor).
>
> Make your airplane have a characteristic impedance of 377 ohms and it too
> will be invisible, at least as far as reflections go, but it will still
> block your view of anything behind it.
>
> You've probably seen pictures of an RF anechoic chamber with all the cones
> and pyramids of absorber.  What this is is a gradual transition from free
> space (at the tips of the cones) to the resistive backing.  They load the
> foam with lossy material in a carefully designed distribution so that
> there's a gradual change.  The length of the cone tells you what
> frequencies you're working at.  I just bought some absorber to do testing
> at 1200-1300 MHz, and the cones are about 18" tall.  The absorber I use for
> X band (8GHz) or Ka-band (32 GHz) is about 4" thick. Mash the tips on the
> absorber, and it stops absorbing so well at higher frequencies, because the
> points are gone, so you have a sudden transition from free space to 
absorber.
>
> So, what's an antenna.. It's a way to transform the impedance of the feed
> line (typically 50 or 75 or 300,etc. ohms) to the impedance of free space
> (377 ohms). If your transmission line happens to be a piece of waveguide,
> for instance, then the antenna can be a flaring horn.
>
> I should comment that B-2's and other stealth vehicles do not make
> themselves invisible by absorbing the incident energy.  What they do is
> reflect (and/or diffract) it in any direction other than the incident
> direction.  For large surfaces this is moderately easy to arrange by
> calculation, but you're stuck with edges of things like fuel doors, inlets,
> etc, and for that, absorber is the way to go.
>
>
>>     [And I have been told in no uncertain times that the figure of 133
>> ohms is wrong.]
>>
>>     This relates to TC's in that, well, what is the impedance match from
>> a toroid out into the air? Does it help a coil to operate to attempt a
>> match? How on earth does one do that?
>
> There isn't any impedance match from toroid to air.  In a TC, you don't
> want to propagate an EM wave, you want a spark.  You actually want a 
mismatch.
>
>
>
>
>>     If you feel like responding directly to my mailbox, that's fine ...
>> [davetracer a t aol dot com]
>>
>>     -- thank you,
>>
>>     Dave
>>
>
>
>
>