View Single Post
  #37   Report Post  
Old May 17th 06, 09:57 PM posted to rec.radio.amateur.antenna
 
Posts: n/a
Default FIGHT? Here is another W8JI myth bone!


Richard Harrison wrote:
Yuri is consistent with Terman, and that is liable to be better than a
bible because it is provable and demands no faith. If wrong, it will be
rewritten with corrections.



If you read what has been said here very carefully you will find Yuri
claims the shield "blocks electric fields" or stops "electrostatic
fields". This is the effect of a Faraday cage or shield.

I did not claim that effect. Terman certainly did not. It is a folklore
or Ham-myth that only appears in amateur circles.

What others (including Terman exactly as you quoted) are trying to
tell Yuri is the shield ONLY affects balance. The shield IS the actual
antenna element that does the radiating. That is written in a half
dozen engineering good engineering references. That is how ANY shield
behaves.

Read he

http://en.wikipedia.org/wiki/Faraday_cage

or he

http://members.aol.com/omlcgm/deteck...gy/faraday.htm

or any of dozens of other places. Unless an off-the-wall hobbyist
publication or rouge opinion, you will see everyone agrees.

What Terman said is absolutely correct. The "shield" (when properly
constructed) balances the capacitance of the antenna to earth. It does
not "stop" the electric field. It certainly does not filter a
time-varying electric field because doing so would by definition of
Maxwell's equations (which everyone who isn't a CFA or EH antenna quack
agrees are true) also stop the time varying magnetic field.

As everyone (including Terman) has tried to explain, the shield only
affects balance. The shield HAS to be the actual antenna element
because by definition of ALL the peer-reviewed textbooks published to
date as well as any description of coaxial cables the inner shield wall
is isolated from the outside by the skin depth of that wall.

This is so very simple to prove, it only takes a moment. It doesn't
even take exotic test gear. These experiments were done in the 18th
century with very crude instruments.

You can take a solid copper sheet for example and place a small loop
antenna near that "wall". If you probe current on the wall near the
loop on the loop side, you will find a current maximum right under that
loop. VERY easy to see. 100% repeatable.

Now if you move the probe to the other side of the wall you will find
current MINIMUM at the sheet center and increasing towards two of the
edges.

This is a TOTALLY open wall with no seal, it isn't even a box.

Shields a few skin depths thick are a virtually perfect barrier to both
magnetic and electric fields. This is true for densely woven coaxial
cable shield or even thin aluminum shields, metallic sheets, or any
good conductive wall.

Saying Terman supports anything to the contrary only proves someone is
misquoting or misunderstanding plain English, since Terman is a very
clear writer. There is no way Terman failed basic physics and his peer
reviewed textbooks are wrong.

Yuri may need to read some basic textbooks, I'd be happy to copy the
applicable pages if there isn't a good library nearby. It is essential
to get the basics down solid.

73 Tom