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VK3ABK > ANTENA 01.01.05 04:23l 81 Lines 4249 Bytes #999 (0) @ WW
BID : 34529_VK3KAY
Read: DL7NDF DG8DG OK1VKZ GUEST
Subj: Antennas and why?
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Sent: 050101/0206Z @:VK3KAY.#WEV.VIC.AUS.OC #:34529 [Wendouree] $:34529_VK3KAY
From: VK3ABK@VK3KAY.#WEV.VIC.AUS.OC
To : ANTENA@WW
Hello to all Thinkers. ( Happy New Year! )
Recent bulletins got me thinking about antennas and what they are supposed
to do. How do they connect our radios to space and to someone at a distance.
Is this the 'action at a distance' as supposed by Einstein and his mates. (?)
Below, is part of a bulletin (I won't credit the sender, to save embarrasment)
that is a good place to start for a look at 'Current Affair'!
The diagram on the left is original; I have added the bit on the right.
As you can see, it represents the same antenna but eliminates the 'strange'
18,000 Ohm impedance at the mid point.
³ ³
³ The 100 æH inductor equates ³
Ü Ü ³ to more than 18kê at 29MHz ³
³ ³ ³ and so becomes a very high ³
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÁ((((ÁÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ : impedance in a low impedance :
³ 100 æH ³ ³ loop. ³
³ ³ ³ ³
³ ³ ³ The 'loop' now shown as two ³
³ ³ ³ 'vertical' radiators, fed ³
³ ³ ³ out of phase, which radiate ³
³ 2.4m square loop ³ ³ a differential of the energy. ³
³ ³ ³ ³
³ ³ ³ This has also caused a shift ³
³ ³ ³ in directivity. ³
³ ³ ³ ³
³ ³ ³ ³
³ ÚÄÄ¿ ³ ³ ÚÄÄ¿ ³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´B1ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´B1ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
ÚÄÄÄÁÄÄÁÄÄÄ¿ ÚÄÄÄÁÄÄÁÄÄÄ¿
³ A T U ³ ³ A T U ³
ÀÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÙ
A 'resonant' loop (ie. a full wave length in circumference) may show a
radiation resistance of 200 Ohm or more depending on construction and height
of the antenna. More critical, would be the proximity of solid objects!
This radiation resistance is important as we are involving another 'impedance'
and hoping to couple to it. This is the impedance of free space which can be
computed knowing the dielectric and magnetic properties, and this comes out
to be about 277 Ohm. This is fairly close to the radiation resistance of a
'resonant' loop antenna, and so gives the loop (quad) an advantage.
But let's now consider a loop much smaller than one at resonance. This will
have a much lower radiation resistance, so much so that it becomes almost a
short circuit across the excitation source. Also, with the much smaller
radiation area, it makes it a most inefficient 'antenna'. In addition, there
would be 'technical difficulties' in construction and coupling such a low
impedance loop to the excitation source. Attempts have been made to overcome
these problems, and the 'Army Loop' made from copper tube of generous
diameter, with 'direct' coupling to the excitation source, is one of them.
This type of antenna must be considered an extreme compromise, and even the
'odd length of wire' would have greater benefit. The whole idea of an antenna
is to 'launch' our energy into space, or to receive it from the same source,
in the most 'effective' way.
Just what does this have to do with Einstein ? !!
Well, even Einstein at one stage was inclined to regard an 'ether' as the
means for conveying the electromagnetic radiations of Hertz through 'empty'
space: that is, a vacuum! But his thinking, involving the problem of gravity,
soon put 'ether' in it's proper place! Oh, yes. 'Action at a distance' as
I mentioned at the beginning, has been a 'bone-of-contention' and is still
being discussed by those opposed to a 'finite' speed of light.
This is enough 'thinking' for one bulletin, but any comments would be welcome.
73. Dick. VK3ABK.
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