[TowerTalk] Steve's antenna study

[Jim Reid]

Hi all,   in response to the following:

>Sorry Jim, but just not so. Krause in volumn 1 specifically proved that
>the maximum gain which could be obtained from an antenna ( he used a
>simple yagi for his discussion ) is at that point where the seperation
>between elements is infinitely small. In such a case, the impedence of
>the antenna was infinitely low. 

I write:

Yes,  the antenna's impedance is a "fall out" from the geometry
which produces the azimuth pattern.  we look at the pattern
and make judgments about the amplitudes vs azimuth.  That
pattern is completely independent from the impedance of the
transmission system and the resulting vswr seen in the
transmission line.  One is an apple,  the other is an orange.
To say the antenna's gain depends upon the efficiency of
the total system is an error.  The antenna will produce the
azimuth pattern regardless of the efficiency of conversion
of what comes out of the transmitter down in the shack, and
what is left by the time the energy (power) at last is able
to accelerate electrons within  the antenna structure.

If you look in any antenna book,  you will read that a dipole,
even an elemental,  minute dipole,  has the same azimuth
radiation pattern as a full size,  center fed,  half-wave dipole.
But the impedance will be nowhere near 50 ohms resistive
within the elemental dipole!  So little radiation would result
when attempting to feed with a 50 ohm transmission line,  but
the pattern,  and hence the gain given to what ever energy
was radiated  will be the same as the full size dipole.  This,
because,  by definition,  gain is the power ratio increase in 
a particular azimuth direction over what it would have been
without the given antenna structure.

Thus gain,  capture area,  and radiation pattern descriptors 
are synonyms,  but efficiency of conversion of the transmitter's
RF output power to radiated EM waves is a different animal,
and does depend upon the entire system including vswr,  
losses,  etc.

73,  Jim,  KH7M


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