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tom · September 4th 09, 05:01 AM posted to rec.radio.amateur.antenna

Jeff Liebermann wrote:
On Thu, 3 Sep 2009 19:11:42 -0700 (PDT), wrote:

How in the heck are you going to
get **ANY** vertical radiator to have a truly isotropic pattern?
It's impossible. An isotropic pattern is a theoretical pattern
in which radiation is equal in all directions. Such a pattern
does not exist with real antennas.

A real isotropic radiator may not exist, but one can get fairly close.
If you believe the model, the total error is 0.44 db. See:
http://802.11junk.com/jeffl/antennas/isotropic/index.html
The NEC2 deck is under the photo labeled "main".

I once built one of these antennas on roughly 444MHz out of cardboard
and magnet wire. The oscillator was a small crystal can oscillator
running from a 9V battery to avoid having the feed coax wrecking the
pattern. The impedance was nowhere near 50 ohms and required a bit of
matching to get the VSWR down. I'm now digging for the photos.

I used a piece of string to maintain a constant radius, a tiny pickup
loop at the end of a length of coax cable running inline with the
string, and eventually going to my antique HP spectrum analyzer. On
the 2dB/div scale, it was a fairly good approximation of an isotropic
radiator with errors mostly caused by indoor reflections and
interference with the bench.

Nice. The 3d pic is impressive.

tom
K0TAR