"Jim - NN7K" wrote in message
m...
Buck--
See: http://www.maxstream.net/helpdesk/article-27
Gain of a dipole is 2.15 dB over an Isotropic - but
qualify that as in a direction perpendicular to the
direction a dipole points! a deep null in signal
occures in the directions that a dipole POINTS.
If vertical polarized , there is a deep null of the top,
and bottom of the dipole- maximum radiation around the
circumfrence , perpendicular to the dipole. (kinda like
a donut) . The only time you will see bigger numbers
is do to sales hype, or- because someone has included
ground reflections into the gain situation ! At least
a couple of antennas on E-BAY, and other places have used
this when "advertiseing" their antennas- If an antenna
pattern looks more like a punk rocker's hair-do, then the
goodyear blimp, or for a dipole/longwire, like a cigar,
the pattern shown has considerable ground reflections,
with undesireable angles of radiation. (at least that
the best way I can describe the situation, someone)
do better??). Jim NN7K

Buck wrote:
What is the gain of a dipole over an imaginary isotropic antenna? I
believe I read 7.x somewhere. Whatever it was, it was greater than
anything I expected. Thanks
Buck


Antenna performance figures are affected by many variables, so performance
claims can be very misleading. To try to keep everybody on the same "level
field", the proper way to define performance is to relate gain to an
isotropic reference in free-space conditions. This is nice physics, but
certainly not real life. Extremely few people can place their 20-meter
dipole in free-space. Most humans string their dipole from tree to tree, or
buy as much metal tower as possible. Those trees, other buildings, tower
metal, guy wires, coax cables, other antennas and the proximity of the
ground all modify the antenna performance in difficult to predict ways. Some
might argue that free-space conditions can never exist in real life, so
antenna testing in their back yard is better than free-space projections.
This would be true, if everybody's back yard was electrically identical to
yours. Thus, until everybody has a standardized back yard, free-space,
isotropically referenced data is your best guide.


Ed
WB6WSN