On 12/9/14, 2:43 PM, Hans Hammarquist via TowerTalk wrote:
Hi Jim,
The next question then is; Do two horizontal, crossed dipoles, feed
90° out of phase have an even power distribution across the
hemisphere, horizontal across the horizon and circular above and
below or is there a direction with a higher power density?
CP along the axis and is the direction of maximum radiation
Linear pol (Horizontal) at the horizon, 3dB down from axial
Elliptical everywhere else
The horizontal pattern is a sort of rounded corner square with about 1
dB variation as I recall.
Look up "turnstile antenna" for tons and tons of analysis and data.
George Brown published/invented it very early as a "omni" broadcast
antenna, especially when multiples are stacked to create a very "flat"
Hpol pattern (for TV and FM broadcast0.
There's a variant where the dipoles are fans or bicones to increase the
bandwidth, and another variant where it radiates elliptical or CP toward
the horizon (to reduce the effect of multipath when received by a H-pol
antenna, giving up some link margin in exchange)
-----Original Message----- From: Jim Lux <jimlux@earthlink.net> To:
Hans Hammarquist <hanslg@aol.com> Sent: Mon, Dec 8, 2014 9:58 pm
Subject: Re: [TowerTalk] Dipole gain?
On 12/8/14, 6:45 PM, Hans Hammarquist via TowerTalk wrote: That way
you should
subtract 3 dB to get a comparison between an antenna gain give
over the isotropic. I was also told that two crossed dipoles were
as close to an isotropic radiator you could get.
Not really. two crossed dipoles fed in phase is the same as a dipole
at 45 degrees. two crossed dipoles fed 90 degrees out of phase is
circularly polarized in the direction normal to the plane containing
the dipoles. Other arrangements of dipoles (e.g. a Lindenblad or a
turnstile) may have better circularity in some directions.
An antenna that is isotropic and has the same polarization in all
directions cannot exist (there's the interestingly named "hairy ball
theorem" about this).
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