[TowerTalk] Fwd: Dipole gain?

[Jim Lux]

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 at earthlink.net> To:
> Hans Hammarquist <hanslg at 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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