[TowerTalk] Dipole gain conclusions

[Hans Hammarquist via TowerTalk]

Gentlemen,


We can conclude that a "full size' dipole has a gain of 2.14 - 2.18 dB over an isotropic radiator in free space (We better "cross the "T"-s and dots the "I"-s here). A very small dipole (with no losses) also called Hertz dipole has a gain of 1.76 dB in fee space. Crossed dipoles fed 90° out of phase have maximum gain in the CP direction.


The radiation pattern is depending on the environment (Was there anybody that didn't know that?) such as closeness to ground, type of ground, surrounding trees, hills etc.


Finally, there is no simple way of make an antenna that has an even power distribution across the hemisphere.


Best 73 and thanks to everybody that contributed to this discussion,


Hans - N2JFS



-----Original Message-----
From: Jim Lux <jimlux at earthlink.net>
To: towertalk <towertalk at contesting.com>
Sent: Wed, Dec 10, 2014 9:17 am
Subject: Re: [TowerTalk] Fwd:  Dipole gain?


> 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)


Actually, an idealized half wavelength long dipole with a sinusoidal 
current distribution has 2.14 dB gain over isotropic.
An infinitely short dipole (Hertzian dipole) has 1.76dB gain over an 
isotrope (numerical gain is 1.5)
> 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