Date: Tue, 09 Dec 2014 21:12:44 -0800
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Dipole gain?
On Tue,12/9/2014 1:46 PM, Ken wrote:
> In actuality, ?gain? in a non rotatable antenna is a myth.
False. Gain for antennas is usually expressed as dBi, meaning dB
referenced to an ideal isotropic radiator. Gain is not a single number
(unless the antenna is isotropic) -- it varies with both vertical and
horizontal angle.
An ideal (lossless) dipole in free space has a PEAK gain (that is,
broadside to it) of 2.18 dBi. Mount that dipole horizontally over real
ground and the earth provides a reflection in the far field that creates
a vertical pattern. Assuming no earth losses, the Peak gain will now be
3-6 dB greater at the vertical angle where the direct wave from the
antenna combines precisely in phase with the wave reflected by the earth.
The shape of the vertical pattern will be entirely dependent upon the
height of the horizontal dipole as a fraction of a wavelength, but will
be almost independent of soil conditions.
73, Jim K9YC
### IOW, the 1st 7.18 to 8.18dbi of gain on any yagi does NOT count.
Your 13.18 dbi gain yagi is only 5 db more than a dipole, with both the yagi
and the
dipole being mounted the same height above ground. Most yagi makers will
assume
5-6 db of ground gain. Just rate the yagi, or any other ant in DBD. We don’t
have isotropics
in real life, nor do we mount yagis or dipoles in free space. Model the yagi
in free space, design
it in free space, then do the final tweak, with the yagi above average ground.
Notice how the FS
ratios on any free space yagi are sky high vs the same yagi above ground.
FS will be a function of height above the ground.
Jim VE7RF
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