[MLB-WIRELESS] FW: [Syd-Wireless] Fresnel Zones

[Robert Tchia]

> -----Original Message-----
> From: syd-wireless-admin at lists.sydneywireless.com
[mailto:syd-wireless-
> admin at lists.sydneywireless.com] On Behalf Of Craig Mead
> Sent: Monday, 12 August 2002 8:42 AM
> To: syd-wireless at lists.sydneywireless.com
> Subject: [Syd-Wireless] Fresnel Zones
> 
> This was forwarded from Michael @ DLink, thought it may help some of
you
> understand Fresnel zones a bit more.
> 
> Enjoy.
> 
> 
> | Hi Craig,
> |
> | As discussed, this is the email I wrote for one of my customers
> yesterday.
> I
> | hope it helps some of the people in the wireless community that are
> interested
> | in understanding Fresnel Zones.
> |
> | In a point-to point, LAN-to-LAN bridging situation, their are two
> cone-shaped
> | signal RF signal patterns which extend from the antenna located at
each
> site.
> | Together these cone shaped signal patterns are known as the Fresnel
Zone
> | (pronounced Freanal Zone).
> |
> | To visualise the shape of the Fresnal Zone between the antenna's,
think
> of
> a big
> | fat cigar sitting between the antennas. If you were to slice the
"cigar"
> at it's
> | thickest point (directly in the middle), this point is known as the
> First
> | Fresnel Zone and this point needs to have 60% of it's radius clear
of
> | obstructions or otherwise your signal will be affected. Have a look
at
> the
> | attached diagram (Figure 1) where the Fresnel Zone is illustrated by
the
> green
> | cigar-shaped signal pattern. The first Fresnel Zone is illustrated
by
> the
> pink
> | circle.
> |
> |
> | An equation for calculating the radius of the Fresnel Zone can be
> expressed as
> | follows (If you do a search on the web for "Fresnel Zone" you will
be
> sure
> to
> | find some calculators for this):
> |
> |                         ____________
> |                        /
> |                       /  D1 x D2
> |                   \  /   _______
> | R = 17.32 x  \/    F x Dt
> |
> | R   =  Radius
> | D1 =  Distance in km from first antenna to point just above
potential
> | obstruction
> | D2 =  Distance in km from point of obstruction to second antenna
> | F = Frequency in GHz (i.e. 2.4)
> | Dt - Total distance in km (D1+D2)
> |
> |
> | Example 1
> |
> | If you wanted to bridge two LAN's 10 kilometres apart, and the land
was
> | perfectly flat, the antenna's would have to be located around 11
metres
> above
> | the ground. This equation shows midpoint of the antenna's which is
the
> | "fattest"point of the Fresnel Zone, so D1 & D2 need to be of equal
> value.
> |                         ____________
> |                        /
> |                       /  5x 5
> |                   \  /   _______
> | R = 17.32 x  \/    2.4x 10
> |
> |
> | Radius of first Fresnel Zone = 17.6771 metres
> |
> | 60% of 17.6771 metres is 10.6062 metres, so therefore the antennas
need
> to
> be
> | around 11 metres above the ground.
> |
> |
> |
> |
> |
> |
> | Example 2
> |
> | Again, if you wanted to bridge two LAN's 10 kilometres apart,
however
> there was
> | a tree 2km from the first antenna. See diagram attached (Figure 1):
> |                         ____________
> |                        /
> |                       /  2x 8
> |                   \  /   _______
> | R = 17.32 x  \/    2.4x 10
> |
> |
> | Radius of the Fresnel Zone at 2km =  14.1417 metres
> |
> | 60% of 14.1417 metres is 8.4850 metres. As the tree is 20 metres
high,
> the
> | antenna's need to be 28.4850 (30 metres) above the ground.
> |
> | You may have noticed that in Example 2 that the value for the
Fresnel
> Zone
> is
> | smaller than in Example 1. As you may have already worked out, this
> because
> | Example 2 is measuring the Fresnel pattern before it has radiated to
> it's
> | maximum point, midway between the two antenna's, as in Example 1.
> |
> |
> |
> |
> | (See attached file: Figure 1.GIF)
> 
> (image available @ http://www.wtf.net.au/images/fresnal.gif  )
> 
> |
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