[TowerTalk] fresnel zone

[Tom Rauch]

> Date:          Fri, 08 May 1998 13:14:56 -0700
> From:          Don Andersen <w7dd@fia.net>
> Subject:       [TowerTalk] fresnel zone
> To:            towertalk@contesting.com

Hi Don and all, 

I haven't read that area of the Handbook, but I hope it doesn't say 
exactly what was written. I try to avoid special jargon to 
describe simple effects, because I get confused.

>The fresnel zone is the earth in the direction of transmission 
>and reception.

The Fresnel region is named after Augustin Fresnel. It is a fancy-
pants name for the near field region. It is any area where the 
pattern of an electromagnetic radiator is still being formed, but 
outside the antenna boundary area. 

Fresnel's (fre nel--French ) original paper was published in 1814, 
and dealt with light of a single color (like a single frequency 
transmitter) and how it could be made to cancel or add with certain 
conditions of reflection.

The Fresnel (or near field) region includes above ground space as 
well as the not-so- deep earth around the antenna where the wave 
freely penetrates. It includes any direction,  so long as the 
radiation pattern is being formed in that area.

The near field (or Fresnel) area is just outside an area called the 
antenna region where things are a real complex mess. (That, and the 
close in  near-field region, is the area where a small "magnetic 
loop" could be called "mostly magnetic". By the time you get a few 
feet away from a loop a few feet in diameter, it quickly looks just 
like any other antenna. A large loop doesn't look magnetic at any 
distance at all, it looks just like a pair of bent dipoles with a 
strong electric field on the vertical sides. Because "magnetic"  
loops start to "look like" any other antenna outside the antenna 
region, and quickly --faster than a dipole-- assume the far field 
pattern, they really aren't anything special at all for noise or 
anything else more than several loop diameters away!)  

The far field is where everything looks like it comes from a single 
point in space. It's also called the Fraunhofer area if you like to 
sound educated. Pass the Grey Pupon!

These areas actually blend smoothly from one area to 
another, and contain more than one field type. For engineering 
purposes, and to confuse us all, they are considered to have 
definite boundaries or cut-off points. 

> The waves hit the earth and reflect at the inverse angle and combine giving a
> 6db maximum gain.  The 6db is not exact due to losses but is much nearer 6db
> that would be expected.  At low angles of radiation, the effiency is
> particularly good.  The frenel zone usually encompasses many acres of land.  If
> this land is in a Box Canyon, its time to call a realtor.

This is where we have to be careful taking things too literally. The 
6 dB perfect ground enhancement is ONLY for selected elevation angles 
and at optimum heights for those angles. It certainly is not a broad 
gain addition covering low angles as well as high. Since it is too 
difficult to do imperfect earth, lets look at a perfectly flat and 
clear sheet of copper..the ideal groundplane.

At 0.35 wl high gain in dB over the same antenna with no groundplane 

50 deg 4.5 dB (maximum gain angle)
25 deg 2.5 dB
15 deg -1 dB (net loss due to reflection)
10 deg -4 dB (net loss due to reflection)

Notice there was NO six dB enhancement, and there was even 4 dB 
loss added at 10 degrees. 

That was for perfect reflection, over real earth, gain will be less. 
This is why a big counterpoise below the antenna helps something like 
an 80 meter yagi to work better, and REALLY helps a low 40 meter 
dipole.

At 0 .5 wl
 
30 deg 6 dB
25 deg 6 dB
15 deg 3.5 dB
10 deg 0.5 dB

Real world gain will again be less, but not as bad as with a lower 
dipole.

> Puesdo-Brewster angle 
>losses are the reason vertical antennas are not able to
> take advantage of fresnal zone enhancement.  Reflections at very low angles are
> not of the correct phase to enhance the wave.

Nor are they in a dipole, unless it is very high. Comparing a 1/4 
wl high vertical to a 1/4 wl high dipole, or even a .35 wl high 
dipole, things aren't so dismal for the vertical at low angles. 

This is especially true on 160 meters, where the above-the-
horizon radiation pattern is pretty well formed before attenuation 
and phase shift completely destroys  re-enforcement. Consider a 160 
meter groundwave signal can reach 50 miles along the earth, and the 
pattern at even very low angles is formed within one mile!

Things happen a  lot faster on 20 meters, so don't use the ARRL 
Handbook's 20 meter example to describe 160 meter systems.  

73, Tom W8JI
w8ji.tom@MCIONE.com

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