Tech Stuff - Wireless - Fresnel Zones and their Effect

Or 'Why Line of Sight is not enough'

This page provides a quick overview of the phenomena known as Fresnel Effect or

Fresnel Zones. Since Monsieur Fresnel was a French physicist/mathematician drop the

's' when pronouncing his name to show you are real savvy (or should that be 'chic').

Bad News: If you can see the receiving antenna the wireless link still might not work -

this is entirely the fault of M. Fresnel (aided and abetted by a few natural laws). If you

prefer some action ("to !#? with the theory just gimme the numbers") go direct to our

Fresnel Calculator.

With apologies to M. Fresnel

Here goes for a quick explanation of the problem (with suitable apologies for the

superficiality). Direct radio waves will obviously travel in a straight line from the

transmitter (TX) to the receiver (RX). But there are other waves inside the signal 'cone'

(or circle if the antenna is a omni). If these other waves don't bump into anything

(there are no obstacles) they will just travel off into the night until overcome with

exhaustion. But if they bump into something (a deflection) they may end up at the RX

antenna and if they are out of phase with the straight line signals they may have a

phase cancelling effect - they will reduce the power of the arriving signal. The

magnitude of the phase cancelling effect is a function of the signal strength (the

longer the distance the waves have travelled the lower the signal strength) and how out

of phase (always described in terms of an angle) is the deflected signal. See diagram 1

below.

Diagram 1 - Phase cancelling effects from deflections (bumps)

Phase Cancellation

Fresnel provided a means to calculate how out of phase the bumps (deflections)

between the transmission source (TX) and the receptor (RX) will be. Each Fresnel zone

is an ellipsoidal shape ('sausage like' to the rest of us) as shown below. Deflections

(bumps) from obstacles which occur anywhere in zone 1 will, if they arrive at the

receptor (RX), create signals that will be 0 to 90o out of phase, in zone 2 they will be 90

to 270o out of phase, in zone 3 they will be 270 to 450o out of phase and so on. Even

numbered zones are bad'ish (they have the maximum phase cancelling effect) and odd

numbered zones are good'ish (they may actually add to the signal power). The signal

strength (and hence the magnitude of the phase cancelling effect) is strongest in zone

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1 and decreases in each successive zone simply because wave paths get longer (and

signals get weaker) the further they are from the direct line from transmitter (TX) to

receptor (RX). Because the Fresnel zone is ellipsoidal, at any arbitrary point which is d1

distance from the transmitter (TX) and d2 distance from the receptor (RX) each Fresnel

zone has a unique radius value of r1. Fresnel's equation allow us to calculate this

unique radius (r1) given the value of either d1 or d2. See diagram 2.

Diagram 2 - Fresnel zones

OK, so what's the big deal

The big deal is this. To maximise RX strength you need to minimise the effect of the out

of phase signals. To do that you must make sure the strongest signals don't bump into

anything - they have the maximum chance of getting to the RX location. The strongest

signals are the ones closest to the direct line between TX and RX and always lie in the

1st Fresnel Zone. The rule of thumb is that 60% of the 1st Fresnel zone must be

clear of obstacles If you have a building, say, that is a distance d1 from the TX

antenna and d2 from the RX antenna then it must not be closer than 60% of r1

measured from the centre line of sight (shown as c1 in diagram 3). You may need to

juggle the antenna positioning to ensure this 60% zone is clear (you will also see this

60% figure written as 0.6 just to confuse). See diagram 3.

Diagram 3 - Fresnel zone clearance

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The earth is in the way

Finally on longer links especially (> 3Km or 2 miles) you will frequently see discussions

about Fresnel zones and the curvature of the earth (though it can have an impact at

very much closer distances). The reason: even if you are shooting a link across flat

ground with no apparent obstacles, eventually the ground itself becomes the obstacle

and can easily get inside the 60% clearance zone. Solution: lift the antenna higher to

make sure the earth stays out of your way! In diagram 4 d1 and d2 are the same (the

mid point) so the distance of the direct line of sight to the earth (c1) must be more than

60% of r1. Our Fresnel Calculator gives the earth's height (or bulge) at the mid point of

the link.

Diagram 4 - Fresnel zone and earth curvature

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(below) or info-support at zytrax. You will have a warm inner glow for the rest of the

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