Talk:Fresnel zone

Untitled

needs more reliable sources....--24.144.100.184 02:10, 2 November 2006 (UTC)

what's the unit of r ? what about the fomula in metric ?

I guessed it is "kilobarleycorns". Probably wrong, I suppose. What are the widths of the subsequent Fresnel zones, those "annular rings"? What are the widths at other than the halfway point?

What do "maximum obstruction" and "recommended obstruction" mean? Gene Nygaard 04:29, 4 May 2005 (UTC)

This article needs to be generalized to include the optics usage. (Yes, I know, sofixit and all that. I don't have time right now, so I'll just mention it here for now.)--Srleffler 04:07, 21 March 2006 (UTC)

Any idea why the formula for the radius on this page gives a different multiplier (43.3 instead of 72.6)? EdDavies 12:36, 3 April 2006 (UTC)

The link to "this page" above appears to be no longer reachable, but I've seen 43.3 elsewhere and had the same question. See this page instead. The 43.3 multiplier (for the radius in feet and distance in miles) may be the "obstacle-free radius", which is often taken as 60% of the Fresnel zone radius. Can someone verify this? 70.89.158.189 21:50, 25 October 2006 (UTC) Gerald Reynolds

General formula is fine, but numerical values only for optics in free-space

I haven't even checked the accuracy of those numerical prefactors, but people should be aware that the concept of a Fresnel zone applies as well to *any* wave phenomenon (my own familiarity with it is in acoustics). I'll change the article if and when I have time, but (1)it should be mentioned that those numerical values which people have been quibbling over are specific to optics in free-space---other values will show up for acoustics problems---still others for other wave-types in different media; or, (2) there should be *no* numerical values given, and simply stress that the formula yields different results based on wave- and media-type. --Smoo222 03:20, 21 March 2007 (UTC)smoo222

zone/region ?

Should this page make any distinction between "Fresnel zone" and "Fresnel region" (commonly used in antenna theory)?

In antenna theory, the Fresnel region generally refers to a radial range of distances between the reactive near field (~2*lambda?) of an aperture, and the Fraunhofer region (2*D^2/lambda, where D is the largest dimension of the aperture). Within the Fresnel region, the radiation pattern of the aperture varies significantly with radial distance since the multitude of sources that constitute a given aperture cannot yet accurately be approximated as having a common phase center.

69.137.170.154 23:42, 21 March 2007 (UTC)Bill Shultz

This is written like a high-school science report. Please recast it in, at the very least, the passive voice.

Error in Attached Image

Currently there's a spurious '15m' above the '20m' on the bottom set of images. It seems to have been left in there by mistake. Xrobau 12:50, 10 September 2007 (UTC)

Article is written to refer only [or predominantly] to radio waves

The article, while apparently sufficient for radio transmitter/receiver problems, mentions almost nothing of other wave types, which of course the entire concept applies to. A specific example is that of mentioning "radio frequency line-of-sight": There is absolutely NO restriction to radio frequencies.

There is also a minor correction which should be made (I have no time to get it totally right), in that it is mentioned that "If unobstructed, radio waves will travel in a straight line from the transmitter to the receiver." This is true at some level of abstraction (that of infinitely high frequency -- the domain of "ray optics"), but *false in the domain where Fresnel zone concepts apply*. In other words, the Fresnel zone is exactly one of several concepts designed to deal with the fact that waves do not travel in straight lines, even when there are no obstructions!

Another suggestion (again, I'll fix it if/when I have time) is to mention the source/receiver reciprocity which is implied by the figures included in the article. For an arbitrary receiver location (imagine the _field_ from the source), the Fresnel zone does not pinch down as is shown in the figures. The "pinching" is a result of the _receiver's_ own Fresnel zone -- that is, its response to an oriented incoming plane wave because the receiver also has a finite width aperture, and an orientation. The cigar-shaped zone in the figures is the *product* of the individual Fresnel zones of the source and receiver, taken individually. Smoo222 (talk) 13:30, 31 March 2009 (UTC)Smoo222

Pronunciation discontinuity

Why is Fresnel pronounced "/frɛnɛl/ fre-NELL" in this article, but "(pronounced /freɪˈnɛl/ fray-NELL)" in the Fresnel lens article? Both refer to the same person. I'd fix it, except that I've always pronounced it /frɛz'nɛl/ (frez-nell). My attempt is probably incorrect, but it makes me unaware of which is correct.

118.208.45.140 (talk) 06:05, 21 October 2010 (UTC)

It seems as if the pronounciation has been "fixed" in the Fresnel lens fashion while the correct (French!) pronounciation should probably be "/frɛnɛl/ fre-NELL"!150.227.15.253 (talk) 15:48, 4 February 2015 (UTC)

Question about phase information

I don't understand the assertion that an obstacle in the first Fresnel zone will result in an interfering signal 0-90 degrees out of phase, 90-270 degrees if in the second zone, etc.

In the first place, surely the phase change associated with a path that goes from one antenna to point P on the surface of the first Fresnel zone to the second antenna is 180 degrees (one half-wavelength path difference).

In the second place, there will likely be a phase change associated with the object causing the interference. For a specular reflection (e.g., a plane surface many wavelengths in extent, such as a flat roof), the phase change will depend on the angle of incidence, polarization and nature of the surface (e.g., dielectric, highly conducting, etc.) and can vary easily from 0-180 degrees.

108.60.121.98 (talk) 22:25, 2 October 2012 (UTC)Douglas Liddell October 2, 2012

You are correct, this can be very confusing given that the information is erroneous. I saw a webpage that could be the source for this wrong information. I'll fix it to say 0-180 180-360 and so on and add that this is the path-length phase difference. Maxbezada (talk) 20:47, 30 January 2013 (UTC)

Isn't this statement self contradictory: "If a reflective object is tangent to the 1st zone, the electromagnetic wave will be shifted 180o because of the increased path length, undergo an additional 180o phase shift due to the reflection, and reinforce the direct wave at the receiver. Consequently, there should be no reflective objects in the 1st Fresnel zone."

I read it to say that reflections tangent to the 1st zone reinforce the direct wave, so don't have any reflective objects there. Seems like they would enhance the signal at the receiver so you should have many. Jimbo (talk) 01:57, 15 December 2016 (UTC)

Confusions

Some of this might already be found in the article or in other comments but I think the article needs substantial clarifications to avoid confusions and misunderstandings.

"Fresnel zones result from diffraction by the circular aperture". No, diffraction by the circular aperture will cause sidelobes in the radiation pattern of circular antennas, but the Fresnel zones are independent of the antennas used (even though both could be important for the end result).

As I understand it the Fresnel zones describes areas of different path differences between the direct beam and an indirect beam that is scattered (e.g. reflected) from some position outside the line of sight. The article may actually say this but not in a very clear way. Since interference depends on phase rather than path differences the path difference is recalculated into a phase difference or difference in wavelengths, lambda. It needs to be clarified whether the zones refers to volumes (the introductory part describes the cross section of the first as circular and the subsequent as annular, which would require the zones to be volumes and a zone in 3D space would normally be interpreted as volume) or if they refer to the limiting surfaces, the distinction does not seem to be well made.

If it is the limiting surface Fn is the zone radius, but if it is a volume it is the outer radius of this volume.

If it is a surface the path difference will be an integer number of wavelengths for the even zones but 0.5*n*lambda for the odd zones. Below I will sometimes refer to the limiting surface as the surface to distinguish it from some zone volume.

If it is a volume the phase difference due to path difference will vary from (n-1)pi to n*pi in the n:th zone, which means that it could either be in-phase or out of phase or have some intermediate phase relation with the direct signal.

In addition to the path difference the scattering process can give rise to phase differences. Often reflexions will lead to a 180 degree phase shift making rays reflected from the n:th surface out of phase with the direct beam. Thus reflexions from the first surface could enhance rather than cancel the signal. Above the Brewster angle reflexions (for beams of vertical polarisation) will not change phase. Scattering may also lead to intermediate phase shifts (typically for lossy media near the Brewster angle) and the end result may also depend on phase shifts of the antenna outside the boresight direction (but since the Fresnel zones are often reasonably narrow compared to the main lobe of the antenna this may rarely be important).

The formula for the Fresnel zone radius appears to be an approximation. Although it is probably a very good approximation in most cases it is good to indicate this with a "curly equal sign" and to clearly state that the radius corresponds to a path difference of 0.5*n*lambda.

One reference that may be worth adding is: 150.227.15.253 (talk) 15:48, 4 February 2015 (UTC)

One of the Worst Technical Articles on Fresnel Zones

This article is wrong on many levels, including its reference to circular apertures and diffraction (concepts also associated with Fresnel, but having no relationship to Fresnel zones). Specifically, the stated formula is trivially derived based on the difference between the direct path length and alternate path lengths being a multiple of a half-wavelength between two points (i.e., the transmitting and receiving antennas):

$F_{n}={\sqrt {\frac {n\lambda d_{1}d_{2}}{d_{1}+d_{2}}}}$

where,

F_n = The nth Fresnel Zone radius in metres

d₁ = The distance of P from one end in metres

d₂ = The distance of P from the other end in metres

$\lambda$ = The wavelength of the transmitted signal in metres

The various Fresnel zone ellipsoids do not "define volumes in the radiation pattern of a (usually) circular aperture": they define the respective terminating surfaces for alternate path lengths that differ by the various multiples of a half-wavelength.

The Confusions section above alludes to this problem: I am declaring BULLSHIT. — Preceding unsigned comment added by 73.3.80.189 (talk) 05:02, 27 May 2015 (UTC)

I would put it this way: the article tells you how to calculate an ellipsoid. What this has to do with wave transmission, reflection from obstacles, interference patterns, or anything like that, you'll have to learn elsewhere, because the author of this article is keeping pretty quiet on those points. But you can visualize the ellipsoid. 178.39.188.74 (talk) 15:50, 10 June 2015 (UTC)

resummarized

That was about the worst summary I've ever seen on an article. I know what fresnel zones are, and even I was confused after reading it. Bad phrasing, extraneous material, parenthetical extras, weasel words, ick. I just rewrote it from scratch and then tried to see if there was anything I left out that was in the original text that could be salvaged. I leave it up to someone else to make it more accurate, but I think now it is at least understandable. --ssd (talk) 04:50, 24 August 2015 (UTC)

Oh, and the illustrations are probably wrong too. --ssd (talk) 04:54, 24 August 2015 (UTC)

Assessment comment

The comment(s) below were originally left at Talk:Fresnel zone/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

It is not necessary to explain this in such technical terms.

Last edited at 23:53, 28 January 2007 (UTC). Substituted at 15:36, 29 April 2016 (UTC)

Shape around the antenna

The antennas are in the focal points of the Fresnel zone, not at its boundary, as stated in "The cross sectional radius of each Fresnel zone is ..., shrinking to a point at the antenna on each end." Petr Matas 14:37, 4 June 2016 (UTC)

Agreed. This should be corrected to state that radius of the first Fresnel Zone ellipsoid reduces to approximately one-fourth of a wavelength at locations near either of the antennae (focal points). Or for the n'th Fresnel zone: $r\approx {\frac {n\lambda }{4}}$. The radius goes to zero at the points ${\frac {n\lambda }{8}}$ beyond each antenna. PhysBrain (talk) 22:55, 10 October 2018 (UTC)

Phase Analogy

The Fresnel zone is a difficult concept to visualize and understand because it involves quite a few different things such as phase, distance, deflection, cycles, multipath, polarity, etc., and it's even more difficult to explain. It may be easier to understand by using an analogy with something that one can sense (e.g. sound waves) rather than something one can only imagine (e.g. radio waves). I'll try an analogy for phase here.

Let's say Alfred is hammering a large nail into a hard piece of wood. He is hammering a blow at exactly one strike per second. Each strike of the hammer of the nail makes a very loud sound.

Let's say Bill is standing next to Alfred. We will call this Location X. We can imagine Alfred's hammer is the transmitter and Bill's ears are the receiver. Each blow of the hammer coincides with the sound Bill hears. The sound is in-sync.

Bill calls Alfred on his cell phone and Alfred answers it with his left hand as he hammers with his right hand. Bill holds the phone to his left ear. It's a perfect cellular network and the hammering Bill hears on the cell phone in his left ear is exactly in-sync with the actual sound Bill hears from his right ear. He hears the hammer in both ears at the same time.

Now let's place Bill several hundred meters away from Alfred. We will call this Location Y.

Alfred is still hammering, but now the sound doesn't coincide to the hammering. When Alfred strikes the nail, Bill hears it with his left ear via the cell phone, but Bill hears nothing with his right ear. But when Alfred lifts the hammer over his head, Bill finally hears the sound with his right ear. This is obviously because the soundwave moves so much slower than a light wave or a radio wave. As Alfred continues to hammer, the pattern remains the same. Instead of hearing the hammer hit the nail once per second, he hears it hit the nail twice per second. In fact, it sounds like the nail is being hit twice as fast. We can say the sound is out of sync.

Now let's move Bill several hundred meters away further still. We will call this Location Z.

Alfred is still hammering, but now everything seems fine just as it was in Location X. Bill can see Alfred hammering, and he can hear Alfred hammering in both ears at the same time. Each sound seems to coincide with the strike of the hammer. The sound is in sync.

The sounds are not the same sounds, they are not the same wave, but they are arriving at Bill's ears at the same time, and seem to be the same sounds.

So this in-sync and out-of-sync might be one way to visualize the concept of in-phase and out-of-phase. The sounds can be associated with the top of a cycle. When the same transmission arrives at a receiver from two different directions due to deflection, it's important that they arrive in-phase. — Preceding unsigned comment added by 96.231.98.253 (talk) 13:35, 8 January 2017 (UTC) Mark The Droner (talk) 22:55, 10 January 2017 (UTC)Mark The Droner

Fairly Major Edit

This page has had major issues for over ten years now. I've read it several times, and although parts of it are very well written, other parts, even within the well-written parts, are poorly or downright badly explained using incorrect words and numbers. So after studying it for some time, I took it upon myself to rewrite parts of the first two sections. I also added a Polarity section, as it is impossible to conclude what happens to a signal bouncing in the Fresnel zone without discussing its polarity. Finally, I really don't like either of the first two graphics. Although they do give the reader an idea of the different regions of the Fresnel zone, there are errors in each which confuse the reader. For example, the first graphic shows n=1, but "n" is nowhere to be found in the graphic. And nothing in the explanation below the graphic makes any sense to me. In the second graphic, the placement of the numbers 1, 2, and 3 are nothing short of horrible and will do nothing but completely confuse the reader. Also, it shows the antennas touching the second Fresnel zone which is impossible. I would redraw both of these graphics but I have no such tools to do so. Perhaps somebody with such illustrative graphic tools and a sound understanding of the Fresnel zone regions could step in and replace these graphics. — Preceding unsigned comment added by Mark The Droner (talk • contribs) 12:03, 11 January 2017 (UTC)

I have done a major copyedit cleanup sweep to fix a number of problems, though more work is still needed, so I tagged some of the remaining problems which I did not have time to fix. I replaced the confusing and non-standard term "polarity" with the more technically-correct term "polarization" (attempting to Wikilink the former term revealed just how confusing the non-standard terminology was). I added a number of Wikilinks to further information on some basic technical terms when they are first introduced. I cleaned up the page layout somewhat, bringing it more into compliance with Wikipedia standards for readability and accessibility.

The WP:Tone of some sections needs to be copyedited for a more encyclopedic Wikipedia style. The number of footnotes needs to be increased beyond the paltry 3 ones at present.

As for improving the graphics, I agree that they need improvement, but I don't have the specialized skills to do this work. I suggest that the Wikipedia:Graphics Lab people be contacted, with specific requests to modify the existing diagrams or to create new ones. Somebody with technical expertise in Fresnel zones will have to work closely with a graphics specialist, unless somebody with both areas of knowledge can be found. Reify-tech (talk) 19:35, 21 January 2017 (UTC)

Ah, yes. (Looks like you forgot to sign). Good work. It has better organization and clarity. One thing that stands out for me though is in the opening sentence: "A Fresnel zone (/freɪˈnɛl/ fray-nel), named for physicist Augustin-Jean Fresnel, is one of a series of concentric [b]prolate[/b] ellipsoidal regions of space between and around a transmitting antenna and a receiving antenna system." It clarifies and confuses me at the same time. It defines what a zone is in that it can be any region. In other words, there are a near-infinite number of Fresnel zones surrounding an antenna system - it's not just one zone with multiple regions. That makes sense and I actually wasn't clear on that point. But I don't understand your apparent addition of the word "prolate" here. The word seems to describe every Freznel zone as that of a shape of a football standing on end. While I agree the entire series of zones together would resemble this, and many of the latter zones would resemble this, but it's misleading and confusing to describe it as (any) one of a series. For example, the first zone would certainly not resemble a football standing on end. It would resemble a football laying on its side. The truth is, a given zone could be prolate or oblate, but the zones together as a whole would be prolate. So I think the opening sentence is better with the word "prolate" omitted.

Re phase shift - it's difficult to describe without a graphic. I hope the use of degrees isn't confusing, as I'm using the word in terms of rotations of a circle (or cycle), not angles. Hence, 360 degrees is a full cycle, 180 degrees is a half cycle, 90 degrees is a quarter cycle, etc. And in this case, the words cycle and wavelength are synonymous. Thanks again. Mark The Droner (talk) 16:56, 21 January 2017 (UTC)

Yeah, I forgot to to sign; this is fixed now. To clarify, the words "prolate spheroid" describe a shape, not a spatial orientation. You may be conflating this term with the words "prone" or "pronate", the first of which means "lying face down". See the article Spheroid for details. The 3D shape of a Fresnel zone derives directly from the geometrical properties of a prolate ellipsoid in relation to reflections which maintain a constant path length (which directly affects phase). Regarding the graphics, the transmitter and receiver should technically be located at the two foci of the ellipse, and not at the very ends of the ellipse or outside of the closed curve. For a long, extended ellipse (a common occurrence in real-world situations), this may be a fine distinction that isn't easy to see, but reflections can cause constructive interference even from behind the receiver, for example. Reify-tech (talk) 19:35, 21 January 2017 (UTC)

Indeed. That fact was missing from the version of a couple weeks ago which is why I added the words "and around" to the opening sentence. After all, certain antennas such as a windsurfer and a parabolic wouldn't work if the Fresnel zone didn't surround the antenna system. Mark The Droner (talk) 13:55, 22 January 2017 (UTC)

Polarization

The following regarding polarization reflects a less than complete understanding of electromagnetic theory, specifically, plane waves and polarization:

   Linear polarization — the sine wave moves on a plane
       Vertical polarization — the sine wave moves on a vertical plane
       Horizontal polarization — the sine wave moves on a horizontal plane
   Circular polarization — the sine wave moves in a tight three-dimensional helix as it leaves the transmitting antenna
       RHCP (right-hand circular polarization) — the sine wave moves clockwise as it leaves the transmitter
       LHCP (left-hand circular polarization) — the sine wave moves counter-clockwise

First, a plane wave's equiphase surface lies on a plane, a spherical wave's equiphase surface lies on a sphere, and a cylindrical wave's equiphase surface lies on a cylinder, regardless of whether the wave's electric-field vector changes its direction along a line (i.e., linear polarization) or rotates (i.e., elliptical or circular polarization). In practice, all electromagnetic waves are essentially spherical waves when far enough away from the source, and over any sufficiently small observation region at that distance, are treated as locally planar. The characterization of a circularly-polarized wave as moving "in a tight three-dimensional helix" is just wrong on many levels. — Preceding unsigned comment added by 107.191.0.144 (talk) 19:24, 20 January 2018 (UTC)

If you can provide a better description or explanation of polarization as it applies to a Fresnel Zone, please do so. — Preceding unsigned comment added by Mark The Droner (talk • contribs) 00:08, 2 March 2018 (UTC)

Removing blatantly incorrect statements is somewhat more important than replacing it with new information, IMO. This is an encyclopedia, and encyclopedias should be factually accurate. What was stated was not a description or explanation of polarization and was simply confusing and wrong. Footlessmouse (talk) 22:44, 21 August 2020 (UTC)

There is nothing incorrect about the polarization description above. It's very possible polarization does not apply to certain waves such as light waves or sound waves. I don't know. But I do know understanding polarization is very important regarding radio wave transmission and receiving. And therefore it is immeasurably important when considering the Fresnel Zone since constructive and destructive interference within a certain zone would depend wholly on polarization. Another point - if polarization didn't matter or didn't exist, the Soviets would not have developed circular polarization. Specific polarizations have their advantages and disadvantages depending on the environment. Mark The Droner (talk) 13:00, 20 April 2021 (UTC)Mark The Droner

Current page still flawed

As a WISP operator of twenty years and counting, ten years supporting AT&T Long Lines microwave paths in the early '70s, and almost fifty years total of radio propagation experience ... I can objectively say the current page has many errors.

There are so many errors and misunderstandings, it's impossible to edit anything on the page. I may start a new paragraph called CORRECTIONS and let the erroneous information as is.

To start ... The equation assumes an isotropic starting and ending points which don't exist in the real world; it mentions obstructions in the path which is incorrect as the equation deals with reflections, not refractions or obstructions.

WISP operators treat the equation as the holy grail of propagation requirements, when in reality I have seen it in practice only once in my life. An operator asked my opinion of his path equations before spending $20,000 in a mid-span repeater link - I ran my version of the equation and showed his initial assessment was flawed ... I saved him $20,000.

40%

In the Clearance calculation section, the article states 40% is maximum allowable obstruction. What?! I guess we're talking about the ellipsoid? Are we talking about its volume or perhaps the "area" of the (curved?) lines between emitter and receiver? I really don't have any idea and the article just tosses it in as if it's obvious.98.21.213.85 (talk) 17:00, 8 February 2024 (UTC)

Yes, it's difficult to understand. The concept of the Fresnel Zone is very complex and difficult to explain. You have to understand what polarity means and what "phase shift" means and why it happens and how it is related to the various Fresnel Zones. You also have to have a clear understanding of what happens to polarity of a secondary signal when it is deflected within the first or second Fresnel zone. The location of the object deflecting (above, below, right, left) matters greatly due to the signal's polarity. If you understand polarity and phase shift, then everything will make sense. The percentages are actually referring to phase shift of a late arriving or deflected secondary signal (other than the primary signal). In other words, a little bit of phase shift is okay, but a lot is not okay. If you understand that, then you'll understand why 20% is okay and 40% is worst case and might be tolerated but would be best to avoid if possible. Anything more than 50% will cause major interference because the two signals (primary and secondary) will begin to cancel each other out. The whole Fresnel Zone concept is difficult to wrap your head around. It will generally take 2-3 reads of the page and some careful reflection to get it. Once you get it, everything will make sense. Mark The Droner (talk) 23:34, 16 February 2024 (UTC)

Flashlight Analogy

I'm adding a flashlight analogy to this talk section to help confused readers who wander in here understand Fresnel Zones. Do not confuse this analogy with the behavior of light waves, as this particular example is specific to radio waves transmitted in vertical polarity (know that most radio waves are transmitted in vertical polarity).

Stand in a rectangular room at ground level. We'll say this entire room is in the 1st Fresnel Zone. Your associate is standing in the far end of the room and is holding a flashlight. You stand in the other end of the room. The flashlight is the transmitter, your eyes are the receiver.

There is a large mirror laying on the floor.

Your associate moves the flashlight up and down and up and down in rhythm. This flashlight mimics the energy of any vertical dipole antenna which would transmit the energy up and down in the form of a sine wave. Your eyes see the light from the flashlight directly. This is the primary signal. The flashlight appears to be moving up and down and up and down in rhythm.

Now look in the mirror on the floor. The same flashlight is moving as seen in the mirror. But it does not appear to be moving up and down and up and down. Instead, the image is inverted. The flashlight appears to be moving down and up and down and up.

This is what happens to a vertical polarity wave when it deflects off the ground. The wave becomes inverted.

Both these "signals" are in the first Fresnel Zone which means, by definition, there is very little change in phase shift. That's because the distance between the lamp and the eyes is very close to the distance between the lamp and the mirror plus the mirror and the eyes. We could say, for example, the primary distance is 100 and the secondary distance is 100.1.

However, because of the deflection of the mirror, the secondary signal, which is inverted (or upside down) is completely out of phase with the primary signal, and effectively cancels out the sine wave of the primary signal. Hence, we have destructive interference due to the deflective surface of the mirror.

In the real world, an antenna installer facing such a problem might consider raising the length of the transmitting antenna, or receiving antenna, or both antennas, which would effectively move the deflective object out of the first Fresnel Zone and into the second Fresnel Zone.

Back to our experiment.

Now, remove the mirror. Run the same experiment. Ignoring the floor, walls, and ceiling, you have a wonderful line-of-sight signal with no interference. Perfect!

Next, place the mirror on the left wall. Run the same experiment. Now you can see the light moving in the mirror. Let's say this distance, again, is 100.1. This time, the flashlight appears to be moving up and down and up and down exactly in tandem with the "primary signal." Hence, the signal is even better than before, it's a stronger signal, and there is no phase shift problem. It's perfect. It's even better than perfect.

All the above has happened in the first Fresnel Zone and therefore is pretty straight forward.

Now let's look at the second Fresnel Zone. Here's where it gets tricky.

The primary signal is exactly the same as before. It's the same distance. Let's say it's 100.

But now the left wall of the house is gone and the mirror is hung on the left side stockade fence outside. The distance that the secondary signal travels is greater than before, it's now 100.5. And this is enough distance to cause a phase shift relative to the primary signal. The phase shift of the deflected secondary signal is one half wave length out of sync with the primary signal, and therefore, cancels out the primary wave and is destructive interference.

Now move the mirror onto the exterior wall of the neighbor's house. This represents the third Fresnel Zone. The distance of the secondary signal is now 101. This is enough distance to cause a phase shift of exactly one wave length - which means there is no effective phase shift. The arriving waves of the primary and secondary signals are exactly aligned. This is constructive interference.

One could create dozens of such scenarios, but the concept is the same.