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Thoughts on the use of water as ground screen
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Hash: SHA1 I have seen what appear to be AM broadcast towers in or on the edges of salt marshes, and it seems pretty obvious to me why that's a good place to go. However, the environmentalists being a little more noticeable than they were once upon a time, this particular method of siting is probably a little more challenging than it used to be. I recognize that salt water is far more conductive than fresh water, but fresh water's still superior to sand and the like. That being said, I am wondering about using a pond as a ground screen and mounting the antenna itself on an island (or a raft) in the middle of the pond. What I don't know is just how large a pond do I need in order for something like this to work? Obviously it depends on type of antenna and band and a bunch of other things, but even a wild-ass guess (with some math or physics behind it) will help make the difference between whether I bother trying or not. For those who absolutely require less variables in their equations, imagine a standard dipole tuned for 20m strung roughly 45 feet above ground level between two trees, one on either side of a fresh water pond. How wide does the pond have to be at that point (and others) for it to work right? Even answers like "the pond will have to be wider than the dipole is long" or "there will be no noticeable impact on performance" are fine if they're based in reality, and ideally in math and physics I can understand. Oh, and another question: what difference, if any, would frozen versus liquid water make in this situation? Jack. (exploring new antenna options.) - -- Jack Twilley jmt at twilley dot org http colon slash slash www dot twilley dot org slash tilde jmt slash -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAl/MBGPFSfAB/ezgRAo5pAKD35cRH0XFUz7p/uqBwbj3SNRP69QCfaz5C 6he5FgG+/q767KjX9g9T75A= =j1k0 -----END PGP SIGNATURE----- |
"Jack Twilley" wrote in message
... -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I have seen what appear to be AM broadcast towers in or on the edges of salt marshes, and it seems pretty obvious to me why that's a good place to go. However, the environmentalists being a little more noticeable than they were once upon a time, this particular method of siting is probably a little more challenging than it used to be. [snip] Environmentalists must be realists if they want to be effective: AM broadcast is one of the lightest footprints in any salt marsh, and AM stations have very little trouble getting EPA clearance for such places. FWIW. YMMV. Bill |
You'd still need a ground radial system, lest the varying water
table constantly detune the array. Also, fresh water is generally a good insulator compared to copper wire. Pete "Jack Twilley" wrote in message ... -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I have seen what appear to be AM broadcast towers in or on the edges of salt marshes, and it seems pretty obvious to me why that's a good place to go. However, the environmentalists being a little more noticeable than they were once upon a time, this particular method of siting is probably a little more challenging than it used to be. I recognize that salt water is far more conductive than fresh water, but fresh water's still superior to sand and the like. That being said, I am wondering about using a pond as a ground screen and mounting the antenna itself on an island (or a raft) in the middle of the pond. What I don't know is just how large a pond do I need in order for something like this to work? Obviously it depends on type of antenna and band and a bunch of other things, but even a wild-ass guess (with some math or physics behind it) will help make the difference between whether I bother trying or not. For those who absolutely require less variables in their equations, imagine a standard dipole tuned for 20m strung roughly 45 feet above ground level between two trees, one on either side of a fresh water pond. How wide does the pond have to be at that point (and others) for it to work right? Even answers like "the pond will have to be wider than the dipole is long" or "there will be no noticeable impact on performance" are fine if they're based in reality, and ideally in math and physics I can understand. Oh, and another question: what difference, if any, would frozen versus liquid water make in this situation? Jack. (exploring new antenna options.) - -- Jack Twilley jmt at twilley dot org http colon slash slash www dot twilley dot org slash tilde jmt slash -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAl/MBGPFSfAB/ezgRAo5pAKD35cRH0XFUz7p/uqBwbj3SNRP69QCfaz5C 6he5FgG+/q767KjX9g9T75A= =j1k0 -----END PGP SIGNATURE----- |
" Uncle Peter" wrote in message news:3cVlc.8415$%o1.2784@lakeread03... You'd still need a ground radial system, lest the varying water table constantly detune the array. Also, fresh water is generally a good insulator compared to copper wire. How much is 75 tons of salt, delivered? :) |
On Tue, 04 May 2004 12:46:02 -0700, Jack Twilley
I recognize that salt water is far more conductive than fresh water, Hi Jack, Unfortunately, this is a persistent illusion that begs real comparison to real conductors. Salt water is miserable as a conductor, and its special place in the pantheon of noble applications has little to do with "conductivity." but fresh water's still superior to sand and the like. Even this is arguable, sand has less loss (as sand is one of the most common precursors to making glass, it is hard to suggest it presents issues of conduction or loss). That being said, I am wondering about using a pond as a ground screen and mounting the antenna itself on an island (or a raft) in the middle of the pond. No doubt you will get a raft of anecdotal support, but not much data. What I don't know is just how large a pond do I need in order for something like this to work? I will do something dangerous and make a presumption. To work, as you suggest it through the example of the AM stations, you need to be operating in the 160M band. OK, so that was a caprice of guesswork, the remainder of your post offers other opportunity to suggest you are building your house on sand. For those who absolutely require less variables in their equations, imagine a standard dipole tuned for 20m strung roughly 45 feet above ground level between two trees, one on either side of a fresh water pond. Well, this is where you are in over your head (water metaphors are abundant in this topic). This, again, requires presumptions insofar as the original observation was driven by the AM example. However, at this point we will depart from the low frequency mandate to examine another mandate: polarization and your presumption of conductivity. A horizontally polarized antenna seeing a horizontally conducting surface is a scenario that describes a self-short-circuit. Horizontally polarized waves meeting the earth (a conductive one) immediately snuff themselves (how long would your car battery last with a screwdriver held across its poles?). On the other hand, vertical antennas do not suffer this fate - and for the same reason: it is a current wave (or at least the magnetic component inducing such a current, in a conductive earth) that spans earth making a perfectly reasonable relationship to continued propagation. How wide does the pond have to be at that point (and others) for it to work right? Even answers like "the pond will have to be wider than the dipole is long" or "there will be no noticeable impact on performance" are fine if they're based in reality, and ideally in math and physics I can understand. Well, once you divorce yourself of the notion of using a horizontal antenna, it becomes a matter of "ray tracing" from the vertical polarized source, out to the reflecting surface. If you want a radiation lobe to bounce away at an angle of 45°; then this surface has to be as far away as the origin of the ray's source is above ground. If you want a radiation lobe to bounce away at an angle of 5°; then this surface has to be 10 or 20 wavelengths away (which is why large bodies of water are so attractive). OK now to drop the other shoe. The reason why this all works can be described through conduction, but that is messy and far from intuitive (why is a poor conductor better than a poorer conductor that is sometimes better than the poor conductor?). The whole matter of near earth conductivity (through the application of trig) hardly matters a whit in regard to DX angles. DX angles are a property of the earth in the far region (at least 5 wavelengths away, which describes the almost certain frustration of planting long enough radials that will make any difference in that regard). The trick is to simply abandon the thought of the antenna except as a point representing the source of the ray to be traced to ground. Think of the wave propagating along that ray. It has left the antenna far behind and is in its native media of 377 Ohms. It then happens to intersect another media - salt water. Salt water's characteristic Z presents that wave with a 10:1 SWR mismatch, and as we all know exceedingly little power passes through that interface, and nearly all of it is reflected (again, in the same angle of reflection already described by the ray we started with). THIS is the power of salt water, marsh water, and other so-called conductive surfaces. The conductivity is for s**t and matters just as much - it is mismatch that does our work. Even average earth presents a mismatch, but not nearly so effective; thus power passes through the interface at its critical angle (another optics concept that is intimately tied into this ray-tracing). We call this (as does the optics community) the Brewster Angle. Optics has a simple method of forecasting the angle, and it is the same math that gives us SWR. a ratio. Well, there is more that could be said, but this is enough to part the waves. 73's Richard Clark, KB7QHC |
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Hash: SHA1 Thank you for the very detailed explanation. So much for that idea. :-) Jack. - -- Jack Twilley jmt at twilley dot org http colon slash slash www dot twilley dot org slash tilde jmt slash -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAmHAvGPFSfAB/ezgRAoBlAKC04X44ElnDqbiN024oNciJ6pSItwCeNuXX B8NCVjYJ3vYQZ9LcQbITBRM= =S6Tn -----END PGP SIGNATURE----- |
Jack Twilley wrote:
Thank you for the very detailed explanation. So much for that idea. :-) Jack. - -- Jack Twilley You aren't the first to think of that idea, and if you stick around the newsgroup long enough, you will find you are not the last! It's not a dumb question -- the only dumb question is the one that isn't asked. Irv VE6BP -- -------------------------------------- Diagnosed Type II Diabetes March 5 2001 Beating it with diet and exercise! 297/215/210 (to be revised lower) 58"/43"(!)/44" (already lower too!) -------------------------------------- Visit my HomePage at http://members.shaw.ca/finkirv/ Visit my Baby Sofia website at http://members.shaw.ca/finkirv4/ Visit my OLDTIMERS website at http://members.shaw.ca/finkirv5/ -------------------- Irv Finkleman, Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP Calgary, Alberta, Canada |
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Hash: SHA1 "Irv" == Irv Finkleman writes: [... I post an idea, Richard explains why it wouldn't work ...] Jack Thank you for the very detailed explanation. So much for that Jack idea. :-) Irv You aren't the first to think of that idea, and if you stick Irv around the newsgroup long enough, you will find you are not the Irv last! It's not a dumb question -- the only dumb question is the Irv one that isn't asked. When I think of something I haven't heard anyone else discuss, and I haven't seen mentioned here or in other online fora, I try to research it a little online and see if I can learn more about it. With this one, it was too tempting (because of the AM example) and the stuff I had read about Sommerfeld grounds in the NEC2 source and also comments seen on Mr. Cebik's site (usually associated with graphs of antenna patterns made with different types of ground) led me to believe that a quasi-infinite plane of salt water goodness would be a huge boost. It sure beats placing a layer of aluminum foil or steel plate over the backyard. ;-) Irv Irv VE6BP Jack. - -- Jack Twilley jmt at twilley dot org http colon slash slash www dot twilley dot org slash tilde jmt slash -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAmQ0IGPFSfAB/ezgRAvr5AKDlREO+WmtdX+Fv5I/g0FF9+cz29ACgkiYX MDO46gVXAQlDPzswMaRPVAI= =emQp -----END PGP SIGNATURE----- |
Jack, Depending on how difficult errecting the thing would be, why not give it a try anyway? The water may not 'help' a signal, but the antenna being away from nearby 'stuff' might. If you have the wire, a pond, a boat(?), etc...? 'Doc |
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Hash: SHA1 "Doc" == w5lz writes: Doc Jack, Depending on how difficult errecting the thing would be, Doc why not give it a try anyway? The water may not 'help' a signal, Doc but the antenna being away from nearby 'stuff' might. If you Doc have the wire, a pond, a boat(?), etc...? 'Doc I plan on bringing along the bits just in case I feel extra motivated. Should anything be learned from the experience, rest assured that I will post. :-) Jack. - -- Jack Twilley jmt at twilley dot org http colon slash slash www dot twilley dot org slash tilde jmt slash -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAmRH3GPFSfAB/ezgRAlfEAKDK02z2wO6uhR/pahhoiPEAiENb+wCgzGpb Bw8oMh6xIi2PWdjjp2OxwjY= =0nRC -----END PGP SIGNATURE----- |
Richard,
Are you sure you meant the statements quoted below? Horizontal polarization bounces just fine from "horizontally conducting surfaces". Indeed, when a mixed polarization wave hits a conducting surface the horizontal polarization in the reflected wave is enhanced, not "short-circuited". This is the same phenomenon that is the related to Brewster's angle. Perhaps you really meant to say that a special guided wave mode, namely the ground wave, does not support horizontal polarization. 73, Gene W4SZ Richard Clark wrote: [Lots of more or less correct stuff snipped] Well, this is where you are in over your head (water metaphors are abundant in this topic). This, again, requires presumptions insofar as the original observation was driven by the AM example. However, at this point we will depart from the low frequency mandate to examine another mandate: polarization and your presumption of conductivity. A horizontally polarized antenna seeing a horizontally conducting surface is a scenario that describes a self-short-circuit. Horizontally polarized waves meeting the earth (a conductive one) immediately snuff themselves (how long would your car battery last with a screwdriver held across its poles?). On the other hand, vertical antennas do not suffer this fate - and for the same reason: it is a current wave (or at least the magnetic component inducing such a current, in a conductive earth) that spans earth making a perfectly reasonable relationship to continued propagation. [More snip] |
On Wed, 05 May 2004 08:49:22 -0700, Jack Twilley
wrote: comments seen on Mr. Cebik's site (usually associated with graphs of antenna patterns made with different types of ground) led me to believe that a quasi-infinite plane of salt water goodness would be a huge boost. Hi Jack, Let's not lose track of why this is true, rather than fixating on the downside of alternative illusory explanations. We have DXers here frequently gushing about their shore installations. We also had reports from one fellow who maintained a regular schedule with an Aussie who worked CW mobile in his car coming home from work. Our man here could literally see the variation in signal strength as he approached or moved away from the shore down under. Now, this report is typically known as anecdotal. However, when you can repeat the observations and correlate them to your hypothesis (as they did), then such reports gain materially. 73's Richard Clark, KB7QHC |
On Wed, 05 May 2004 22:38:06 GMT, Gene Fuller
wrote: Richard, Are you sure you meant the statements quoted below? Horizontal polarization bounces just fine from "horizontally conducting surfaces". Indeed, when a mixed polarization wave hits a conducting surface the horizontal polarization in the reflected wave is enhanced, not "short-circuited". This is the same phenomenon that is the related to Brewster's angle. Perhaps you really meant to say that a special guided wave mode, namely the ground wave, does not support horizontal polarization. 73, Gene W4SZ Richard Clark wrote: [Lots of more or less correct stuff snipped] A horizontally polarized antenna seeing a horizontally conducting surface is a scenario that describes a self-short-circuit. Horizontally polarized waves meeting the earth (a conductive one) immediately snuff themselves (how long would your car battery last with a screwdriver held across its poles?). Hi Gene, Vertical polarization is the only mode that the Brewster Angle works for (that's why polarized sunglasses work so well, they are contra-polarized for what DOES reflect). To test your hypothesis, use EZNEC over a perfect ground and note the distinct difference at low angles (less than 5 degrees). The horizontal radiation lobe is an example of Lambertian (another Optics term) distribution where the maximal gain is observed directly overhead, and only when phases positively combine (due to the high surface conduction presenting a second source). Other phases give rise to this Lambertian distribution which is much like the lobe characteristics of a headlight glowing in the fog. 73's Richard Clark, KB7QHC |
On Thu, 06 May 2004 00:01:19 GMT, Richard Clark
wrote: To test your hypothesis, use EZNEC over a perfect ground and note the distinct difference at low angles (less than 5 degrees). I might add, compare this horizontal's low angle performance to its free space performance (a world of difference from that nearby "conductivity" and none of it remarkably "good" even for the most perfect of grounds). You have to hoist your horizontal pretty high to bring the phase gains into play. 73's Richard Clark, KB7QHC |
Richard,
Is that you, or did your evil twin steal your role on RRAA? Try reading my comment again. If you still disagree, then perhaps you should crack open any elementary physics or optics textbook. I did not mention antennas or lobes. I was commenting on your assertion that the horizontal polarization is "shorted out" at a conducting surface. Utter nonsense. 73, Gene W4SZ Richard Clark wrote: On Wed, 05 May 2004 22:38:06 GMT, Gene Fuller wrote: Richard, Are you sure you meant the statements quoted below? Horizontal polarization bounces just fine from "horizontally conducting surfaces". Indeed, when a mixed polarization wave hits a conducting surface the horizontal polarization in the reflected wave is enhanced, not "short-circuited". This is the same phenomenon that is the related to Brewster's angle. Perhaps you really meant to say that a special guided wave mode, namely the ground wave, does not support horizontal polarization. 73, Gene W4SZ Richard Clark wrote: [Lots of more or less correct stuff snipped] A horizontally polarized antenna seeing a horizontally conducting surface is a scenario that describes a self-short-circuit. Horizontally polarized waves meeting the earth (a conductive one) immediately snuff themselves (how long would your car battery last with a screwdriver held across its poles?). Hi Gene, Vertical polarization is the only mode that the Brewster Angle works for (that's why polarized sunglasses work so well, they are contra-polarized for what DOES reflect). To test your hypothesis, use EZNEC over a perfect ground and note the distinct difference at low angles (less than 5 degrees). The horizontal radiation lobe is an example of Lambertian (another Optics term) distribution where the maximal gain is observed directly overhead, and only when phases positively combine (due to the high surface conduction presenting a second source). Other phases give rise to this Lambertian distribution which is much like the lobe characteristics of a headlight glowing in the fog. 73's Richard Clark, KB7QHC |
Richard Clark wrote:
"Salt water is miserable as a conductor, and its special place in the pantheon of noble applications has little to do with conductivity." I praise the god of conductivity for the ocean`s behavior as a beniign enabler of medium wave propagation over extraordinary distances as compared with ordinary earth. The reason the ocean`s surface allows long distance propagation is explained in part on page 15 by Ed Laport in "Radio Antenna Engineering": "Nothing can be done about the electrical characteristics of the grouind or topography between transmitting and receiving antennas. By choice, it is possible to locate the antennas in areas of the best available soil conductivity, thus to increase the terminal efficiency to some extent, and to increase this efficiency still further by proper design of the grounding system. (For ground waves vertical systems are imperative as there is zero horizontally polarized ground wave propagation.) Optimum ground-wave propagation is obtained over salt water because of its conductivity (many times that of the best soils to be found on the land) and uniform topography." Laport has a ground-wave propagation table on page 17 of "Radio Antenna Engineering". It is for low frequencies which best exploit ground-waves. At 1000 miles over seawater, a wave at 400 KHz is attenuated by 98 dB. Over good soil, 111 dB. Over poor soil, 160 dB. Soil resistance determines penetration depth nto the soil and loss that the soil extracts from a wave, especially if the frequency isn`t too high. At 10 MHz and above, over real earth, the earth`s capacitance offers so much less opposition than the earth`s resistance that the earth`s resistive opposition really does have very little to do with conductivity. However low the soil conductivity is, it is effectively bypassed by the earth`s susceptance. Sea water is so good at limiting medium-wave penetration and loss that broadcast stations can be heard far out at sea during daylight hours. This range is much greater than over any type of land. I recall hearing the steel guitars of Hawaiian music when we were approaching from the U.S.A. during WW-2. We were still days away and the sun could be high in the sky. It wasn`t all that far as we only traveled about 250 miles a day in good seas. Field strength increases by 6 dB every time you cut the remaining distamnce by half as you approach the station. At half the distance the volts per meter double. Best regards, Richard Harrison, KB5WZI |
Gene, W4SZ wrote:
"---I was commenting on your assertion that the horizontal polarization is "shorted out" at a conductive surfacce." Richard Clark`s description may be indelicate but as I recall, Terman says rouighly the same in several instances. Wish I had a copy at hand. Terman says that a horizontally polarized low-angle wave suffers a phase reversal upon reflection and as the difference in path length is negligible between incident and reflected waves at low angles, the waves being of opposite phase add to zero. Best regards, Richard Harrison, KB5WZI |
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Richard,
I am well aware of the properties of the phase reversal, cancellation of direct and reflected waves, and so on. I have no substantive disagreements with Terman. I suspect Richard Clark was exercising a bit of poetic license by stating that the horizontal polarization was "shorted" at the conducting ground plane, perhaps in a vain attempt to simplify his explanation to the original poster. However, this statement is simply wrong. If it were true there would be no NVIS nor any reflections at all from a normal incidence wave on a conducting surface. Radar would not work. Mirrors would not work. Wave cancellation is not such a difficult topic (except on RRAA). There is no need to invoke phony arguments about waves "shorting". 73, Gene W4SZ Richard Harrison wrote: Gene, W4SZ wrote: "---I was commenting on your assertion that the horizontal polarization is "shorted out" at a conductive surfacce." Richard Clark`s description may be indelicate but as I recall, Terman says rouighly the same in several instances. Wish I had a copy at hand. Terman says that a horizontally polarized low-angle wave suffers a phase reversal upon reflection and as the difference in path length is negligible between incident and reflected waves at low angles, the waves being of opposite phase add to zero. Best regards, Richard Harrison, KB5WZI |
"Richard Clark" wrote
The electric dipole moment is clearly bridged by a conductor, by definition. As such, at the interface, it must collapse completely into a current which gives rise to counter emf, the two waves cancel as a function of phase - the proof again is found in the Lambertian distribution that vanishes completely with the removal of ground (why horizontal antennas are held up in the air). The more remote the ground, the greater the variation of phase and the distribution, and yet the low angles never fully recover (the death embrace of ground is always there). Richard, would the dipole's performance thus be improved by bedding the ground with sand, and hurt by adding ground radials? Same true if the dipole was at some compromise between 1/4 wave and the desired 1/2 wave above ground? Regards, Jack Painter Virginia Beach, Va (where mostly sand exists anyway) |
Richard Harrison wrote:
Richard Clark wrote: "Salt water is miserable as a conductor, ..... I praise the god of conductivity for the ocean`s behavior as a beniign enabler of medium wave..... Great info again Richard, I recall during my early days in the Navy of being able to listen to AM radio stations from Victoria B.C. all the way to Hawaii. I always attributed it to the salt medium, but never really knew enough about antennas and propagation other than to tell myself that salt water was a better conductor. When you think in terms of 'salt water because of its conductivity (many times that of the best soils)' and then show the differences in terms of Db attenuation it all makes sense. This applies in many areas of discussion on this newsgroup. One worries about ideal height above ground, transmission line losses, radiation pattern, etc,. whereas the main objective is to get a signal out in the ether one way or another, and damn the technical naysayers, full speed ahead. Even with a limited knowledge of antennas and propagation, which thanks to the knowledge gained on this newsgroup, I have always managed to get a signal out of the shack and into the air without regard to the warnings of the pundits, and thoroughly enjoyed making contacts -- many DX -- blissfully unaware of how effective or efficient my antenna system may have been! It frequently troubles me that when a novice asks a simple question about antennas they are often distracted and possibly prevented from trying something because of theoretical albeit often practical arguments against. If you don't get the wire out there, you don't get the signals out either. 60db attenuation still beats infinity, and on a clear day you can hear forever! By golly, maybe I should rethink salting the back 40! :-) Irv VE6BP -- -------------------------------------- Diagnosed Type II Diabetes March 5 2001 Beating it with diet and exercise! 297/215/210 (to be revised lower) 58"/43"(!)/44" (already lower too!) -------------------------------------- Visit my HomePage at http://members.shaw.ca/finkirv/ Visit my Baby Sofia website at http://members.shaw.ca/finkirv4/ Visit my OLDTIMERS website at http://members.shaw.ca/finkirv5/ -------------------- Irv Finkleman, Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP Calgary, Alberta, Canada |
"Richard Clark" wrote
The electric dipole moment is clearly bridged by a conductor, by definition. As such, at the interface, it must collapse completely into a current which gives rise to counter emf, the two waves cancel as a function of phase - the proof again is found in the Lambertian distribution that vanishes completely with the removal of ground (why horizontal antennas are held up in the air). The more remote the ground, the greater the variation of phase and the distribution, and yet the low angles never fully recover (the death embrace of ground is always there). Richard, would the dipole's performance thus be improved by bedding the ground with sand, and hurt by adding ground radials? Same true if the dipole was at some compromise between 1/4 wave and the desired 1/2 wave above ground? Regards, Jack Painter Virginia Beach, Va (where mostly sand exists anyway) |
On Thu, 6 May 2004 12:47:58 -0400, "Jack Painter"
wrote: "Richard Clark" wrote The electric dipole moment is clearly bridged by a conductor, by definition. As such, at the interface, it must collapse completely into a current which gives rise to counter emf, the two waves cancel as a function of phase - the proof again is found in the Lambertian distribution that vanishes completely with the removal of ground (why horizontal antennas are held up in the air). The more remote the ground, the greater the variation of phase and the distribution, and yet the low angles never fully recover (the death embrace of ground is always there). Richard, would the dipole's performance thus be improved by bedding the ground with sand, and hurt by adding ground radials? Same true if the dipole was at some compromise between 1/4 wave and the desired 1/2 wave above ground? Hi Jack, A good question, and one that brings out the one of my elliptical statements about having disproven you don't have to worry, because there is nothing you can do. In fact you can do something, however, it separates the discussion of ground insofar as near field and far field issues. IF you add a ground screen below a horizonal antenna, you CAN improve your communications efficiency (your contact, with sufficient resolution, could see an improved, stronger signal). This, of course, has no strength in its argument in the far field, the same problem exists of the complete collapse of the electric field through its polarization giving rise to a canceling current. The near field application (where the media does NOT exhibit a 377 Ohm characteristic) is one of shielding the source from loss (which is largely a dielectric loss, not a conductive, Ohmic loss). Richard Harrison, KB5WZI, has already recalled Terman's treatment, but having no reference handy, he hadn't really pulled it together. The point of the matter is that for a conductive ground, the electric fields are laid across a short. The obvious occurs and that electric field collapses into a magnetic field (through the short circuit current that necessarily follows) at the interface. This simple statement is enough to evidence the reversal of fortune (magnetic replacing electric in the face of its initiating source spells short circuit city). At a distance (along the magic 0° DX launch angle), BOTH the source and its reflection (or image) in the ground below it, are at an equal distance to the observer. Thus the distant observers (if they could) see TWO sources that are 180° out of phase. Thus everywhere along this meridian, those two signal completely cancel. With tongue in cheek, let's call this 100dB down. This happens ONLY for horizontal polarized signals. By shielding ground beneath the horizontal antenna, you are doing nothing to change this star fixed fate; but you are improving efficiency with a net positive gain, relatively speaking. You simply have two stronger signals canceling. At higher angles, lets call them 5° or higher (sometimes much higher) the path lengths of the two sources diverge from equality (a phase shift is introduced) as the signal strength attempts to pull toward the free space value, some 30dB higher. If you pull your attention successively higher, you eventual come to the point where the two path lengths introduce enough phase difference that they combine to a net signal that is greater than the free space value. This, by the way, does not constitute DX opportunity and is crowed about as the great NVIS advantage (in other words, the sufferer has no options and is content to make lemonade). This exercise describes the Lambertian distribution, a classic example of Optical sources. Raising the horizontal is much the same gain story. It removes itself from the cold embrace of earth's loss, and it introduces a new phase combination. Thus the lobes may lower from the Zenith, but you will never see them pulled all the way down to the horizon, such is the fate of horizontality. ;-) 73's Richard Clark, KB7QHC |
Gene, W4SZ wrote:
"There is no need to invoke phony arguments about waves "shorting"." Shorting waves does not annihilate them. It merely reflects them. A short is a low-resistance conductor. A transmission line short is a low-impedance U-turn for for the wave`s current which forces the voltage between conductors to zero. Cancellation of the electric field sends its energy for an instant to the magnetic field. As these two conjoined fields continuously regenerate each other, the electric field is immediately recreated by the enhanced magnetic field. The electric field goes from zero at the short to double the incidet just 1/4-wave back from the short due to addition of the incident and reflected wave vectors (phasors). For a complete reflection in a short, you need zero resistance. Otherwise, resistance consumes some of the available energy. When a radio wave strikes the ground, it is reflected. Angle of reflection equals the incidence angle but because the earth is an imperfect reflector the reflection is ncomplete. Reflection depends on incidence amgle, wave polarization, frequency, and type of earth. The reflection occurs as if the R-F wave were an optical wave. NVIS is simple to do by using a horizontal dipole up 1/4-wave above the earth. The wave is delayed 90-degrees in travel to the earth. It is delayed 180-degrees by earth reflection. Then, another 90-degrees of delay is experienced in the reflected wave`s return to the vicinity of the dipole. The 360-degree total round-trip delay puts the reflected wave back in-phase with newly emerging radiation from the dipole in its travel toward the zenith. If the ionosphere can reflect this high-angle energy, it can cause reception fairly close to the transmitter. Best regards, Richard Harrison, KB5WZI |
Every time I see a post that begins "-----BEGIN PGP SIGNED MESSAGE-----", I
can't help but think of a kid saying "hey, you want to see my secret decoder ring?" "Jack Twilley" wrote in message ... -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I have seen what appear to be AM broadcast towers in or on the edges of salt marshes, and it seems pretty obvious to me why that's a good place to go. However, the environmentalists being a little more noticeable than they were once upon a time, this particular method of siting is probably a little more challenging than it used to be. I recognize that salt water is far more conductive than fresh water, but fresh water's still superior to sand and the like. That being said, I am wondering about using a pond as a ground screen and mounting the antenna itself on an island (or a raft) in the middle of the pond. What I don't know is just how large a pond do I need in order for something like this to work? Obviously it depends on type of antenna and band and a bunch of other things, but even a wild-ass guess (with some math or physics behind it) will help make the difference between whether I bother trying or not. For those who absolutely require less variables in their equations, imagine a standard dipole tuned for 20m strung roughly 45 feet above ground level between two trees, one on either side of a fresh water pond. How wide does the pond have to be at that point (and others) for it to work right? Even answers like "the pond will have to be wider than the dipole is long" or "there will be no noticeable impact on performance" are fine if they're based in reality, and ideally in math and physics I can understand. Oh, and another question: what difference, if any, would frozen versus liquid water make in this situation? Jack. (exploring new antenna options.) - -- Jack Twilley jmt at twilley dot org http colon slash slash www dot twilley dot org slash tilde jmt slash -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.4 (FreeBSD) iD8DBQFAl/MBGPFSfAB/ezgRAo5pAKD35cRH0XFUz7p/uqBwbj3SNRP69QCfaz5C 6he5FgG+/q767KjX9g9T75A= =j1k0 -----END PGP SIGNATURE----- |
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