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#11
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On Thu, 2 Sep 2004 13:36:14 -0500, "Richard Fry"
wrote: So you are saying that sea water paths provide far better groundwave propagation than overland paths because sea water a such a good reflector? RF Hi OM, What is the contention that is your point? 73's Richard Clark, KB7QHC |
#12
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"Richard Clark" wrote message
... On Thu, 2 Sep 2004 13:36:14 -0500, "Richard Fry" wrote: So you are saying that sea water paths provide far better groundwave propagation than overland paths because sea water a such a good reflector? RF Hi OM, What is the contention that is your point? 73's Richard Clark, KB7QH __________________ That it isn't the reflective nature of sea water that is responsible for its good propagation of groundwave signals. That is due to its conductivity. RF |
#13
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#14
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On Thu, 2 Sep 2004 16:07:29 -0500, "Richard Fry"
wrote: That it isn't the reflective nature of sea water that is responsible for its good propagation of groundwave signals. That is due to its conductivity. Hi OM, Then the response is quite simple: you are wrong. 73's Richard Clark, KB7QHC |
#15
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"Richard Clark" wrote
If all would review the standard FCC groundwave propagation curves, they would notice that they offer low AM Band signal strengths in terms of "conductivity" and that the differences in strength for the 5,000 millimhos per meter and that of 40 millimhos per meter (125 fold difference) DO NOT achieve the same proportional difference in received signal strength. In fact, the difference is so narrow you could shave with a razor as sharp as it. Even at the high end of the band the difference has to be out 700 miles to show the "conductive" ratio. Of course, over that range of transmission ONLY Sea Water would support that forecast as continental soil varies vastly in smaller spans - hence the reputation of the Sea. _______________ For a reality check, here are the approx distances to the 1 mV/m contour for 1kW of radiated power from a 90 degree vertical with a good radial ground system. The values were determined from the FCC's standard curves. Freq Conductivity/Miles 540 kHz 8/66, 40/124, 5,000/140 1,600 kHz 8/22, 40/56, 5,000/126 The average ground conductivity in the U.S. is fairly low, probably somewhere between 8 and 16 mS/m. The difference in ground wave propagation over such paths is dramatically poorer than over sea water. It is also clear from the above values how much better the low freq MW broadcast channels perform. RF |
#16
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"Richard Clark" wrote in message
... On Thu, 2 Sep 2004 16:07:29 -0500, "Richard Fry" wrote: That it isn't the reflective nature of sea water that is responsible for its good propagation of groundwave signals. That is due to its conductivity. Hi OM, Then the response is quite simple: you are wrong. 73's Richard Clark, KB7QHC __________ Believe as you wish, but you won't have much company... |
#17
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On Thu, 2 Sep 2004 17:17:00 -0500, "Richard Fry"
wrote: It is also clear from the above values how much better the low freq MW broadcast channels perform. Hi OM, You have a remarkable capacity to find controversy where there is none. Again, what is the contention that is your point? 73's Richard Clark, KB7QHC |
#18
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On Thu, 2 Sep 2004 17:20:32 -0500, "Richard Fry"
wrote: Believe as you wish, but you won't have much company... Hi OM, Your reaction is the shock of a belief being challenged. ;-) As for having company? I am not under the illusion that science is a democracy, much less a madison avenue concept. 73's Richard Clark, KB7QHC |
#19
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Richard Clark wrote:
On Thu, 2 Sep 2004 14:27:37 +0000 (UTC), "Reg Edwards" wrote: Incidentally, ground loss is not only smaller in sea water, it is also smaller with soil resistivities of several thousand ohms and greater. There's a maximum somewhere in between. Hi George, The statement above falls into the category of "Old Wives' Tales." Given the choice for conductors, Sea Water ranks 6 or 7 orders of magnitude in worse conductivity than any metal (or even carbon) you would care to pick. By this logic, you should do everything in your power to operate in an open pit coal mine. ;-) . . . It's not an "old wives' tale" at all. We have to remember that there are two loss mechanisms involved with antennas. One is the resistance encountered by ground current flowing to one of the feedline terminals in a ground-mounted antenna. This can be reduced to an arbitrarily small value by installing radials. The other is loss incurred when the field strikes the ground and reflects. This second loss is generally negligible for horizontally polarized antennas except at high radiation angles, but is very significant for verticals at low angles. This loss occurs mainly beyond the far edge of most radial fields, so there's usually nothing you can do to reduce the reflection loss except to move the antenna. Perfectly conducting ground has no loss, and perfectly insulating ground has no loss. When considering ground system loss (resistance encountered by local ground currents), the loss is maximum somewhere in between. Doing some experimental modeling with EZNEC/4 (NEC-4 based), I've found that the maximum loss for a radial ground system in the HF range unfortunately hits somewhere around average soil conductivity. It's not an abrupt maximum -- the loss varies fairly gently with conductivity. Reflection loss has to be considered a little differently. While the same statement about perfectly conducting and perfectly insulating ground is still true, if you had perfectly insulating ground, radiated power would be lost to useful radiation by penetrating the ground, whether or not it got dissipated as heat. (In reality, it would of course eventually get dissipated, since no ground is perfectly insulating.) Looking just at the amount of radiation that ends up above the horizon, and neglecting conductive ground current loss, the reflection "loss" does indeed seem to increase monotonically as the ground conductivity decreases. The ground reflection loss can easily be evaluated with any version of EZNEC, including the demo. Choose a vertical antenna such as example file Vert1.ez, set the ground type to Real/MININEC, wire loss to zero, and the plot type to 3D. There should be no resistive loads in the model. Then look at the Average Gain shown at the bottom of the main window after doing a plot calculation. The deviation from a value of 1 (0 dB) represents the fraction of the applied power that's lost in the ground reflection. Roy Lewallen, W7EL |
#20
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"Richard Clark" wrote
"Richard Fry" wrote: It is also clear from the above values how much better the low freq MW broadcast channels perform. Hi OM, You have a remarkable capacity to find controversy where there is none. Again, what is the contention that is your point? 73's Richard Clark, KB7QHC _______________ You quote only a part of my post with which you happen to agree, and then say I find controversy when there is none. The point of my last post on this subject, and our real controversy here relates to which characteristic of sea water is responsible for its lower groundwave path loss, as developed in the thread. You wrote that the reason is because sea water is a good reflector. I wrote that it is because of its good conductivity. This difference in our positions should be evident by reading the thread. RF |
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