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Yacht Rf ground and radials
I want to set up a hf antenna for my sailboat.
I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will |
Yacht Rf ground and radials
I don't think Icom would jack you around about this, do you really?
Marine radios are a large part of their business and they do know their business. Uhmmm...this isn't one of those ionosphere posts is it? Butch Will wrote: I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will |
Yacht RF ground and radials
Will wrote:
"Does sea water make a good enough ground without radials?" It`s the best you can get in a boat at sea, The point is to get a good connection to the sea. You don`t need radials for that. That`s why a thin copper plate is recommended. Copper is durable and poisonous to sea organisns which may foul the surfaces of other materials. Skin effect applies. Bolting to a spot inside a metal hull means the RF must travel from the bolt location inside the hull (it can`t penetrate the hull) to an edge where it reaches from the inside surface to the outside surface and thence to the waterline. DC resistsance of a conductor is resistivity x length divided by crossection. AC resistance is more but proportioned to the DC resistance. A large crossection or area produces a low resistance. That`s why the plate is better for contacting the water than a wire. Its also why the seawater has a low resistance despite a higher resistivity than copper. The huge crossection of seawater has very low resistance in most cases and its reactance is low too. Low resistance and low reactance make a good path for RF. Best regards, Richard Harrison, KB5WZI |
Yacht Rf ground and radials
A 6" square plate makes an adequate ground when immersed in salt sea
water. That is unless the transmitter power exceeds 10 kW. |
Yacht Rf ground and radials
On Tue, 23 May 2006 23:44:50 +1000, Will
wrote: They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Hi Will, Below is better than above, to say the least. How much below is immaterial. Is sea water equal to copper wire radials as a RF ground system? Skip the pursuit of the Holy Grail in radials, this may lead you to start carrying buckets of dirt which screws up buoyancy. Why do i have to use copper foil when most other people suggest using ordinary copper wire? Probably more surface area. All ideas and comments appreciated. How good (or poor) sea water is for matching and loss, is seeing the glass 3/4's empty. How good sea water is for propagation is seeing the pitcher nearby and filling your glass several times. 73's Richard Clark, KB7QHC |
Yacht Rf ground and radials
In article ,
Butch Magee wrote: I don't think Icom would jack you around about this, do you really? Marine radios are a large part of their business and they do know their business. Uhmmm...this isn't one of those ionosphere posts is it? Butch Will wrote: I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will No, Not an Ionsphere Post, just a guy who has no clue about the technology that he wants to use, and he is asking questions, trying to learn. From the replys he has received so far, he is finding out that 99% of the hams, don't have a clue about MF/HF Marine Radio Antenna systems Design, either. I suggest that he head on over to rec.boat.electronics, and ask Larry, Gary S., Old Chief Lynn, or one of the other Old Salts, that have been doing these installations for decades, and have the experience in the technology being asked about. Most hams think that MF/HF Marine Radio Antenna Systems design should follow the same rules that Land Startions use. Well that isn't the case, and usually ends up is a "**** Poor", marginal system that only talks "when the Band is open", and "when the Band is open" even a wet noodle will radiate enough to communicate. Me |
Yacht Rf ground and radials
Richard Clark wrote:
On Tue, 23 May 2006 23:44:50 +1000, Will wrote: They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Hi Will, Below is better than above, to say the least. How much below is immaterial. Is sea water equal to copper wire radials as a RF ground system? Skip the pursuit of the Holy Grail in radials, this may lead you to start carrying buckets of dirt which screws up buoyancy. Why do i have to use copper foil when most other people suggest using ordinary copper wire? Probably more surface area. All ideas and comments appreciated. How good (or poor) sea water is for matching and loss, is seeing the glass 3/4's empty. How good sea water is for propagation is seeing the pitcher nearby and filling your glass several times. 73's Richard Clark, KB7QHC A few additional questions along these lines for the group (with some paraphrasing): 1. What is the skin depth in salt water at 14 MHz? How would this affect a ground plate at four feet below the surface? 2. What would the ohmic losses be over a one square foot by 33 foot path through salt water? 3. How well would the ground plate work on fresh water bodies, such as much of the Chesapeake, the Great Lakes, and various rivers and tributaries often used by cruisers? How would it compare with radials over fresh water? 4. Can anyone cite a published and reproducible study in which the RF losses through salt water were measured and compared with losses through one or more copper wire "radials" on or below deck of a typical cruising vessel? Or is there a published theoretical analysis of this comparison? Looking for more than the casual, anecdotal stuff. 5. Will a four foot length of wire dropped into sal****er provide a "good" RF "ground" and on what is the answer based? I need enlightenment! Thanks, and 73, Chuck NT3G |
Yacht Rf ground and radials
Icom needs to modernize their thinking and get rid of their "old school" installation guides. Wire radials are the way to go on your sailboat. Not copper foil or wide copper strips, just plain old 14 gauge wire radials. I suggest you read some of the threads about this on the Maritime Mobile Ham Forum from people with real world experience with marine HF installations. You'll find the answers to your questions the http://cruisenews.net/cgi-bin/mmham/webbbs_config.pl Eric "Will" wrote in message ... I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will |
Yacht Rf ground and radials
You mean the "Maritime" mis information forum? Looks like some real world BS artists on there. Would you make those radials on the boat resonant or not? If resonant, how would you know? How much more inductance would a ground wire have than copper foil. 73 Gary K4FMX On Tue, 23 May 2006 21:11:29 -0400, "Eric Fairbank" wrote: Icom needs to modernize their thinking and get rid of their "old school" installation guides. Wire radials are the way to go on your sailboat. Not copper foil or wide copper strips, just plain old 14 gauge wire radials. I suggest you read some of the threads about this on the Maritime Mobile Ham Forum from people with real world experience with marine HF installations. You'll find the answers to your questions the http://cruisenews.net/cgi-bin/mmham/webbbs_config.pl Eric "Will" wrote in message ... I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will |
Yacht Rf ground and radials
Will writes:
I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Well, I don't have personal experience with this. There is a chapter in the ARRL antenna book, and it basically agrees with ICOM. As to radials - two comments. First - how do you make sure that they don't get in the way. And that RF currents won't be a hazard to people. Second - there are plenty of wires on a yacht. How do the wires know whether or not they are supposed to act as radials? *If* there is a better alternative to the traditional backstay using seawater as ground, it might be the vertical dipole. Feeding would be tricky, but for single band operation, you could probably feed it like a J-Pole. I don't see how to make a multiband variant. And you get the high voltage points close to the deck, which doesn't sound like a good idea in a damp and salty environment. There are also people who hoist a horizontal dipole when needed. OK for recreational radio, not if you need to be able to communicate in rough weather. Of course, square riggers are beautiful, and you could use the yard-arms as a stacked yagi. 73 LA4RT Jon |
Yacht Rf ground and radials
I was told, years ago, that sea water is a very good conductor of radio
signals..... Sooo, take your ground wire and simple drop it into the water was a bit of a weight.. and see, or should I say, hear what happens... the length should not matter as the water is your conductor... Of course make sure your swr is low... Larry ve3fxq Will wrote: I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will |
Yacht Rf ground and radials
http://www.photolib.noaa.gov/nurp/nur09010.htm
This photo is of the barge over the Tektite II habitat; Summer 1970. I operated W2YRQ from inside the habitat with a Hy Gain 14 AVQ attached to this steel barge. We also dropped some heavy cable in the water with the conductors unwound. Worked great. 73 H. NQ5H PS Reg is usually right. "Reg Edwards" wrote in message ... A 6" square plate makes an adequate ground when immersed in salt sea water. That is unless the transmitter power exceeds 10 kW. |
Yacht Rf ground and radials
In article ,
"Eric Fairbank" wrote: Icom needs to modernize their thinking and get rid of their "old school" installation guides. Wire radials are the way to go on your sailboat. Not copper foil or wide copper strips, just plain old 14 gauge wire radials. I suggest you read some of the threads about this on the Maritime Mobile Ham Forum from people with real world experience with marine HF installations. You'll find the answers to your questions the http://cruisenews.net/cgi-bin/mmham/webbbs_config.pl Eric Radials are the WORST type of RF Ground for ANY MF/HF Marine Antenna System. Anyone who has any sense at all can understand this, just thinbk about it. First, they have to be very long when dealing with frequencies below 4 Mhz, and unless you have a BIG vessel, you don't have room for 1/4 wave radials. Second, Radials need to be resonant to do any good, and that limits them to one or two frequencies, where Marine Radios need to have antennas that can operate on many Bands, which is why non-resonate, low impedance, RF Grounds, are used by most all Commercial Marine Antenna Systems. I suggest that you go out and get 30 Years of Commercial Marine Radio Installation and Operation Experience, and then come back and explain it all to us, again.......in detail.....if you live that long.... Me been there, done that...... |
Yacht Rf ground and radials
Eric Fairbank wrote:
Icom needs to modernize their thinking and get rid of their "old school" installation guides. Wire radials are the way to go on your sailboat. Not copper foil or wide copper strips, just plain old 14 gauge wire radials. I suggest you read some of the threads about this on the Maritime Mobile Ham Forum from people with real world experience with marine HF installations. You'll find the answers to your questions the http://cruisenews.net/cgi-bin/mmham/webbbs_config.pl Eric "Will" wrote in message ... I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will As others have pointed out in other contexts, it is more useful to think about an HF "radial" on a boat as the "other half" of a vertical dipole, rather than as an "RF ground". Even with substantial asymmetry (which is almost a necessity, given the proximity of the horizontal "radial" to the sea), most autotuners can match an off-center-fed, L-shaped configuration easily. It is not necessary that the horizontal "radial" be resonant for good performance. Disregarding the balanced, center-fed dipole, the most common alternative approaches to a vertical antenna are 1) the use of the sea as a large ground plane, usually with a Dynaplate, and 2) the use of a large conducting surface (traditionally specified as a minimum of 100 square feet) inside the hull. This surface can also be thought of as the other half of an asymmetrical dipole. It is often connected to the sea through under water metal parts such as the keel, in which case it constitutes a kind of hybrid involving both 1 and 2. Comparative test data on these systems are quite difficult to obtain and virtually all of the information available is anecdotal, rarely reproducible, and often contradictory. Analytical studies of these systems, particularly as applied to fiberglass yachts, are also scarce. Unless you really know what you are doing, I second Me's advice to find someone with a lot of experience for assistance. Of course, experimentation is a good thing, too. Good luck. Chuck |
Yacht Rf ground and radials
Will wrote:
"How can this work when the Dynaplate is below seawater?" In the old days, an Apelco radio, a Webster antenna, and a Dynaplate ground put you in the marine radio business just fine. Best regards, Richard Harrison, KB5WZI |
Yacht Rf ground and radials
On Tue, 23 May 2006 23:44:50 +1000, Will
wrote: I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will First, radials on a boat are not usually better than a good ground to seawater. If you do use radials they need to be resonant which means they need to be ¼ wavelength long at each frequency of operation or they need to be tuned with a loading coil to make them resonant. The reason is that if they are not resonant you will get little current into them. Your antenna system will be unbalanced and being that the radials will usually be closer to other wires etc. on the boat they will couple into them before they couple to the sea. That will make the tuner coax and control cables radiators as well, because of the higher impedance of the radials. If the radials are mounted in the bottom of the hull right near the water they have a chance of coupling to the sea. But if you go to that trouble it is much easier to couple directly to the seawater with some metal under the boat. (dynaplate, through hulls etc.) Radials that do not couple the energy to the sea act as part of the antenna. That wouldn't be bad if that radiation went where you wanted it to go but a large part of it will get into all sorts of things on the boat that you don't want it to. Radials on a boat are different than when laid over earth. When laid over earth there is tight coupling to the earth and length is not important. When they are elevated above earth they need to be resonant in order to work. Short radials do little good in this situation. A boat installation is similar to an elevated installation on land. I see some guys on the Maritime mis-information forum spouting about using radials on a boat. You are much better off getting a connection to the sea for your ground. It is one of the best ground planes available. To do so you need a very short ground lead from the tuner to the sea connection. The best way to accomplish that is to mount the tuner down in the hull right next to the sea ground connection within a foot or so. Then run your antenna lead from the tuner to the antenna as much in the clear as you can. Remember that no matter where the tuner is mounted the antenna starts at the ground connection. If you have a 10 foot lead from the tuners ground connection to the sea water connection that 10 feet will radiate like the antenna. Problem is so will the coax and tuner control lines radiate and couple into all sorts of places you don't want it to. By placing the tuner at the ground connection you have control over what radiates and what does not. Copper foil for the ground lead to the tuner makes a lower impedance path than doe's wire. You want the lowest impedance path you can get for the ground lead. 73 Gary K4FMX |
Yacht Rf ground and radials
Gary Schafer wrote:
On Tue, 23 May 2006 23:44:50 +1000, Will wrote: I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? Is sea water equal to copper wire radials as a RF ground system? Does sea water make a good enough ground without radials? How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Why do i have to use copper foil when most other people suggest using ordinary copper wire? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. All ideas and comments appreciated. Will Hello Gary, Although I'm not advocating "radials" or any other marine HF grounding approach, I am trying to understand the reasoning used by those who do advocate. I've tried to set up instrumentation that would allow comparative measurements of alternative "ground" properties for yachts, but the problems involved have been overwhelming. I think I understand why the world is not awash in empirical data in this area, especially compared to what is available for land-based verticals. First, radials on a boat are not usually better than a good ground to seawater. I think the word "radials" in this thread ought to be in quotes, since we're really not talking about conventional symmetrical radials from which radiation is substantially canceled. As you point out, the "radials" being discussed in this thread are simply a horizontal part of the radiating antenna and not really radials at all. Having said that, I would welcome learning your basis for the conclusion, and any info you can offer on how much better and in what way. Do you have a measurement of the HF "ground resistance" provided by seawater using a Dynaplate? Would your conclusion change for a vessel on the Chesapeake where salinity is quite low? If you do use radials they need to be resonant which means they need to be ¼ wavelength long at each frequency of operation or they need to be tuned with a loading coil to make them resonant. The reason is that if they are not resonant you will get little current into them. Your antenna system will be unbalanced and being that the radials will usually be closer to other wires etc. on the boat they will couple into them before they couple to the sea. That will make the tuner coax and control cables radiators as well, because of the higher impedance of the radials. We don't seem to require that the backstay (or whatever vertical radiator we are using) be a physical 1/4 wavelength when we use a tuner. Why would we impose that requirement on the horizontal part of the radiating system? Isn't the famous 100 square feet of copper approach analytically equivalent to a nonresonant "radial"? It is true that proximity of the horizontal radiator to other wiring can cause problems, and this may be an unequivocal disadvantage to the approach. Of course, an entire sailboat is in the reactive near-field region of the vertical radiating element regardless of the "ground" used and so the coupling issue is a matter of degree. If the radials are mounted in the bottom of the hull right near the water they have a chance of coupling to the sea. But if you go to that trouble it is much easier to couple directly to the seawater with some metal under the boat. (dynaplate, through hulls etc.) Radials that do not couple the energy to the sea act as part of the antenna. That wouldn't be bad if that radiation went where you wanted it to go but a large part of it will get into all sorts of things on the boat that you don't want it to. There are two issues he interference (already discussed above) and efficiency. Assuming no communication value is given to near-vertical radiation angles (a mistake, in my opinion, for the lower frequencies), the efficiency of an L-shaped, center-fed radiator is probably no worse than 3 dB below that of the same vertical element worked against a perfect ground plane. That is based on simply assuming that 100% of the power that would have been radiated by the horizontal element is dissipated in the "ground". In practice, the typical Dynaplate-based ground plane will be less than perfect, even over salt water, and the horizontal element will most definitely radiate some power. Hence, my suggestion than 3 dB is worst case and I would not be shocked to see the Dynaplate-based system less efficient. Data, data, data. Radials on a boat are different than when laid over earth. When laid over earth there is tight coupling to the earth and length is not important. If we're now talking about real radials, I wonder if the statement is true. Real radials elevated three feet above seawater might couple very tightly. It would be interesting to see some analysis or measurements of this. But we're talking about "radials" in this thread and the statement is not relevant to them. When they are elevated above earth they need to be resonant in order to work. Short radials do little good in this situation. A boat installation is similar to an elevated installation on land. As mentioned above, I believe elevated radials (real radials) need to be resonant and symmetrical in order to cancel radiation from them. The problem is semantic, of course. A radiating structure with horizontal and vertical components can be 100% efficient with the horizontal components nowhere near resonant lengths. There will be effects on horizontal and vertical patterns but that is impossible to generalize. I see some guys on the Maritime mis-information forum spouting about using radials on a boat. You are much better off getting a connection to the sea for your ground. It is one of the best ground planes available. I'm curious about how these ground plates work. The attenuation in seawater of RF at 10 MHz is on the order of 30 dB/foot. If a ground plate is four feet below the surface, how does it work? Copper's conductivity is orders of magnitude better than seawater. Do we know the break even point for a copper wire ground plane vs. seawater? Reflection properties of seawater OTOH are well-known and documented. http://ecjones.org/physics.html Ionospheric Physics of Radio Wave Propagation The usual instructions for installing ground plates recommend bonding the RF ground connection to metal tanks, the engine, lifelines, the mast and associated steel rigging (other than the insulated backstay) etc. Sometimes I wonder if all that metal is not forming the "other half of the dipole", pretty much independently of the ground plate's existence. Since a good many installations disregard your recommendation to mount the tuner close to the ground plate, rather than close to the antenna, that 10 feet or so may contribute to the "size" of the counterpoise when a ground plate is used. I suppose this is heresy, but disproving it would require testing a ground plate with NO connections to the tuner other than transmitter, antenna and ground plate. A manual tuner would make sense for the test. Of course the transmitter would have to be run from an isolated battery so as not to bond the tuner to the boat's DC ground wiring. To make sense of the feedpoint impedance, the antenna should be a quarter wavelength to provide a known radiation resistance, but much of it would be buried in the hull, etc. It quickly becomes complex. Another factor is that at the higher frequencies, a typical backstay approaches or exceeds a half-wavelength, in which case even a "short" length of wire, with or without a ground plate, will work well. If the ground plate is providing one of the best ground planes available, its impedance should actually decrease with decreases in frequency, given the attenuation of seawater as a function of frequency. Thus, the ground plate should be spectacular at 160 meters. Alas, I have not seen even anecdotal information on that. To do so you need a very short ground lead from the tuner to the sea connection. The best way to accomplish that is to mount the tuner down in the hull right next to the sea ground connection within a foot or so. Then run your antenna lead from the tuner to the antenna as much in the clear as you can. Remember that no matter where the tuner is mounted the antenna starts at the ground connection. If you have a 10 foot lead from the tuners ground connection to the sea water connection that 10 feet will radiate like the antenna. Problem is so will the coax and tuner control lines radiate and couple into all sorts of places you don't want it to. Without some kind of isolation, that is true. The flip side, though, is that (as you pointed out) the antenna starts 10 feet "lower" and a good deal of radiation from that 10 foot length is now inside the hull. If your objection to the use of "radials" is unwanted coupling, then this must surely fall into the same potential (no pun) category. By placing the tuner at the ground connection you have control over what radiates and what does not. Copper foil for the ground lead to the tuner makes a lower impedance path than doe's wire. You want the lowest impedance path you can get for the ground lead. 73 Gary K4FMX Be interested in your thoughts on these issues, Gary. Sorry for the length. 73, Chuck NT3G |
Yacht Rf ground and radials
In article t,
chuck wrote: \ Hello Gary, Although I'm not advocating "radials" or any other marine HF grounding approach, I am trying to understand the reasoning used by those who do advocate. I've tried to set up instrumentation that would allow comparative measurements of alternative "ground" properties for yachts, but the problems involved have been overwhelming. I think I understand why the world is not awash in empirical data in this area, especially compared to what is available for land-based verticals. First, radials on a boat are not usually better than a good ground to seawater. I think the word "radials" in this thread ought to be in quotes, since we're really not talking about conventional symmetrical radials from which radiation is substantially canceled. As you point out, the "radials" being discussed in this thread are simply a horizontal part of the radiating antenna and not really radials at all. What you need to understand is just what kind of antenna are you trying to describe, here as a MF/HF Marine Antenna? Are you thinking a Marconi, Vertical Dipole, Offcenter Feed Marconi, or just what? The standard MF/HF Marine Antenna, usually is considered a Marconi, and that is what MOST both commercial and non-commercial MF/HF Marine antennas end up being. So lets discuss Marine Marconi Antennas, and what makes good ones and bad ones. Marconi Antennas are charactorized by 1/4 Lambda Vertical Radiating Element, sitting in close proximity to a LOW Impedance RF Ground System Perpendicular to the 1/4wave Vertical. In the MF/HF Marine enviorment we add a Tuner, Auto or Manual so as to be able to tune the Vertical to a 1/4 Wave Electrical Length and Resonance on each frequency that the vessel maybe required to use in the MF/HF Marine Frequency Bands, which cover 1.6Mhz to 25 Mhz in frequency. Ok now lets look at what the vertical is. A Backstay, a Whip, a Loaded Whip, a Loaded Whip with wire under it. It really doesn't matter, as they all will be tuned to resonance, in either 1/4Wave, or 3/4Wave by the tuner, against the impedance of whatever RF Ground is connected to the tuner. Hence the "Old RadioMans Addage", "If you have a Good RF Ground, anything will radiate a good signal, even a wet noodle, but even the best antenna will radiate poorly if it is working against a poor RF Ground." It is the RF Ground, that determines how WELL Marconi Antennas work. Always has, and always will. Having said that, I would welcome learning your basis for the conclusion, and any info you can offer on how much better and in what way. Do you have a measurement of the HF "ground resistance" provided by seawater using a Dynaplate? Would your conclusion change for a vessel on the Chesapeake where salinity is quite low? Ok, lets look at what RF Ground really means in MF/HF Marine Antenna Systems. Put on your "Bruce's Special RF Glasses" and look at you vessel, and lets see what the Marine enviorment really looks like to RF. Lets take a look at the wood or plastic hulled vessel over there, what DO WE SEE? Well, we see the vertical radiator, we see the salt water, we see the metal on the boat that is grounded electrically together, (bonded) we see the engine if it has one, we see the piping and wiring that is all connected electrically, or bonded to the engine, and nothing else. Now remember that for a Marconi Antenna, IT IS THE RF GROUND that determines the efficency of the system. What makes a good low impedance RF Gound? Large Flat area, perpendicular to the Radiator, very electrically conductive. Sounds just like Salt Water, doesn't it. Hmmmm, wonder why most really good systems use the WATER as the RF Ground? Dahhhh. Again, remember that we are looking for a LOW Impendance RF Ground that doesn't have much of a reactive component to it, so that it will be good just about anywhere in the 1.6- 25Mhz range. If you do use radials they need to be resonant which means they need to be ¼ wavelength long at each frequency of operation or they need to be tuned with a loading coil to make them resonant. The reason is that if they are not resonant you will get little current into them. Your antenna system will be unbalanced and being that the radials will usually be closer to other wires etc. on the boat they will couple into them before they couple to the sea. That will make the tuner coax and control cables radiators as well, because of the higher impedance of the radials. We don't seem to require that the backstay (or whatever vertical radiator we are using) be a physical 1/4 wavelength when we use a tuner. Why would we impose that requirement on the horizontal part of the radiating system? Isn't the famous 100 square feet of copper approach analytically equivalent to a nonresonant "radial"? Ok, now lets look at what we have to use to build a good LOW Impedance RF Ground with what we see using "Bruce's Special RF Glasses". One thing to think about, Do we really want to make a direct connection to the WATER, for our RF Ground, and if so do we want to make that Direst Connection a DC CONNECTION? This question is where "Elctrolysis" comes into play, and is beyound the scope of this lecture, so for convience lets assume we want NO DC Path to the WATER. So how do we then couple the RF to the water effectivly? Well, we use a Capacator, and a capacitor is made up of "Two Plates seperated by an insulator. Our capacitor has one plate as the Salt Water, and the insulator is the Wood or Plastic Hull, and the other plate we need to build out of what we have aboard that we can see with our "Special Glasses". Now what are the factors that increase the capacative coupling in a capacator? Plate surface area, and plate speration, so we want as much surface area as we can get, as close to the Salt Water, as we can get. The higher the coupling, the lower the impedance of the RF Gound System, and the BETTER the antenna system wiull function. It is true that proximity of the horizontal radiator to other wiring can cause problems, and this may be an unequivocal disadvantage to the approach. Of course, an entire sailboat is in the reactive near-field region of the vertical radiating element regardless of the "ground" used and so the coupling issue is a matter of degree. Not a real big issue here. Near field is basically unimportant in Marconi Antenna Systems, except for Near Field Grounded Verticals within a few feet of the Vertical Antenna. Chuck NT3G End of Lecture Part 1 MF/HF Marine Radio Antenna System Design / Simplified It's the Ground, dummy, the RF Ground........ Bruce in alaska an Old MF/HF Marine RadioMan from way back..... -- add a 2 before @ |
Yacht Rf ground and radials
I put this aside until I could do a little modeling. A lot of postings
have been made in the interim, but I don't see too much in the way of answers. I'll try to answer some of your questions. Will wrote: I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? I don't know anything about Dynaplates, but if it's on the hull, it's very near the surface of the water. Any current it conducts will flow along the top of the water displaced by the hull. If, on the other hand, it's really under any depth of water at all, it'll be invisible to RF and might as well not be there. Is sea water equal to copper wire radials as a RF ground system? Yes. Does sea water make a good enough ground without radials? Yes. A foot-long wire "ground rod" below the antenna provides a nearly lossless ground connection at HF. How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Radial wires are used for land based systems because of the poor conductivity of soil. Radial wires reduce the resistance of the path current takes going to and from the antenna base. Salt water is a good conductor and doesn't need -- and won't benefit from -- radial wires. Why do i have to use copper foil when most other people suggest using ordinary copper wire? You don't. And won't copper corrode rapidly in salt water? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. None. A simple wire down into the water is adequate. Or use a small plate very near the surface if you prefer. Roy Lewallen, W7EL |
Yacht Rf ground and radials
chuck wrote:
A few additional questions along these lines for the group (with some paraphrasing): 1. What is the skin depth in salt water at 14 MHz? About 2.4 inches. How would this affect a ground plate at four feet below the surface? A ground plate at that depth would be invisible to RF. It might as well not be there. This is, of course, assuming it actually has 4 feet of water above it and not a boat's hull and air. 2. What would the ohmic losses be over a one square foot by 33 foot path through salt water? Let's see, salt water conductivity is about 5 S/m, which is 1.524 S/ft. So the *DC* resistance of that piece of sea water would be 1.524 * 33 / (1 * 1) ~ 50 ohms. But the RF resistivity would be much greater because only the outer few inches would carry any current. 3. How well would the ground plate work on fresh water bodies, such as much of the Chesapeake, the Great Lakes, and various rivers and tributaries often used by cruisers? How would it compare with radials over fresh water? Fresh water is quite a different story. The skin depth in *pure* fresh water at 14 MHz is 156 feet. But "fresh water" is far from pure. Unfortunately I don't have any ready data on "typical" "fresh water". So the skin depth is somewhere between 2.4 inches and 156 feet. Not much help. If the water is pretty pure, radials near the surface would be an improvement over a ground plate. 4. Can anyone cite a published and reproducible study in which the RF losses through salt water were measured and compared with losses through one or more copper wire "radials" on or below deck of a typical cruising vessel? Or is there a published theoretical analysis of this comparison? Looking for more than the casual, anecdotal stuff. No. An NEC-4 model shows a one-foot wire to provide a nearly perfect ground in salt water. But that falls far short of your requirement. 5. Will a four foot length of wire dropped into sal****er provide a "good" RF "ground" and on what is the answer based? Yes, but one foot does just as well -- any current on the wire will drop to essentially zero within the first foot, so the remainder might as well not be there. This is from an NEC-4 model. I need enlightenment! So do we all. Roy Lewallen, W7EL |
Yacht Rf ground and radials
Roy Lewallen wrote:
chuck wrote: . . . 2. What would the ohmic losses be over a one square foot by 33 foot path through salt water? Let's see, salt water conductivity is about 5 S/m, which is 1.524 S/ft. So the *DC* resistance of that piece of sea water would be 1.524 * 33 / (1 * 1) ~ 50 ohms. . . Oops. The DC resistance would be 33 / (1 * 1) / 1.524 ~ 22 ohms. Roy Lewallen, W7EL |
Yacht Rf ground and radials
Roy Lewallen wrote:
I put this aside until I could do a little modeling. A lot of postings have been made in the interim, but I don't see too much in the way of answers. I'll try to answer some of your questions. I think we all would be interested in how a small piece of metal buried in sea water can provide an efficient ground versus one or 2 elevated radials. I also dont see how its efficient concentrating all your current in such a small area. Since the salinity of salt water is not constant using one or 2 radials on yacht would be more efficient. Will wrote: I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? I don't know anything about Dynaplates, but if it's on the hull, it's very near the surface of the water. Any current it conducts will flow along the top of the water displaced by the hull. If, on the other hand, it's really under any depth of water at all, it'll be invisible to RF and might as well not be there. Is sea water equal to copper wire radials as a RF ground system? Yes. Does sea water make a good enough ground without radials? How can salt water which would have some resistance even though its conductivity is high compared to earth behave better than copper wire when returning antenna currents to the feedpoint. The small amount of current flowing in a 100 watt signal i would not want to waste sending it into salt water. Salt makes good resistors, why would you introduce a loss into the equation which radials seem to eliminate even though we dont have ground loss over sea water? We also have the issue of the skin depth of sea water to consider. Yes. A foot-long wire "ground rod" below the antenna provides a nearly lossless ground connection at HF. This is not how most yachts connect their ground connections. They connect to the sea cocks well below the top of the water anywhere for 3ft to 5 ft down. Some even use slim flat ground shoes again well below the water line. Its impractical for a any sail vessel to maintain a connection to sea water close to the surface because loading and the yacht heeling when sailing. How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Radial wires are used for land based systems because of the poor conductivity of soil. Radial wires reduce the resistance of the path current takes going to and from the antenna base. Salt water is a good conductor and doesn't need -- and won't benefit from -- radial wires. Indeed radials do perform this way. I would still want to use radials wires even 1 or radials wires even on a yacht since the length of the radials will have a greater capacity to sea water ground. It also would be more efficient in providing a current return. Why do i have to use copper foil when most other people suggest using ordinary copper wire? You don't. And won't copper corrode rapidly in salt water? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. None. A simple wire down into the water is adequate. Or use a small plate very near the surface if you prefer. Again yachts bury their ground connection well below the skin depth. Some even run the ground wire from the tuner down to the keel which is well submersed in salt water. All they are using is one short piece of foil that is behaving like a small radial. We will see what the models say. 2 elevated radials over seawater versus a 1 ft square piece of metal buried below the skin depth. My money would be on the radials. Bob Roy Lewallen, W7EL |
Yacht Rf ground and radials
Roy Lewallen wrote:
Roy Lewallen wrote: chuck wrote: . . . 2. What would the ohmic losses be over a one square foot by 33 foot path through salt water? Let's see, salt water conductivity is about 5 S/m, which is 1.524 S/ft. So the *DC* resistance of that piece of sea water would be 1.524 * 33 / (1 * 1) ~ 50 ohms. . . Oops. The DC resistance would be 33 / (1 * 1) / 1.524 ~ 22 ohms. Roy Lewallen, W7EL Thank you for the detailed response, Roy. A couple of issues still trouble me however. If the skin depth at 14 MHz is about 2.4 inches, can we roughly assume that the RF resistance of that path is no less than 52.8 ohms (2.4*22 ohms)? This assumes most of the RF current would occur in the top one inch (attenuation at one inch would be about 15 dB), and that the resistance at 14 MHz is equal to the DC resistance. A path one inch deep by 16 feet long (1/4 wavelength at 14 MHz) would then have no less than 26.4 ohms resistance at 14 MHz. Now imagine a system of multiple one foot wide by 16 feet long copper radials on the ground with 26.4 ohm resistance distributed uniformly in each radial. Obviously such a system will be lossy, with an average radial resistance of 13.2 ohms. While the analogy is a stretch, it illustrates the difficulty I am having in understanding how seawater can be considered more efficient than even a single slightly elevated radial, which is reported to be less than 1 dB worse than 120 quarter wavelength buried radials (ignoring slight pattern distortion). So even if seawater does constitute a less lossy ground plane than a single radial (yeah, apples and oranges, but we can weigh their juices I think) it would be better by less than 1 dB. . Then there is the issue of the one foot long "grounding rod" immersed in the sea. If the above back-of-the-envelope analysis is valid, it would seem that a even one inch long rod would be more than sufficient. If we were dealing with a pool of liquid mercury or silver, this would have considerable intuitive appeal for me. But the seawater model is troubling. I imagine seawater to be a lot like earth, except more homogeneous and with orders of magnitude higher conductivity. And I imagine a perfect ground plane to have conductivity orders of magnitude higher than seawater. I imagine even a modest system of copper radials to appear more like liquid mercury than seawater does. Where am I going astray? 73, Chuck NT3G |
Yacht Rf ground and radials
bob wrote:
Roy Lewallen wrote: I put this aside until I could do a little modeling. A lot of postings have been made in the interim, but I don't see too much in the way of answers. I'll try to answer some of your questions. I think we all would be interested in how a small piece of metal buried in sea water can provide an efficient ground versus one or 2 elevated radials. I also dont see how its efficient concentrating all your current in such a small area. Since the salinity of salt water is not constant using one or 2 radials on yacht would be more efficient. Imagine for a moment that instead of salt water that the ocean was covered by a thick metal plate. How would you effectively use that as a ground? Salt water isn't as good a conductor as metal, but it acts more like that than dirt. I don't know how much the salinity or conductivity of sea water varies, but suspect that even at its worst it's quite a good conductor. How can salt water which would have some resistance even though its conductivity is high compared to earth behave better than copper wire when returning antenna currents to the feedpoint. Cross sectional area. Replacing all the sea water with copper would improve it, but scattering a bunch of copper radials out and replacing only tiny parts of it wouldn't make much difference. And the loss is so low to begin with that even replacing it with copper wouldn't make any difference. The small amount of current flowing in a 100 watt signal i would not want to waste sending it into salt water. It won't go in very far. It'll stay very close to the top. And the waste is negligble. Salt makes good resistors, why would you introduce a loss into the equation which radials seem to eliminate even though we dont have ground loss over sea water? We also have the issue of the skin depth of sea water to consider. Solid salt is actually a decent dielectric, I believe. Again, the trick is cross sectional area. The current is spread over a large area of water, so the overall loss is negligible. The analysis I did took skin effect into consideration. The skin depth is even less in metal, yet metal has low RF loss. Yes. A foot-long wire "ground rod" below the antenna provides a nearly lossless ground connection at HF. This is not how most yachts connect their ground connections. They connect to the sea cocks well below the top of the water anywhere for 3ft to 5 ft down. Some even use slim flat ground shoes again well below the water line. Its impractical for a any sail vessel to maintain a connection to sea water close to the surface because loading and the yacht heeling when sailing. I'm sorry to hear that, because any connection below a few inches is ineffective at HF. How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Radial wires are used for land based systems because of the poor conductivity of soil. Radial wires reduce the resistance of the path current takes going to and from the antenna base. Salt water is a good conductor and doesn't need -- and won't benefit from -- radial wires. Indeed radials do perform this way. I would still want to use radials wires even 1 or radials wires even on a yacht since the length of the radials will have a greater capacity to sea water ground. It also would be more efficient in providing a current return. More efficient than a deep plate, for sure. Not any more efficient than a foot long uninsulated wire extending downward from the surface. But by all means use whatever makes you feel well grounded. Why do i have to use copper foil when most other people suggest using ordinary copper wire? You don't. And won't copper corrode rapidly in salt water? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. None. A simple wire down into the water is adequate. Or use a small plate very near the surface if you prefer. Again yachts bury their ground connection well below the skin depth. Some even run the ground wire from the tuner down to the keel which is well submersed in salt water. If the wire is uninsulated, the first few inches of the wire will provide the ground connection. If it's insulated, they'll have no HF ground connection at all except what's provided by capacitive coupling through the first few inches of insulation. All they are using is one short piece of foil that is behaving like a small radial. We will see what the models say. By all means, do some modeling. The only program I know of which will allow modeling submerged conductors is NEC-4 and derivatives. 2 elevated radials over seawater versus a 1 ft square piece of metal buried below the skin depth. My money would be on the radials. Certainly elevated radials would be better than metal more than a skin depth or two deep. Better yet is a wire extending from the surface to a few skin depths. Why isn't that possible? Incidentally, I'm not proposing replacing the standard grounding system, which I'm sure is important for other uses including, probably, lightning protection. It will just need to be supplemented if you want an effective HF ground. Roy Lewallen, W7EL |
Yacht Rf ground and radials
chuck wrote:
If the skin depth at 14 MHz is about 2.4 inches, can we roughly assume that the RF resistance of that path is no less than 52.8 ohms (2.4*22 ohms)? This assumes most of the RF current would occur in the top one inch (attenuation at one inch would be about 15 dB), and that the resistance at 14 MHz is equal to the DC resistance. What you'd need to do is look at the I^2 * R loss for every little pie slice of water the current flows through. It's greatest near the antenna (assuming a vertical) where the current density is greatest. In that region, the current density is greatest and R is also greatest, so that's where the majority of loss occurs. (Which is why a radial wire field is useful for land installations -- its resistance is least near the antenna.) So you can't just calculate a single value of R or I based on the current and cross section at some point -- the entire area over which the current is flowing must be taken into account. The modeling program does just that. Don't get too worried about the skin depth. Shallower skin depth is an indication of a better conductor. The skin depth in metal is extremely thin, yet it's a better conductor yet. A path one inch deep by 16 feet long (1/4 wavelength at 14 MHz) would then have no less than 26.4 ohms resistance at 14 MHz. True but irrelevant. The current at the far end is much less than the current at the near end. Now imagine a system of multiple one foot wide by 16 feet long copper radials on the ground with 26.4 ohm resistance distributed uniformly in each radial. Obviously such a system will be lossy, with an average radial resistance of 13.2 ohms. While the analogy is a stretch, it illustrates the difficulty I am having in understanding how seawater can be considered more efficient than even a single slightly elevated radial, which is reported to be less than 1 dB worse than 120 quarter wavelength buried radials (ignoring slight pattern distortion). So even if seawater does constitute a less lossy ground plane than a single radial (yeah, apples and oranges, but we can weigh their juices I think) it would be better by less than 1 dB. . The problem is that the analogy is too much of a stretch. Too many incorrect assumptions were made, resulting in an invalid conclusion. Then there is the issue of the one foot long "grounding rod" immersed in the sea. If the above back-of-the-envelope analysis is valid, it would seem that a even one inch long rod would be more than sufficient. As it turns out, a one inch rod is nearly as good, even though it doesn't extend to the entire depth where significant current is flowing. Half the total current is below about 1.7 inches deep. To connect directly with essentially all the current requires at least several skin depths. Here's the relative current on a foot long wire directly below a quarter wave vertical at 14 MHz: Depth (in.) I 0.5 0.81 1.5 0.53 2.5 0.35 3.5 0.23 4.5 0.15 5.5 0.10 6.5 0.07 .. . . 10.5 0.01 11.5 0.006 If we were dealing with a pool of liquid mercury or silver, this would have considerable intuitive appeal for me. But the seawater model is troubling. I imagine seawater to be a lot like earth, except more homogeneous and with orders of magnitude higher conductivity. And I imagine a perfect ground plane to have conductivity orders of magnitude higher than seawater. I imagine even a modest system of copper radials to appear more like liquid mercury than seawater does. At RF, taking skin depth into account, there's about 5 orders of magnitude difference between the conductivities of copper and average soil. Sea water is 30 times more conductive (at RF) than average soil, so it's still far short of copper. But Suppose we had a conductor which was 10 orders of magnitude more conductive than copper -- would it make any difference if our ground plane was made out of that or out of copper? How about 3 orders of magnitude less conductive? The fact is that in this application, 30 times better than soil is adequate for the water to behave a lot more like copper than like soil. A modest system of radials in soil looks like very, very small cross sections of copper (remember the skin depth in copper!) separated by very large regions of soil. Out of curiosity, I altered the conductivity of the water in my computer model. Dropping it by a factor of 10 at DC (about 3 at RF) results in a reduction of about one dB in field strength, or about 25% in efficiency when using a single ground wire. So salt water has just about the minimum conductivity you can get by with if you want really good efficiency with a single ground wire. Where am I going astray? In oversimplifying the problem and using analogies which aren't quite right. Roy Lewallen, W7EL |
Yacht Rf ground and radials
The permittivity, K, of water is about 80.
The relative velocity of propagation along a wire immersed in water is about VF = 1/Sqrt( K ) = 0.11 At a frequency of 7.5 MHz, a 1/4-wavelength of wire immersed in water is only 1.1 metres = 43 inches long. Furthermore, in salt sea water, considering a wire as a transmission line, dielectric loss is so high there is little or no current flowing at the end of a quarterwave radial wire. Longer wires can be disregarded because they carry no current. So, at 7.5 MHz, there is no point in considering a system which has more than a radius of 1.1 metres. At higher frequencies the radius is even less. A copper coin, 1" in diameter, immersed in a large volume of salt water, has an impedance low enough to be used as an efficient ground for a 1/4-wave HF vertical antenna. It is limited by its power handling capacity. I have made measurements years ago but have no records as I didn't attach any importance to them at the time. And still don't. Unpolluted, clean, fresh pond water, is a different kettle of fish. Permittivity is still about 80 but the resistivity is very much greater. About 1000 ohm-metres is a reasonable value. ---- Reg. |
Yacht Rf ground and radials
Reg Edwards wrote:
The permittivity, K, of water is about 80. The relative velocity of propagation along a wire immersed in water is about VF = 1/Sqrt( K ) = 0.11 At a frequency of 7.5 MHz, a 1/4-wavelength of wire immersed in water is only 1.1 metres = 43 inches long. Furthermore, in salt sea water, considering a wire as a transmission line, dielectric loss is so high there is little or no current flowing at the end of a quarterwave radial wire. Longer wires can be disregarded because they carry no current. So, at 7.5 MHz, there is no point in considering a system which has more than a radius of 1.1 metres. At higher frequencies the radius is even less. A copper coin, 1" in diameter, immersed in a large volume of salt water, has an impedance low enough to be used as an efficient ground for a 1/4-wave HF vertical antenna. It is limited by its power handling capacity. I have made measurements years ago but have no records as I didn't attach any importance to them at the time. And still don't. Unpolluted, clean, fresh pond water, is a different kettle of fish. Permittivity is still about 80 but the resistivity is very much greater. About 1000 ohm-metres is a reasonable value. ---- Reg. Interesting info, Reg. I also made some kitchen table-top sal****er measurements about a year ago, but at much lower frequencies than you discuss. My measurements are not handy at the moment, but they don't comport with yours. I utilized a variety of electrode geometries: concentric, 4 pole, parallel plate, etc. Measurements of electric field strength, conductivity, path conductance, etc. are not difficult but interpretation of the data stumped me. As you remember, the conductance of a sal****er path is a direct function of the path's cross-sectional area. A penny doesn't produce much of a cross-sectional area at its end of the path. Maybe your pennies are better than ours, Certainly worth more. 73. Chuck |
Yacht Rf ground and radials
Reg Edwards wrote:
The permittivity, K, of water is about 80. The relative velocity of propagation along a wire immersed in water is about VF = 1/Sqrt( K ) = 0.11 . . . When the material is conductive, like salt water, you also have to consider the conductivity in determining velocity factor. The velocity factor in salt water is 0.0128 at 7.5 MHz, 0.0175 at 14 MHz (based on conductivity of 5 S/m and dielectric constant of 81). Incidentally, you can get this information directly from EZNEC, including the demo program. Select a real ground type, then find the velocity factor, skin depth, and other information in Utilities/Ground Info. Roy Lewallen, W7EL |
Yacht Rf ground and radials
chuck wrote:
Reg Edwards wrote: The permittivity, K, of water is about 80. The relative velocity of propagation along a wire immersed in water is about VF = 1/Sqrt( K ) = 0.11 At a frequency of 7.5 MHz, a 1/4-wavelength of wire immersed in water is only 1.1 metres = 43 inches long. Furthermore, in salt sea water, considering a wire as a transmission line, dielectric loss is so high there is little or no current flowing at the end of a quarterwave radial wire. Longer wires can be disregarded because they carry no current. So, at 7.5 MHz, there is no point in considering a system which has more than a radius of 1.1 metres. At higher frequencies the radius is even less. A copper coin, 1" in diameter, immersed in a large volume of salt water, has an impedance low enough to be used as an efficient ground for a 1/4-wave HF vertical antenna. It is limited by its power handling capacity. I have made measurements years ago but have no records as I didn't attach any importance to them at the time. And still don't. Unpolluted, clean, fresh pond water, is a different kettle of fish. Permittivity is still about 80 but the resistivity is very much greater. About 1000 ohm-metres is a reasonable value. ---- Reg. Interesting info, Reg. I also made some kitchen table-top sal****er measurements about a year ago, but at much lower frequencies than you discuss. My measurements are not handy at the moment, but they don't comport with yours. I utilized a variety of electrode geometries: concentric, 4 pole, parallel plate, etc. Measurements of electric field strength, conductivity, path conductance, etc. are not difficult but interpretation of the data stumped me. As you remember, the conductance of a sal****er path is a direct function of the path's cross-sectional area. A penny doesn't produce much of a cross-sectional area at its end of the path. Maybe your pennies are better than ours, Certainly worth more. 73. Chuck Hi Chuck So what would be the best size cross sectional area to achieve a close to perfect RF ground from 1 to 30 mhz over sea water? Considering things like corrosion, fowling, growth on the plate over time and any other factors that would deteriorate the effectiveness of this connection. You would want adequate safety margin when using this kind of simple direct contact. Bob |
Yacht Rf ground and radials
Roy Lewallen wrote:
bob wrote: Roy Lewallen wrote: I put this aside until I could do a little modeling. A lot of postings have been made in the interim, but I don't see too much in the way of answers. I'll try to answer some of your questions. I think we all would be interested in how a small piece of metal buried in sea water can provide an efficient ground versus one or 2 elevated radials. I also dont see how its efficient concentrating all your current in such a small area. Since the salinity of salt water is not constant using one or 2 radials on yacht would be more efficient. Imagine for a moment that instead of salt water that the ocean was covered by a thick metal plate. How would you effectively use that as a ground? Salt water isn't as good a conductor as metal, but it acts more like that than dirt. I don't know how much the salinity or conductivity of sea water varies, but suspect that even at its worst it's quite a good conductor. How can salt water which would have some resistance even though its conductivity is high compared to earth behave better than copper wire when returning antenna currents to the feedpoint. Cross sectional area. Replacing all the sea water with copper would improve it, but scattering a bunch of copper radials out and replacing only tiny parts of it wouldn't make much difference. And the loss is so low to begin with that even replacing it with copper wouldn't make any difference. The small amount of current flowing in a 100 watt signal i would not want to waste sending it into salt water. It won't go in very far. It'll stay very close to the top. And the waste is negligble. Salt makes good resistors, why would you introduce a loss into the equation which radials seem to eliminate even though we dont have ground loss over sea water? We also have the issue of the skin depth of sea water to consider. Solid salt is actually a decent dielectric, I believe. Again, the trick is cross sectional area. The current is spread over a large area of water, so the overall loss is negligible. The analysis I did took skin effect into consideration. The skin depth is even less in metal, yet metal has low RF loss. Yes. A foot-long wire "ground rod" below the antenna provides a nearly lossless ground connection at HF. This is not how most yachts connect their ground connections. They connect to the sea cocks well below the top of the water anywhere for 3ft to 5 ft down. Some even use slim flat ground shoes again well below the water line. Its impractical for a any sail vessel to maintain a connection to sea water close to the surface because loading and the yacht heeling when sailing. I'm sorry to hear that, because any connection below a few inches is ineffective at HF. How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Radial wires are used for land based systems because of the poor conductivity of soil. Radial wires reduce the resistance of the path current takes going to and from the antenna base. Salt water is a good conductor and doesn't need -- and won't benefit from -- radial wires. Indeed radials do perform this way. I would still want to use radials wires even 1 or radials wires even on a yacht since the length of the radials will have a greater capacity to sea water ground. It also would be more efficient in providing a current return. More efficient than a deep plate, for sure. Not any more efficient than a foot long uninsulated wire extending downward from the surface. But by all means use whatever makes you feel well grounded. Why do i have to use copper foil when most other people suggest using ordinary copper wire? You don't. And won't copper corrode rapidly in salt water? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. None. A simple wire down into the water is adequate. Or use a small plate very near the surface if you prefer. Again yachts bury their ground connection well below the skin depth. Some even run the ground wire from the tuner down to the keel which is well submersed in salt water. If the wire is uninsulated, the first few inches of the wire will provide the ground connection. If it's insulated, they'll have no HF ground connection at all except what's provided by capacitive coupling through the first few inches of insulation. All they are using is one short piece of foil that is behaving like a small radial. We will see what the models say. By all means, do some modeling. The only program I know of which will allow modeling submerged conductors is NEC-4 and derivatives. I dont have NEC4 is it too much to ask you to run the model. Radials over sea water versus a direct connection? 2 elevated radials over seawater versus a 1 ft square piece of metal buried below the skin depth. My money would be on the radials. Certainly elevated radials would be better than metal more than a skin depth or two deep. Better yet is a wire extending from the surface to a few skin depths. Why isn't that possible? Theres no easy way of making sure that the wires will submerge precisely or close to the ideal skin depth. The loading and heeling of the yacht would affect this depending on the sailing position wind speed and other factors. The motion of the waves and swell conditions will also be another variable. It would work great when you anchored. Incidentally, I'm not proposing replacing the standard grounding system, which I'm sure is important for other uses including, probably, lightning protection. It will just need to be supplemented if you want an effective HF ground. Roy Lewallen, W7EL Well if you read the many sailing web pages and the Icom marine guides they all advocate installing your RF ground system well below the skin depth of salt water. They also advocate bonding all your on board metals to submerged objects like the keel and copper ground shoes, which is clearly wrong. A yacht with elevated radials installed below the deck would radiate a better signal in my view. However what constitutes an effective radial system over seawater for frequencies between 1 and 30 mhz using a random wire backstay antenna versus a direct connection to sea water i cant answer without the modeling software. Bob |
Yacht Rf ground and radials
bob wrote:
chuck wrote: Reg Edwards wrote: The permittivity, K, of water is about 80. The relative velocity of propagation along a wire immersed in water is about VF = 1/Sqrt( K ) = 0.11 At a frequency of 7.5 MHz, a 1/4-wavelength of wire immersed in water is only 1.1 metres = 43 inches long. Furthermore, in salt sea water, considering a wire as a transmission line, dielectric loss is so high there is little or no current flowing at the end of a quarterwave radial wire. Longer wires can be disregarded because they carry no current. So, at 7.5 MHz, there is no point in considering a system which has more than a radius of 1.1 metres. At higher frequencies the radius is even less. A copper coin, 1" in diameter, immersed in a large volume of salt water, has an impedance low enough to be used as an efficient ground for a 1/4-wave HF vertical antenna. It is limited by its power handling capacity. I have made measurements years ago but have no records as I didn't attach any importance to them at the time. And still don't. Unpolluted, clean, fresh pond water, is a different kettle of fish. Permittivity is still about 80 but the resistivity is very much greater. About 1000 ohm-metres is a reasonable value. ---- Reg. Interesting info, Reg. I also made some kitchen table-top sal****er measurements about a year ago, but at much lower frequencies than you discuss. My measurements are not handy at the moment, but they don't comport with yours. I utilized a variety of electrode geometries: concentric, 4 pole, parallel plate, etc. Measurements of electric field strength, conductivity, path conductance, etc. are not difficult but interpretation of the data stumped me. As you remember, the conductance of a sal****er path is a direct function of the path's cross-sectional area. A penny doesn't produce much of a cross-sectional area at its end of the path. Maybe your pennies are better than ours, Certainly worth more. 73. Chuck Hi Chuck So what would be the best size cross sectional area to achieve a close to perfect RF ground from 1 to 30 mhz over sea water? Considering things like corrosion, fowling, growth on the plate over time and any other factors that would deteriorate the effectiveness of this connection. You would want adequate safety margin when using this kind of simple direct contact. Bob Hello Bob, Sorry, but I'm not able to answer your question as I'm still struggling to find an appropriate mental construct. For the moment, I'm suspending disbelief, as they say. Roy, W7EL, has reported model results showing that a wire (probably a few millimeters in diameter) only one foot long will produce near-perfect (my words) results. The greater the cross-sectional area, the better, of course, but it would seem not to be a critical factor based on what both Roy and Reg reported, A one inch diameter copper pipe would probably give you some margin based on those reports. Make it a couple of feet long and slip it through six inches or so of one of those foam "noodles" the kids use when swimming. That will keep it afloat, ensure it is visible, and protect the hull from damage when it collides. Remember that you will have to figure out how to attach this pipe to your tuner. In a lot of installations, that will mean six feet or more of wire (from tuner to pipe) hanging over the gunwale. That wire is effectively part of your antenna, and it will radiate. For convenience, it would make sense to let the pipe float away from the hull by six feet or so, but that makes the connecting wire even longer. If you hang something like that over the side you'll doubtless want to secure it with some kind of UV resistant line to take the strain off the wire, especially when under way (ugh!). On some boats, using the stainless rudder shaft could be a better solution if you can attach to it. It is often near the surface of the water. On other boats, the rudder shaft exits the hull well below sea level and that probably wouldn't work. Experiment by all means, and if you go the copper pipe route, just remember that your zinc will be protecting whatever copper you immerse. If you use a lot of copper (like a 1 foot diameter by 1 foot long cylinder made of copper flashing with lots of holes drilled in it), expect accelerated depletion of zinc. Good luck. Chuck ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Yacht Rf ground and radials
On Sun, 28 May 2006 04:59:49 +1000, bob wrote:
Well if you read the many sailing web pages and the Icom marine guides they all advocate installing your RF ground system well below the skin depth of salt water. They also advocate bonding all your on board metals to submerged objects like the keel and copper ground shoes, which is clearly wrong. Hi Bob, As I offered some time ago, how deep is fairly immaterial and your dismissal of "many sailing web pages, Icom" and so on to then come to the conclusion that they are "clearly wrong" is not quite so clear why. There are only two paths to that ground system well below the skin depth of salt water: (1) Through the water; (2) Through a lot of air within the boat. For (1), that already takes care of itself, but is a very odd method to getting to that dynaplate. Besides, a wire tacked to the outside of the hull, or worse simple thrashing in the surf, has got to add to drag. Following this wet path automatically snubs how much current will make it to the plate anyway. As Reggie offers, after 40 inches at 7MHz it is immaterial and that wet path to the plate makes the plate simply a tie-point. In short, you have to penetrate that skin depth to get beneath it. Penetrating it solves the "problem" of going too deep. For (2), what do you stand to lose with a deeper connection that is approached through the interior of the boat? This is a matter of matching characteristics, which lead to issues of loss. You have the same connection loss anyway you look at it. So, what value is there in these page's recommendations? It insures a connection. Of course, I could be wrong. I've only had experience in electronics on Big Gray Boats in salt water, or in Big Gray Submarines beneath salt water. Ground was everywhere and death as far away as a power lead (several dozen nuclear warheads were only slightly further away). 73's Richard Clark, KB7QHC |
Yacht Rf ground and radials
bob wrote:
I dont have NEC4 is it too much to ask you to run the model. Radials over sea water versus a direct connection? Sure. How many, how high, and how long? The foot-long wire produces efficiency of virtually unmeasurably close to 100%. So radials can't be significantly better. What sort of efficiency are you expecting from the radials? [I wrote:] Certainly elevated radials would be better than metal more than a skin depth or two deep. Better yet is a wire extending from the surface to a few skin depths. Why isn't that possible? Theres no easy way of making sure that the wires will submerge precisely or close to the ideal skin depth. The loading and heeling of the yacht would affect this depending on the sailing position wind speed and other factors. The motion of the waves and swell conditions will also be another variable. It would work great when you anchored. Why can't you extend a wire or strip all around your boat, or make it several feet long? There's no penalty for having it extend beyond several skin depths. Fishermen somehow manage to keep their lines and nets in the water -- surely you can work out a way to keep a wire in the water. Well if you read the many sailing web pages and the Icom marine guides they all advocate installing your RF ground system well below the skin depth of salt water. They also advocate bonding all your on board metals to submerged objects like the keel and copper ground shoes, which is clearly wrong. Yes. There's a vast amount of incorrect information on the web. Have you ever browsed around audiophile pages dealing with speaker wire? I'm sure there are innumerable astrology pages, too. A yacht with elevated radials installed below the deck would radiate a better signal in my view. However what constitutes an effective radial system over seawater for frequencies between 1 and 30 mhz using a random wire backstay antenna versus a direct connection to sea water i cant answer without the modeling software. All I can do is present the results that physical laws dictate. It's not uncommon for that to be inadequate to change a person's beliefs. Roy Lewallen, W7EL |
Yacht Rf ground and radials
To illustrate the order of magnitude of the effects :
An ideal shape of ground electrode is a hemisphere, of diameter D metres, pressed into the soil, flush with the soil surface.. Its resistance to the soil is easily proved and calculated : R = S / Pi / D ohms, where S is the soil's resistivity in ohm-metres and D is the diameter in metres. Pi = 3.14. The metric system is by far the most simple. The resistivity of salt sea water is 0.22 ohm-metres, constant wherever you may sample and test. Unaffected by the melting of the glaciers. So with a diameter of 0.22 metres = 9 inches, the electrode resistance = 1 ohm. Low enough? If the ground electrode is a ball with diameter = 9 inches, immersed in sea water at a sensible depth, then the electrode resistance will be halved. At radio frequencies the impedance of the connection to ground will be that of the connecting wire only, even before the resistance of the connecting wire to the water is taken into account. The high permittivity of water will also tend to decrease impedance at RF. Another illustration, following Lord Kelvin : The resistance of a ground rod to soil is given by : R = S / 2 / Pi / L * ArcSinh( 2 * L / D ) ohms, where S = soil resistivity, L = rod length in metres and D is rod diameter. ArcSinh is the inverse hyperbolic Sine function you will find on your pocket scientific calculators. So in sea water, at low frequencies, a rod 12 inches in length and a diameter of 1 inch will have a resistance of 1.2 ohms. At HF, because of the very low propagation velocity in water, propagation effects predominate and the rod must be considered as a very lossy transmission line. But its impedance to ground is still very low because Zo is very small. So the hull of a metal boat makes an excellent ground. Just connect to it with an alligator clip at the end of a length of wire and stop worrying about it. By the way, the practical units of resistivity in ohm-metres should be much preferred to the academic units of milli-Siemens. When dealing with milli-Siemens I find I have to stand on my head and look backwards. 1 milli-Siemens = 1000 ohm-metres. The clock tells me it's 7.30 in the morning in Birmingham, the idle, depressed ex-industrial city of the Midlands, where there used to be 10,000 factories, now superceded by the hardworking Chinese, and I'm already half way down a bottle of Spanish Campaneo red. Hic! ---- Reg, G4FGQ. |
Yacht Rf ground and radials
Roy Lewallen wrote: I don't know anything about Dynaplates, but if it's on the hull, it's very near the surface of the water. Any current it conducts will flow along the top of the water displaced by the hull. That hole in the water is pretty important. It not only allows the plate a few feet down on the hull to make a good connection, it also makes the boat float better. I'm sure sooner or later someone will tell you there isn't a hole because where they put their boat, because when they take the boat out of the water they can find the hole. 73 Tom |
Yacht Rf ground and radials
An arithmetical correction. I forgot to divide by Pi.
The resistance of a hemispherical electrode, 9 inches diameter, in salt sea water, is even smaller. It is only 0.32 ohms. Incidentally. the resistance of a flat circular disk of diameter D metres, in contact with the soil surface, is given by : R = S / 2 / D ohms, where S = soil resistivity in ohm-metres. In sea water, a disk of 12 inches diameter has a resistance of 0.37 ohms. Which is negligible in comparison with the radiation resistance of a 1/4-wave vertical antenna of 36 ohms. Careful readers should make a note of these hints and tips, free to USA citizens, in their notebooks. My own notebooks extend from volumes A to letter S. I'm wondering who to leave them to in my Will & Last Testament. ---- Reg, G4FGQ. |
Yacht Rf ground and radials
wrote:
I'm sure sooner or later someone will tell you there isn't a hole because where they put their boat, because when they take the boat out of the water they can find the hole. Didn't they notice that the water lever dropped? -- 73, Cecil http://www.qsl.net/w5dxp |
Yacht Rf ground and radials
"Reg Edwards" wrote:
The clock tells me it's 7.30 in the morning in Birmingham, the idle, depressed ex-industrial city of the Midlands, where there used to be 10,000 factories, now superceded by the hardworking Chinese, and I'm already half way down a bottle of Spanish Campaneo red. Hic! Reg, FYI: "NAPA, Calif. - French and California winemakers marked the 30th anniversary of the storied Paris tasting with another sip-and-spit showdown. California won - and by more than a nose. Native wines took the top five of 10 spots, with a 1971 Ridge Monte Bello cabernet sauvignon from the Santa Cruz mountains coming out on top Wednesday." -- 73, Cecil, W5DXP |
Yacht Rf ground and radials
Cecil Moore wrote: wrote: I'm sure sooner or later someone will tell you there isn't a hole because where they put their boat, because when they take the boat out of the water they can't find the hole. Didn't they notice that the water lever dropped? Not usually. They fill the hole in the water with so much money they never notice anything else. :-) |
Yacht Rf ground and radials
Roy Lewallen wrote:
I put this aside until I could do a little modeling. A lot of postings have been made in the interim, but I don't see too much in the way of answers. I'll try to answer some of your questions. Will wrote: I want to set up a hf antenna for my sailboat. I have read various guides from Icom etc. They suggest running copper foil to a Dynaplate and use sea water as the ground. How can this work when the Dynaplate is below sea water? I don't know anything about Dynaplates, but if it's on the hull, it's very near the surface of the water. Any current it conducts will flow along the top of the water displaced by the hull. If, on the other hand, it's really under any depth of water at all, it'll be invisible to RF and might as well not be there. Roy, after re-reading the above comment this morning, I realized you probably meant that RF currents flowing to the plate could simply travel from the plate along the hull/water boundary to the water line, and from there, change direction and travel over the water's surface. Much as currents would travel up and down the surface of a large ocean wave (except for hull/water vs. air/water boundary differences). If so, then your model results show the Dynaplate's location is not critical as long as it is attached to the hull and it need not be mounted at the waterline? Is my reading of your comment correct? Thanks. 73, Chuck Is sea water equal to copper wire radials as a RF ground system? Yes. Does sea water make a good enough ground without radials? Yes. A foot-long wire "ground rod" below the antenna provides a nearly lossless ground connection at HF. How can a piece of copper metal about 1 ft square equal several radials laying on the boats deck? Radial wires are used for land based systems because of the poor conductivity of soil. Radial wires reduce the resistance of the path current takes going to and from the antenna base. Salt water is a good conductor and doesn't need -- and won't benefit from -- radial wires. Why do i have to use copper foil when most other people suggest using ordinary copper wire? You don't. And won't copper corrode rapidly in salt water? Over seawater what would be the best number of radials to use considering that maximum length i can run is 40 ft. I am planning to use a backstay antenna with a SGC 230 Tuner. None. A simple wire down into the water is adequate. Or use a small plate very near the surface if you prefer. Roy Lewallen, W7EL ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
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