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In article . net,
chuck wrote: You seem to be saying that sea water (which is one heck of a lot less conductive than copper -- I mean orders of magnitude less conductive) is the only rf ground (return path) that works? And that the only way to utilize it in a fiberglass hull is with capacitive coupling? By your reasoning, radio communication from a vessel on fresh water is impossible. Or, as a corollary, radio communication from a vessel on the hard is impossible because the capacitive coupling to the sea is over a distance of more than 12". And of course, for VHF, we all use the equivalent of copper that is not capacitively coupled to the sea. Since we all agree that VHF works fine that way, can you tell us at what frequency the laws of electromagnetic radiation "jump ship" and no longer work the same as at VHF? A reference would be most welcome. The issue here is radiation, not propagation. Can you also provide a reference to the published and repeatable testing of 400 sq. ft. of copper ground that didn't work any better than having nothing at all? Radio stations thousands of miles from the sea would be amazed to learn that their ground systems don't work better than nothing at all. Why not post your theory on rec.radio.amateur.antenna and see how it is received there? Suggest you reef those sails a tad, Me. Chuck Well Chuck, now lets look at the subject a bit and see what the difference is between your VHF analogy, and MF/HF RF Ground systems. Copper vs Salt Water at VHF....Very significant difference in conductivity, but it isn't conductivity that we are dealing with here. At VHF the RF Ground side of the antenna is built into the antenna, and needs only to be .5 meters long. (roughly) Also consider that at VHF an antenna can operate over a very wide Frequency Range (155-159 Mhz) without any major RF Impedance changes. This is NOT even close to being true at MF/HF frequencies. Move your frequency 250Khz and your antenna tuning is totally blown away. Copper vs Salt Water at MF/HF.....Still some what significant in Conductivity, but again it isn't conductivity we are talking about here. When was the last time you saw a vessel dragging around a pile of copper so as to have a Good RF Ground, no matter what the hull was made of? At MF/HF Frequencies the RF Impedance of the Ground System is inversely proportional to coupling to the Salt water, if the vessel is floating in salt water. Obviously you have never tried to communicate using MF/HF Frequencies in the Great Lakes and Mississippi River Systems from plastic or wood hulled vessels. It is a bitch to design an effective RF Ground for such vessels, as there just isn't enough there to make it work, like in Salt Water. Also remember that RF Impedance changes with Frequency, a rather LARGE point to be made here. When you are designing a Marine RF Ground system it must work across one order of magnitude, or better in frequency span, and provide a Low Impedance across the whole span. Now tell us all just how you accomplish this feat of magic with a plastic or cellulose hull sitting in fresh water? Like I said, it is a bitch, and a compromise in the best situation. The Laws of Physics aren't the problem here, it is how the Frequency interacts with RF Ground System Impedance that determine how well things work. No "Jump ship" about it, just simple Physics, that you apparently don't have a good handle on. Radio Stations in the MF Frequency Band use Marconi Antennas, and or Phased Marconi Antennas, for Transmission. These have EXTENSIVE RF Ground Systems, usually made up of over 100 1/4 Lambda length radial wires buried 6 to 10" below the surrounding surface, depending on the Radiation Pattern for that station, as designated on the License issued by the FCC. These RF Grounds are engineered specifically for the site and Radiation Pattern needed. The RF Ground for such Systems covers significantly more area than 400 Sq Ft, and the addition of such a small area of copper screen, wouldn't make any significant difference to the Impedance of such a Ground System. What few HF Stations that are licensed in the US these days, mostly use Balanced Antennas instead of Marconi Antennas and the RF Ground isn't nearly as significant to their operation as it is for Marconi Antennas. The significance is basically limited to Takeoff Angles. Also of some significance here is that all these situations are single frequency antennas, and therefor don't need to be Low Impedance across a wide spread of frequencies (an order of magnitude or better when opertional. If you had some practical knowledge in the field, you would notice that if a wet, swampy field, or marshland could be found, that is where these antenna systems are built. Can you guess why? Having been in both the Bradcast Engineering, and Marine Radio Engineering end of things for MANY years, and having designed, installed, and maintained both classes of systems, I have dealt with these issues for MANY years, and the moral to the story IS, " It's the RF Ground, Sonny, the RF Ground". Me been there, done that......for a long long time..... |
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