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Yes, "resistance" is traditionally used to mean the real part of an
impedance, which in turn is expressed as a complex number. An equivalent circuit for an impedance is then a series R - X, with the R and X corresponding to the real and imaginary parts of the impedance respectively. Parallel equivalents are of course also used, but usually explicitly described as a parallel equivalent, or given as a complex admittance (G + jB, with G and B representing the shunt conductance and susceptance). As an antenna gets smaller, the impedance does rise, causing a requirement for more voltage for a given power. The rise, however, is due to increasing (series) reactance, not radiation resistance. If you make the antenna long enough to reach resonance, then continue making it longer, the impedance again rises until you hit "anti-resonance" (parallel resonance). In that region it's due to both an increasing reactance and an increasing radiation resistance. A real consequence of the low radiation resistance of a small antenna is that the conductor current is very high for a given applied power. This results in increased I^2 * R loss in the conductors. The loss can be very substantial in small antennas. Roy Lewallen, W7EL John Larkin wrote: On Wed, 17 Sep 2003 15:41:33 -0700, Roy Lewallen wrote: Well, actually, no. The radiation resistance generally decreases as an antenna gets smaller, assuming it's small compared to a wavelength. Roy Lewallen, W7EL At any given frequency, you can analyze the impedance of a small antanna as a series R-C or a shunt R-C. Viewed as a shunt resistance, Rr increases as the antenna gets smaller, and as a series network, it gets smaller. I guess the standard convention must be to treat Rr as a series element, so it gets smaller as the antenna gets smaller. Either way, it takes more volts (or, if you prefer, more amps) to force a small antenna to radiate as much as a larger one. John |
Roy Lewallen wrote:
[...] As an antenna gets smaller, the impedance does rise, causing a requirement for more voltage for a given power. The rise, however, is due to increasing (series) reactance, not radiation resistance. If you make the antenna long enough to reach resonance, then continue making it longer, the impedance again rises until you hit "anti-resonance" (parallel resonance). In that region it's due to both an increasing reactance and an increasing radiation resistance. A real consequence of the low radiation resistance of a small antenna is that the conductor current is very high for a given applied power. This results in increased I^2 * R loss in the conductors. The loss can be very substantial in small antennas. Roy Lewallen, W7EL Thanks for the excellent description. It's not clear if the device works like a regular antenna - apparently it is encased in ferrite the size of a grapefruit. Here is the url again: http://www.aftenposten.no/english/lo...ticleID=609108 Now we all know ferrite antennas work great in AM radios, especially where a strong interfering signal can be nulled out. But it's not clear how well they work as transmitting antennas. Wouldn't they have the same inefficiency problems as a small loop antenna? In other words, very low radiation resistance? If so, it doesn't seem possible it would have long range. Perhaps it is meant for local communication over short distances, which can be useful for secure links. There is an interesting article on magnetic induction, where the field strength falls off as 1/r^6 instead of 1/r^2. This group talks about using very small antennas for distances up to 2 meters: http://www.auracomm.com/Downloads/webwireless.pdf |
Roy Lewallen wrote:
[...] As an antenna gets smaller, the impedance does rise, causing a requirement for more voltage for a given power. The rise, however, is due to increasing (series) reactance, not radiation resistance. If you make the antenna long enough to reach resonance, then continue making it longer, the impedance again rises until you hit "anti-resonance" (parallel resonance). In that region it's due to both an increasing reactance and an increasing radiation resistance. A real consequence of the low radiation resistance of a small antenna is that the conductor current is very high for a given applied power. This results in increased I^2 * R loss in the conductors. The loss can be very substantial in small antennas. Roy Lewallen, W7EL Thanks for the excellent description. It's not clear if the device works like a regular antenna - apparently it is encased in ferrite the size of a grapefruit. Here is the url again: http://www.aftenposten.no/english/lo...ticleID=609108 Now we all know ferrite antennas work great in AM radios, especially where a strong interfering signal can be nulled out. But it's not clear how well they work as transmitting antennas. Wouldn't they have the same inefficiency problems as a small loop antenna? In other words, very low radiation resistance? If so, it doesn't seem possible it would have long range. Perhaps it is meant for local communication over short distances, which can be useful for secure links. There is an interesting article on magnetic induction, where the field strength falls off as 1/r^6 instead of 1/r^2. This group talks about using very small antennas for distances up to 2 meters: http://www.auracomm.com/Downloads/webwireless.pdf |
It's really hard to tell anything from a press release, and even harder
to believe most of what you can tell from it. One thing that stands out is that the antenna being described is apparently operating at a pretty low frequency, where all antennas are very small in terms of wavelength, and very inefficient. Without any technical information, I can only offer some generalities. A small antenna (in terms of wavelength) is going to be inefficient, and a tiny antenna is going to be very inefficient. And efficiency is important in a transmitting antenna. (I noted an exception to this in another posting about a terminated vee or rhombic antenna, but the exception doesn't apply here.) Often, for a given size small antenna, you can get lower loss by putting it on a ferrite core. But unless the chunk of ferrite is truly awesome in size or the frequency is pretty high, the antenna you're starting with will be tiny in terms of wavelength. That is, you might improve a very, very, very inefficient antenna to a very, very inefficient antenna. And that's probably the reason you don't usually see ferrite cores in transmitting antennas -- it's better to make them a bit bigger, too big to practically use a ferrite core. Somebody mentioned saturation as a consideration. I'd have to work through the numbers with an actual core size and material, frequency, number of turns, and power level to see if it would be a problem. But the very large air gap in the magnetic path (from one end of the rod to the other -- which is, in fact, where the radiating field escapes) greatly reduces the flux density, so you could probably run a surprising amount of power without saturation. In many ferrites routinely used at RF, losses cause objectionable heating at flux densities well below saturation, and can often shatter the ferrite before saturation is even approached. Among amateurs, core heating seems to be almost universally blamed on saturation, due to a lack of understanding of the mechanism really causing the heating. There are certainly many applications for small, inefficient transmitting antennas. Even a small improvement in efficiency is beneficial, and that might be what the folks in the article have accomplished. Roy Lewallen, W7EL No Spam wrote: Thanks for the excellent description. It's not clear if the device works like a regular antenna - apparently it is encased in ferrite the size of a grapefruit. Here is the url again: http://www.aftenposten.no/english/lo...ticleID=609108 Now we all know ferrite antennas work great in AM radios, especially where a strong interfering signal can be nulled out. But it's not clear how well they work as transmitting antennas. Wouldn't they have the same inefficiency problems as a small loop antenna? In other words, very low radiation resistance? If so, it doesn't seem possible it would have long range. Perhaps it is meant for local communication over short distances, which can be useful for secure links. There is an interesting article on magnetic induction, where the field strength falls off as 1/r^6 instead of 1/r^2. This group talks about using very small antennas for distances up to 2 meters: http://www.auracomm.com/Downloads/webwireless.pdf |
It's really hard to tell anything from a press release, and even harder
to believe most of what you can tell from it. One thing that stands out is that the antenna being described is apparently operating at a pretty low frequency, where all antennas are very small in terms of wavelength, and very inefficient. Without any technical information, I can only offer some generalities. A small antenna (in terms of wavelength) is going to be inefficient, and a tiny antenna is going to be very inefficient. And efficiency is important in a transmitting antenna. (I noted an exception to this in another posting about a terminated vee or rhombic antenna, but the exception doesn't apply here.) Often, for a given size small antenna, you can get lower loss by putting it on a ferrite core. But unless the chunk of ferrite is truly awesome in size or the frequency is pretty high, the antenna you're starting with will be tiny in terms of wavelength. That is, you might improve a very, very, very inefficient antenna to a very, very inefficient antenna. And that's probably the reason you don't usually see ferrite cores in transmitting antennas -- it's better to make them a bit bigger, too big to practically use a ferrite core. Somebody mentioned saturation as a consideration. I'd have to work through the numbers with an actual core size and material, frequency, number of turns, and power level to see if it would be a problem. But the very large air gap in the magnetic path (from one end of the rod to the other -- which is, in fact, where the radiating field escapes) greatly reduces the flux density, so you could probably run a surprising amount of power without saturation. In many ferrites routinely used at RF, losses cause objectionable heating at flux densities well below saturation, and can often shatter the ferrite before saturation is even approached. Among amateurs, core heating seems to be almost universally blamed on saturation, due to a lack of understanding of the mechanism really causing the heating. There are certainly many applications for small, inefficient transmitting antennas. Even a small improvement in efficiency is beneficial, and that might be what the folks in the article have accomplished. Roy Lewallen, W7EL No Spam wrote: Thanks for the excellent description. It's not clear if the device works like a regular antenna - apparently it is encased in ferrite the size of a grapefruit. Here is the url again: http://www.aftenposten.no/english/lo...ticleID=609108 Now we all know ferrite antennas work great in AM radios, especially where a strong interfering signal can be nulled out. But it's not clear how well they work as transmitting antennas. Wouldn't they have the same inefficiency problems as a small loop antenna? In other words, very low radiation resistance? If so, it doesn't seem possible it would have long range. Perhaps it is meant for local communication over short distances, which can be useful for secure links. There is an interesting article on magnetic induction, where the field strength falls off as 1/r^6 instead of 1/r^2. This group talks about using very small antennas for distances up to 2 meters: http://www.auracomm.com/Downloads/webwireless.pdf |
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The radiating and receiving efficiencies of ALL small ferrite- rod antennas
is very, very small. It's crudely about the same as a whip 10 times longer than the rod. The rod has the advantage over the receiving whip because it can be fitted inside a medium-wave pocket transistor radio and contains its own tuner and matching transformer. In addition, the rod does not need a ground. The MF receiving rod appears to behave very well only because MW broadcast stations are high power, relatively local, and pocket-transistor receivers have a higher gain than your shack transceiver which at MF is intended to be connected to a 260-feet length of wire at a height of 60 feet. If efficiency of a small transmitting rod should be as high even as 1 percent, regardless of whether the loss is in the wire or in the ferrite, it is obvious 99 percent of the transmitter power will be dissipated in the antenna and only a 20-watt transmitter would wreck it. PS: As with a magloop the ampere-turns are high but nearly all loss occurs in the ferrite. A serious problem is the high permeability temperature coefficient. As the core gets hot the antenna detunes itself. If you MUST have a dinky 160m pocket transceiver then try a dust-iron antenna. Six rods round one rod or a number of stacked slabs will have fewer suicidal tendencies. By far the fastest way of finding the number of turns is to wind on some wire and if there are too many take some off again. ---- Reg, G4FGQ ------------------------------------------------------------ "Roy Lewallen" wrote It's really hard to tell anything from a press release, and even harder to believe most of what you can tell from it. One thing that stands out is that the antenna being described is apparently operating at a pretty low frequency, where all antennas are very small in terms of wavelength, and very inefficient. Without any technical information, I can only offer some generalities. A small antenna (in terms of wavelength) is going to be inefficient, and a tiny antenna is going to be very inefficient. And efficiency is important in a transmitting antenna. (I noted an exception to this in another posting about a terminated vee or rhombic antenna, but the exception doesn't apply here.) Often, for a given size small antenna, you can get lower loss by putting it on a ferrite core. But unless the chunk of ferrite is truly awesome in size or the frequency is pretty high, the antenna you're starting with will be tiny in terms of wavelength. That is, you might improve a very, very, very inefficient antenna to a very, very inefficient antenna. And that's probably the reason you don't usually see ferrite cores in transmitting antennas -- it's better to make them a bit bigger, too big to practically use a ferrite core. Somebody mentioned saturation as a consideration. I'd have to work through the numbers with an actual core size and material, frequency, number of turns, and power level to see if it would be a problem. But the very large air gap in the magnetic path (from one end of the rod to the other -- which is, in fact, where the radiating field escapes) greatly reduces the flux density, so you could probably run a surprising amount of power without saturation. In many ferrites routinely used at RF, losses cause objectionable heating at flux densities well below saturation, and can often shatter the ferrite before saturation is even approached. Among amateurs, core heating seems to be almost universally blamed on saturation, due to a lack of understanding of the mechanism really causing the heating. There are certainly many applications for small, inefficient transmitting antennas. Even a small improvement in efficiency is beneficial, and that might be what the folks in the article have accomplished. Roy Lewallen, W7EL No Spam wrote: Thanks for the excellent description. It's not clear if the device works like a regular antenna - apparently it is encased in ferrite the size of a grapefruit. Here is the url again: http://www.aftenposten.no/english/lo...ticleID=609108 Now we all know ferrite antennas work great in AM radios, especially where a strong interfering signal can be nulled out. But it's not clear how well they work as transmitting antennas. Wouldn't they have the same inefficiency problems as a small loop antenna? In other words, very low radiation resistance? If so, it doesn't seem possible it would have long range. Perhaps it is meant for local communication over short distances, which can be useful for secure links. There is an interesting article on magnetic induction, where the field strength falls off as 1/r^6 instead of 1/r^2. This group talks about using very small antennas for distances up to 2 meters: http://www.auracomm.com/Downloads/webwireless.pdf |
The radiating and receiving efficiencies of ALL small ferrite- rod antennas
is very, very small. It's crudely about the same as a whip 10 times longer than the rod. The rod has the advantage over the receiving whip because it can be fitted inside a medium-wave pocket transistor radio and contains its own tuner and matching transformer. In addition, the rod does not need a ground. The MF receiving rod appears to behave very well only because MW broadcast stations are high power, relatively local, and pocket-transistor receivers have a higher gain than your shack transceiver which at MF is intended to be connected to a 260-feet length of wire at a height of 60 feet. If efficiency of a small transmitting rod should be as high even as 1 percent, regardless of whether the loss is in the wire or in the ferrite, it is obvious 99 percent of the transmitter power will be dissipated in the antenna and only a 20-watt transmitter would wreck it. PS: As with a magloop the ampere-turns are high but nearly all loss occurs in the ferrite. A serious problem is the high permeability temperature coefficient. As the core gets hot the antenna detunes itself. If you MUST have a dinky 160m pocket transceiver then try a dust-iron antenna. Six rods round one rod or a number of stacked slabs will have fewer suicidal tendencies. By far the fastest way of finding the number of turns is to wind on some wire and if there are too many take some off again. ---- Reg, G4FGQ ------------------------------------------------------------ "Roy Lewallen" wrote It's really hard to tell anything from a press release, and even harder to believe most of what you can tell from it. One thing that stands out is that the antenna being described is apparently operating at a pretty low frequency, where all antennas are very small in terms of wavelength, and very inefficient. Without any technical information, I can only offer some generalities. A small antenna (in terms of wavelength) is going to be inefficient, and a tiny antenna is going to be very inefficient. And efficiency is important in a transmitting antenna. (I noted an exception to this in another posting about a terminated vee or rhombic antenna, but the exception doesn't apply here.) Often, for a given size small antenna, you can get lower loss by putting it on a ferrite core. But unless the chunk of ferrite is truly awesome in size or the frequency is pretty high, the antenna you're starting with will be tiny in terms of wavelength. That is, you might improve a very, very, very inefficient antenna to a very, very inefficient antenna. And that's probably the reason you don't usually see ferrite cores in transmitting antennas -- it's better to make them a bit bigger, too big to practically use a ferrite core. Somebody mentioned saturation as a consideration. I'd have to work through the numbers with an actual core size and material, frequency, number of turns, and power level to see if it would be a problem. But the very large air gap in the magnetic path (from one end of the rod to the other -- which is, in fact, where the radiating field escapes) greatly reduces the flux density, so you could probably run a surprising amount of power without saturation. In many ferrites routinely used at RF, losses cause objectionable heating at flux densities well below saturation, and can often shatter the ferrite before saturation is even approached. Among amateurs, core heating seems to be almost universally blamed on saturation, due to a lack of understanding of the mechanism really causing the heating. There are certainly many applications for small, inefficient transmitting antennas. Even a small improvement in efficiency is beneficial, and that might be what the folks in the article have accomplished. Roy Lewallen, W7EL No Spam wrote: Thanks for the excellent description. It's not clear if the device works like a regular antenna - apparently it is encased in ferrite the size of a grapefruit. Here is the url again: http://www.aftenposten.no/english/lo...ticleID=609108 Now we all know ferrite antennas work great in AM radios, especially where a strong interfering signal can be nulled out. But it's not clear how well they work as transmitting antennas. Wouldn't they have the same inefficiency problems as a small loop antenna? In other words, very low radiation resistance? If so, it doesn't seem possible it would have long range. Perhaps it is meant for local communication over short distances, which can be useful for secure links. There is an interesting article on magnetic induction, where the field strength falls off as 1/r^6 instead of 1/r^2. This group talks about using very small antennas for distances up to 2 meters: http://www.auracomm.com/Downloads/webwireless.pdf |
Why not in metal can? You can use for example alumina. But no
ferroelectrics! (If it's a magnetic antenna) - Henry Arie de Muynck schrieb in Nachricht ... "John Miles" wrote http://www.aftenposten.no/english/lo...ticleID=609108 I've always assumed that the performance of ferrite-rod antennas in transmitting applications was limited by core saturation. Wonder if there's anything to this "invention"? I especically like the statement: "Our tiny antenna can be placed in the car or cast in metal, and is at least as good" Great, an antenna working even if cast in metal.... Arie. |
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