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#1
September 11th 15, 12:06 AM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
wrote in message
... On Thursday, September 10, 2015 at 5:27:41 AM UTC-5, gareth wrote: What is the nature of free space such that it requires antennae to be at least 1/4 wave before accepting radiation efficiently? Ding, ding, ding. Totally false statement. Don't pass go, don't collect $200. Convince yourself by calculating the retarded potential from a far field when the same field strength comes from a dipole and when it comes from a short antenna. You find that there has to be more power fed into the short antenna (irrespective of feed impedances and ohmic resistances) to achieve that same field strength. |
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#2
September 11th 15, 01:17 AM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
"gareth" wrote in message ... wrote in message ... On Thursday, September 10, 2015 at 5:27:41 AM UTC-5, gareth wrote: What is the nature of free space such that it requires antennae to be at least 1/4 wave before accepting radiation efficiently? Ding, ding, ding. Totally false statement. Don't pass go, don't collect $200. # Convince yourself by calculating the retarded potential from a far field # when # the same field strength comes from a dipole and when it comes from a short # antenna. #You find that there has to be more power fed into the short antenna # (irrespective of # feed impedances and ohmic resistances) to achieve that same field strength. Well, I don't know what you are calling "retarded potential". At any rate, if the radiated power is equal for two different antennas, why would the field strength be different, except as related to pattern differences? |
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#3
September 11th 15, 11:23 AM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
"Wayne" wrote in message
... Well, I don't know what you are calling "retarded potential". It is the term for the electric potential that must have existed on an antenna when analysing the radiated signal at some distant point. It is termed, "retarded" because the radiation originated at some time in the past. |
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#4
September 11th 15, 05:40 PM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
"gareth" wrote in message ... "Wayne" wrote in message ... Well, I don't know what you are calling "retarded potential". It is the term for the electric potential that must have existed on an antenna when analysing the radiated signal at some distant point. It is termed, "retarded" because the radiation originated at some time in the past. OK now I know. I hadn't heard the term before. But, how about my question? " if the radiated power is equal for two different antennas, why would the field strength be different, except as related to pattern differences?" The radiated power just is spread out differently. |
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#5
September 11th 15, 06:10 PM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
"Wayne" wrote in message
... "gareth" wrote in message ... "Wayne" wrote in message ... Well, I don't know what you are calling "retarded potential". It is the term for the electric potential that must have existed on an antenna when analysing the radiated signal at some distant point. It is termed, "retarded" because the radiation originated at some time in the past. OK now I know. I hadn't heard the term before. But, how about my question? " if the radiated power is equal for two different antennas, why would the field strength be different, except as related to pattern differences?" The radiated power just is spread out differently. Quoting from Electromagnetism by F.N.H.Robinson in the Oxfors Physic Series, 1973 edition ISBN 0 19 851806 4, Chapter 11, Radiation and page 100 .. The radiated power is proportional to the current times the antenna length divided by the wavelenght, and all squared. Therefore, to achieve the same radiated power from a short antenna, the current in the antenna has to be higher. HTH |
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#6
September 11th 15, 06:18 PM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
"gareth" wrote in message ... "Wayne" wrote in message ... "gareth" wrote in message ... "Wayne" wrote in message ... Well, I don't know what you are calling "retarded potential". It is the term for the electric potential that must have existed on an antenna when analysing the radiated signal at some distant point. It is termed, "retarded" because the radiation originated at some time in the past. OK now I know. I hadn't heard the term before. But, how about my question? " if the radiated power is equal for two different antennas, why would the field strength be different, except as related to pattern differences?" The radiated power just is spread out differently. Quoting from Electromagnetism by F.N.H.Robinson in the Oxfors Physic Series, 1973 edition ISBN 0 19 851806 4, Chapter 11, Radiation and page 100 .. The radiated power is proportional to the current times the antenna length divided by the wavelenght, and all squared. Therefore, to achieve the same radiated power from a short antenna, the current in the antenna has to be higher. But wasn't it implicit in your original statement that the power was equal in both the 1/4 wave and the shorter antenna? So for your assertion, we don't need to know the current. |
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#7
September 11th 15, 06:21 PM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
"Wayne" wrote in message
... "gareth" wrote in message ... "Wayne" wrote in message ... "gareth" wrote in message ... "Wayne" wrote in message ... Well, I don't know what you are calling "retarded potential". It is the term for the electric potential that must have existed on an antenna when analysing the radiated signal at some distant point. It is termed, "retarded" because the radiation originated at some time in the past. OK now I know. I hadn't heard the term before. But, how about my question? " if the radiated power is equal for two different antennas, why would the field strength be different, except as related to pattern differences?" The radiated power just is spread out differently. Quoting from Electromagnetism by F.N.H.Robinson in the Oxfors Physic Series, 1973 edition ISBN 0 19 851806 4, Chapter 11, Radiation and page 100 .. The radiated power is proportional to the current times the antenna length divided by the wavelenght, and all squared. Therefore, to achieve the same radiated power from a short antenna, the current in the antenna has to be higher. But wasn't it implicit in your original statement that the power was equal in both the 1/4 wave and the shorter antenna? No, for I was referring to the power arriving at some point in the far field |
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#8
September 11th 15, 06:22 PM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
On 9/11/2015 1:10 PM, gareth wrote:
"Wayne" wrote in message ... "gareth" wrote in message ... "Wayne" wrote in message ... Well, I don't know what you are calling "retarded potential". It is the term for the electric potential that must have existed on an antenna when analysing the radiated signal at some distant point. It is termed, "retarded" because the radiation originated at some time in the past. OK now I know. I hadn't heard the term before. But, how about my question? " if the radiated power is equal for two different antennas, why would the field strength be different, except as related to pattern differences?" The radiated power just is spread out differently. Quoting from Electromagnetism by F.N.H.Robinson in the Oxfors Physic Series, 1973 edition ISBN 0 19 851806 4, Chapter 11, Radiation and page 100 .. The radiated power is proportional to the current times the antenna length divided by the wavelenght, and all squared. Therefore, to achieve the same radiated power from a short antenna, the current in the antenna has to be higher. There is no contradiction there. Current is not power. Power is voltage times current. Since the impedance of a short antenna is not the same as the impedance of a larger antenna, it makes perfect sense that the current for a given power level will not be the same. -- Rick |
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#9
September 11th 15, 06:48 PM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
"rickman" wrote in message
... There is no contradiction there. Current is not power. Power is voltage times current. Since the impedance of a short antenna is not the same as the impedance of a larger antenna, it makes perfect sense that the current for a given power level will not be the same. Feed 1kW into your 472kHz antenna and get only 1W erp, most of the high current driving the ohmic resistance and not the radiation resistance |
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#10
September 11th 15, 06:24 PM
posted to rec.radio.amateur.antenna
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The nature of Free Space (Once called, "The Lumeniferous Aether")
gareth wrote:
"Wayne" wrote in message ... "gareth" wrote in message ... "Wayne" wrote in message ... Well, I don't know what you are calling "retarded potential". It is the term for the electric potential that must have existed on an antenna when analysing the radiated signal at some distant point. It is termed, "retarded" because the radiation originated at some time in the past. OK now I know. I hadn't heard the term before. But, how about my question? " if the radiated power is equal for two different antennas, why would the field strength be different, except as related to pattern differences?" The radiated power just is spread out differently. Quoting from Electromagnetism by F.N.H.Robinson in the Oxfors Physic Series, 1973 edition ISBN 0 19 851806 4, Chapter 11, Radiation and page 100 .. The radiated power is proportional to the current times the antenna length divided by the wavelenght, and all squared. Therefore, to achieve the same radiated power from a short antenna, the current in the antenna has to be higher. HTH Congratulations, you have just confirmed Ohm's law. -- Jim Pennino |
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