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Reg Edwards wrote:
"Ian Jackson" wrote - Are you sure it's as high as that, Reg? I once did a Smith Chart plot of the impedance at the centre of a dipole, the valued being taken from a table 'compiled by Wu' (LK Wu?). These only catered for a lengths up to a few wavelengths. As the plot progressed round and round the Smith Chart, it seemed to be heading for something around 350 to 400 ohms. I've just done a search on 'Wu+dipole+impedance', and one of the results is http://www.fars.k6ya.org/docs/antenn...nce-models.pdf I'll have a read of it today. =================================== The characteristic impedance of an infinitely long wire is Zo. If we cut the line and measure between the two ends we obtain an input impedance of twice Zo. Which is the answer to our problem. Zo is a function of wavelength, conductor diameter and conductor resistance R where R includes the uniformly distributed radiation resistance. On a high Zo line the radiation resistance is small compared with Zo and the only effect of the radiation resistance is to give Zo a small negative angle. Which when estimating Zo can be ignored. (It is conductor resistance which at HF gives Zo of ALL lines a very small negative angle). In the problem posed, the current is also uniformly distributed along the low-loss line and radiation resistance is not the value we are familiar with and what we might do with it. And so we get approximately - Rin = 120 * ( Ln( Wavelength / 2 / d ) - 1 ) At a wavelength of 2 metres and a conductor diameter of 10mm the input resistance = 433 ohms. I cannot guarantee the above formula to be correct. But is it low enough for you? ;o) Mr Wu calculates radiation resistance which is not the same as input impedance unless correctly referenced. It is usual in technical papers to calculate Radres at one end of the antenna. Or it may be the distributed value. I havn't the time to find and study the full text. From past experience, with me, it usually ends up as a wild goose chase. ---- Reg. Sounds reasonable, Reg. To put it for simple people like me, it would mean it's a transmission line of diameter x with an infinite diameter shield. Then we feed 2 of them, balanced, colinear, and that's our R sub r. Did I misunderstand? tom K0TAR |
#2
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Tom Ring wrote:
Sounds reasonable, Reg. To put it for simple people like me, it would mean it's a transmission line of diameter x with an infinite diameter shield. Then we feed 2 of them, balanced, colinear, and that's our R sub r. Did I misunderstand? Minus the radiation, of course. Now, the question is, how much does that change things. |
#3
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"Tom Ring" wrote in message . .. Tom Ring wrote: Sounds reasonable, Reg. To put it for simple people like me, it would mean it's a transmission line of diameter x with an infinite diameter shield. Then we feed 2 of them, balanced, colinear, and that's our R sub r. Did I misunderstand? Minus the radiation, of course. Now, the question is, how much does that change things. ================================== Tom, Twas not I who posed the original "input impedance of an infinitely long dipole" question. The radiation resistance does not enter very much into the solution. But in any case the radiation resistance is not the one we are accustomed to because the current distribution along the wire is not of sinewaveform but decays rather slowly exponentially. I've just remembered the name of the real antenna which best fits the problem. It is the terminated Beverage which is just a very long horizontal wire some distance above ground. The Zo of the Beverage is 60 * Ln( 4 * Height / d ) ohms, where d is wire diameter. When terminated its input resistance at LF is Zo (see the Bible, the ARRL Antenna Book). A typical value of Zo is 550 ohms. Note that radiation resistance does not enter the formula although it cannot be denied radiation does occur. The formula is a close approximation which serves present purposes. Putting two Beverages back-to-back to make a dipole we get an input impedance of 1100 ohms. The infinite dipole is in the same high impedance ballpark. To calculate Zo of an isolated infinite dipole we shall have to change dimensions Height above ground disappears and is replaced by wavelength (or frequency). Zo = 60 * ( Ln( Wavelength / 2 / d ) - 1 ) ohms, approximately. A more exact formula involves inverse hyperbolic functions and wavelength, height, wire length, and wire diameter, but nobody ever uses it. You won't find it in Terman. On the favourite American 40-meter band with a 14 AWG infinitely long wire Zo = 505 ohms. Which makes the dipole input impedance = 1010 ohms but a nice, round 1000 ohms is near enough for me. Your 'infinite shield' is a fair description for the return path but the end-effect is fairly large. I prefer 'the rest of the Universe'. But the nearest point is still the Earth's surface. I leave the people, who attempted to use Smith Charts and Eznec to solve the interesting problem, to fathom out where they (or the charts or Eznec) went wrong. ---- Reg. |
#4
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Reg Edwards wrote: The Zo of the Beverage is 60 * Ln( 4 * Height / d ) ohms, where d is wire diameter. When terminated its input resistance at LF is Zo (see the Bible, the ARRL Antenna Book). A typical value of Zo is 550 ohms. Note that radiation resistance does not enter the formula although it cannot be denied radiation does occur. The formula is a close approximation which serves present purposes. Putting two Beverages back-to-back to make a dipole we get an input impedance of 1100 ohms. The infinite dipole is in the same high impedance ballpark. To calculate Zo of an isolated infinite dipole we shall have to change dimensions Height above ground disappears and is replaced by wavelength (or frequency). Zo = 60 * ( Ln( Wavelength / 2 / d ) - 1 ) ohms, approximately. A more exact formula involves inverse hyperbolic functions and wavelength, height, wire length, and wire diameter, but nobody ever uses it. You won't find it in Terman. On the favourite American 40-meter band with a 14 AWG infinitely long wire Zo = 505 ohms. Which makes the dipole input impedance = 1010 ohms but a nice, round 1000 ohms is near enough for me. Your 'infinite shield' is a fair description for the return path but the end-effect is fairly large. I prefer 'the rest of the Universe'. But the nearest point is still the Earth's surface. I leave the people, who attempted to use Smith Charts and Eznec to solve the interesting problem, to fathom out where they (or the charts or Eznec) went wrong. ---- Reg. |
#5
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In message , Reg
Edwards writes "Tom Ring" wrote in message ... Tom Ring wrote: Sounds reasonable, Reg. To put it for simple people like me, it would mean it's a transmission line of diameter x with an infinite diameter shield. Then we feed 2 of them, balanced, colinear, and that's our R sub r. Did I misunderstand? Minus the radiation, of course. Now, the question is, how much does that change things. ================================== Tom, Twas not I who posed the original "input impedance of an infinitely long dipole" question. The radiation resistance does not enter very much into the solution. But in any case the radiation resistance is not the one we are accustomed to because the current distribution along the wire is not of sinewaveform but decays rather slowly exponentially. I've just remembered the name of the real antenna which best fits the problem. It is the terminated Beverage which is just a very long horizontal wire some distance above ground. The Zo of the Beverage is 60 * Ln( 4 * Height / d ) ohms, where d is wire diameter. When terminated its input resistance at LF is Zo (see the Bible, the ARRL Antenna Book). A typical value of Zo is 550 ohms. Note that radiation resistance does not enter the formula although it cannot be denied radiation does occur. The formula is a close approximation which serves present purposes. Putting two Beverages back-to-back to make a dipole we get an input impedance of 1100 ohms. The infinite dipole is in the same high impedance ballpark. To calculate Zo of an isolated infinite dipole we shall have to change dimensions Height above ground disappears and is replaced by wavelength (or frequency). Zo = 60 * ( Ln( Wavelength / 2 / d ) - 1 ) ohms, approximately. A more exact formula involves inverse hyperbolic functions and wavelength, height, wire length, and wire diameter, but nobody ever uses it. You won't find it in Terman. On the favourite American 40-meter band with a 14 AWG infinitely long wire Zo = 505 ohms. Which makes the dipole input impedance = 1010 ohms but a nice, round 1000 ohms is near enough for me. Your 'infinite shield' is a fair description for the return path but the end-effect is fairly large. I prefer 'the rest of the Universe'. But the nearest point is still the Earth's surface. I leave the people, who attempted to use Smith Charts and Eznec to solve the interesting problem, to fathom out where they (or the charts or Eznec) went wrong. ---- Reg. I'd like to say thanks to all who responded to my original question. There have been a variety of answers. The only answers I feel might be wrong are those where there is a reactive component. Surely, this can be removed by lengthening the antenna by the appropriate amount - but as it is already infinite, this might be difficult. Anyway, all very interesting. Cheers, Ian. -- |
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