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 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. |
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. |
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. -- |
"Cecil Moore" bravely wrote to "All" (19 Sep 05 09:27:25)
--- on the heady topic of " 73 Ohms, How do you get it?" CM Subject: 73 Ohms, How do you get it? CM Xref: core-easynews rec.radio.amateur.antenna:216868 CM pezSV7BAXdag wrote: The limit for Z does not exist or is (in general) the complex infinity. CM As the length of a dipole is increased, for the same CM power input, more energy is radiated during the first CM transcient cycle and less is available for reflection CM from the ends of the dipole. Reflected energy is what CM is causing the feedpoint impedance to change. As the CM length of the dipole is incrementally increased, the CM magnitude of the reflected energy is incrementally CM decreased. I believe Balanis alludes to this characteristic CM of standing-wave antennas. CM The feedpoint impedance is Zfp =3d (Vfor+Vref)/(Ifor+Iref) CM using phasor addition. CM The limit of that equation as Vref and Iref go to zero CM is Vfor/Ifor. That's what happens for an infinitely CM long dipole. That's also what happens during the transient CM phase of a finite dipole. Thus, Vfor/Ifor can be thought CM of as the characteristic impedance of the dipole. Seems CM to me, Vfor/Ifor could actually be measured during the CM transient phase of a long finite dipole. Will a TDR CM report the ratio of V/I for an RF pulse? Cecil, an infinitely long antenna is simply an impedance transformation between different mediums. i.e. wire to free space. A*s*i*m*o*v |
Asimov wrote:
Cecil, an infinitely long antenna is simply an impedance transformation between different mediums. i.e. wire to free space. We know one of the impedances to be 377 ohms. Question is, what is the other impedance? -- 73, Cecil, http://www.qsl.net/w5dxp |
It's so simple to derive the value of 73 ohms.
Just divide 146 by 2. Or multiply 36.5 by 2. There are other ways of doing it. |
Cec, if you divide the well-known 377 ohms by 5.164, which is the
square-root of 26.67, you get precisely 73 ohms. What's the problem? --- Reg |
Reg Edwards wrote:
Cec, if you divide the well-known 377 ohms by 5.164, which is the square-root of 26.67, you get precisely 73 ohms. But, but, but, Reg, you said the feedpoint impedance of an infinite dipole is 1200 ohms, or some such. That's very close to 377 ohms times pi. -- 73, Cecil http://www.qsl.net/w5dxp |
But, but, but, Reg, you said the feedpoint impedance of
an infinite dipole is 1200 ohms, or some such. That's very close to 377 ohms times pi. ================================ Cec, for once, you ageed with me. But, but, but, coincidences in arithmetic are certain to occur. ---- Reg. |
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