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#11
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Antenna design question
Richard Clark wrote:
Lest there be any confusion: an antenna IS a transmission line. In fact, there is a formula for calculating the Z0 of a single horizontal transmission line wire above ground. #14 wire at 30 feet is very close to 600 ohms. #14 wire at 30 feet describes a lot of dipoles. -- 73, Cecil http://www.w5dxp.com |
#12
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Antenna design question
In message , Cecil Moore
writes Richard Clark wrote: Lest there be any confusion: an antenna IS a transmission line. In fact, there is a formula for calculating the Z0 of a single horizontal transmission line wire above ground. #14 wire at 30 feet is very close to 600 ohms. #14 wire at 30 feet describes a lot of dipoles. Are there any calculations or charts for centre impedance of a dipole in free space, starting from zero length, and going out to infinity? -- Ian |
#13
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Antenna design question
In article ,
Ian Jackson wrote: Are there any calculations or charts for centre impedance of a dipole in free space, starting from zero length, and going out to infinity? I think that what you're looking for is in Kraus "Antennas for All Applications", page 446 - "Self-impedance of a thin linear antenna". The formula given is based on the induced-EMF method... it's an approximation which apparently works well for cylindrical antennas whose length is at least 100x the diameter. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#14
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Antenna design question
On Tue, 21 Oct 2008 13:30:39 +0100, Ian Jackson
wrote: Are there any calculations or charts for centre impedance of a dipole in free space, starting from zero length, and going out to infinity? Institutional memory here is so slight: "Theory of antennas of arbitrary size and shape," Proc. I.R.E., 29, 493, 1941 and S. A. Schelkunoff, "Advanced Antenna Theory, " John Wiley and Sons, Inc., New York, (1952) Accessible reference work can be found by searching the PTO with his patent number: 2235506. 73's Richard Clark, KB7QHC |
#15
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Antenna design question
In message , Dave Platt
writes In article , Ian Jackson wrote: Are there any calculations or charts for centre impedance of a dipole in free space, starting from zero length, and going out to infinity? I think that what you're looking for is in Kraus "Antennas for All Applications", page 446 - "Self-impedance of a thin linear antenna". The formula given is based on the induced-EMF method... it's an approximation which apparently works well for cylindrical antennas whose length is at least 100x the diameter. Thanks for that. I've found a free download of a PDF copy (18MB) at: http://www.badongo.com/file/9893801 I'll have a look to see if it is what I want. I would have thought that the feed impedance of a dipole at a wide range of frequencies/lengths (ie 'very short' to 'very long') would have been fairly typical rule-of-thumb required information for those interested in antennas. However, it does not seem to be! -- Ian |
#16
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Antenna design question
In message , Richard Clark
writes On Tue, 21 Oct 2008 13:30:39 +0100, Ian Jackson wrote: Are there any calculations or charts for centre impedance of a dipole in free space, starting from zero length, and going out to infinity? Institutional memory here is so slight: "Theory of antennas of arbitrary size and shape," Proc. I.R.E., 29, 493, 1941 and S. A. Schelkunoff, "Advanced Antenna Theory, " John Wiley and Sons, Inc., New York, (1952) Accessible reference work can be found by searching the PTO with his patent number: 2235506. 73's Richard Clark, KB7QHC Thanks. As I said in my reply to Dave Platt, I would have thought that the feed impedance of a dipole over a wide range of frequencies/lengths (ie 'very short' to 'very long') would have been fairly typical rule-of-thumb required information for those interested in antennas. However, this does not seem to be the case. -- Ian |
#17
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Antenna design question
In article ,
Ian Jackson wrote: I would have thought that the feed impedance of a dipole at a wide range of frequencies/lengths (ie 'very short' to 'very long') would have been fairly typical rule-of-thumb required information for those interested in antennas. However, it does not seem to be! Oh... if rule-of-thumb is good enough for your needs, then it's not too difficult to summarize. There's a nice chart on page 2-3 of the ARRL Antenna Book. You should consider the resistive, and reactive portions of the feedpoint impedance separately. The resistive part rises from zero, up through a nominal 50 ohms or so at resonance (just under 1/2 wavelength), up to several thousand ohms at second (or anti-) resonance. If you plot the impedance-vs.- resistance relationship with the doublet length on a linear scale and the resistance on a logarithmic scale, it's not too far from being a straight line through much of this range. Between second and third resonance, the resistance drops back down to around 100 ohms... between third and fourth, up to several thousand ohms again, and so forth. As the doublet continues to get longer, the feedpoint resistance oscillates between low (odd-resonant) and high (even- or anti-resonant) values, with the oscillation becoming less and less as the doublet gets longer (think of a damped sine wave). In theory it'll eventually settle down to 377 ohms. The reactive portion of the impedance also oscillates as the doublet gets longer and longer. Between an even-numbered and odd-numbered resonance it's capacitive, dropping from thousands of ohms of negative reactance, to zero at the odd resonance. It then becomes inductive, rising to several thousand ohms just before the next even (anti-) resonant length is reached. As the even-numbered resonance length is passed it falls abruptly from very positive (inductive) to very negative (capacitive), and then begins to return slowly to zero at the next odd resonance. These excursions from positive (inductive) to negative (capacitive) continue, and also fall in their absolute value as the doublet gets longer and longer. Once the doublet is "sufficiently long" its reactance pretty much vanishes and it looks like a 377-ohm resistance. Near the resonant lengths, the value of the reactance is changing rather more rapidly than the value of the resistance. The same basic principles apply fairly well to doublets that aren't in free space, but ground reflections, mutual coupling with other antenna elements, etc. have a big effect on the actual values. Few of us have the luxury of stringing up an 80-meter longwire doublet in free space, alas :-) -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#18
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Antenna design question
Ian wrote:
"Are there any calculations or charts for centre impedance of a dipole in free space, starting from zero length, and going out to infinity?" It gets repetitive after a while. Arnold B. Bailey has "Graph of the resistance of a center-fed antenna near first resonance and below" on page 343 in "TV and Other Receiving Antennas". Then on page 348 Bailey has: "Various orders of resonance of thin center-fed antennas, showing the current loops and approximate radiation and antenna resistance in each case.". Best regards, Richard Harrison, KB5WZI |
#19
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Antenna design question
Richard Clark wrote:
On Mon, 20 Oct 2008 17:57:03 -0700, Jim Lux wrote: All of this kind of confusion is trying to make one sort of model (a transmission line) fit something else (a radiator). Hi Jim, I've seen this kind of assertion made before, generally as a blanket prohibition/warning/incantation/supplication/condemnation - but never with any demonstrable problem that wasn't an example of designed-in failure suited to the argument. Lest there be any confusion: an antenna IS a transmission line. You're right, but in many situations, it's not a uniform transmission line, by any means. The Schelkunoff analysis is quite elegant. |
#20
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Antenna design question
Dave Platt wrote:
I think that what you're looking for is in Kraus "Antennas for All Applications", page 446 - "Self-impedance of a thin linear antenna". The formula given is based on the induced-EMF method... it's an approximation which apparently works well for cylindrical antennas whose length is at least 100x the diameter. And, interestingly, a LOT of amateur antennas don't meet this slenderness constraint. Wire dipoles hanging in the air do. Fans, cages, etc., often don't. No problem with the model, just awareness of the footnotes and limitations (which often get omitted in the less rigorously reviewed internet literature..) |
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