| Page 1 of 2 | 1 | 2 |
|
|
Antenna design question
While looking for a way to get a little more bandwidth out of an 80
meter antenna, I mocked up an antenna in EZNEC that was thick. I plan on making an antenna with spreaders and run 4 wires on each leg of the dipole. I emulated this in EZNEC by simply making the wire thickness quite thick, ranging from 4 inches to a foot. The interesting thing was that as the thickness increased, the antenna length decreased for min VSWR. Is this a real thing? |
Antenna design question
Yes.
- 'Doc |
Antenna design question
On Mon, 20 Oct 2008 08:40:35 -0400, Michael Coslo
wrote: While looking for a way to get a little more bandwidth out of an 80 meter antenna, I mocked up an antenna in EZNEC that was thick. I plan on making an antenna with spreaders and run 4 wires on each leg of the dipole. I emulated this in EZNEC by simply making the wire thickness quite thick, ranging from 4 inches to a foot. The interesting thing was that as the thickness increased, the antenna length decreased for min VSWR. Is this a real thing? Yep. It's called a "cage dipole". See: http://www.smeter.net/antennas/wire-cage-dipole.php with a program to help with the numbers: http://www.smeter.net/software/dipcage2.exe Even the ARRL uses them: http://www.arrl.org/news/stories/2001/05/03/2/ -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna design question
Yes, Mike, both Jeff and Doc are correct, the thicker dipole will be shorter
than a thin one for resonance at the same frequency.. Walt, W2DU |
Antenna design question
Michael Coslo wrote:
While looking for a way to get a little more bandwidth out of an 80 meter antenna, I mocked up an antenna in EZNEC that was thick. I plan on making an antenna with spreaders and run 4 wires on each leg of the dipole. I emulated this in EZNEC by simply making the wire thickness quite thick, ranging from 4 inches to a foot. The interesting thing was that as the thickness increased, the antenna length decreased for min VSWR. Is this a real thing? yes. It's often explained as "extra capacitance from the bigger size", but I think that's not what's really going on. |
Antenna design question
On Mon, 20 Oct 2008 08:40:35 -0400, Michael Coslo
wrote: Is this a real thing? Hi Mike, Push it further. There is a copy of my EZNEC file at: http://home.comcast.net/~kb7qhc/ante.../Cage/cage.htm 73's Richard Clark, KB7QHC |
Antenna design question
"Jim Lux" wrote in message ... Michael Coslo wrote: While looking for a way to get a little more bandwidth out of an 80 meter antenna, I mocked up an antenna in EZNEC that was thick. I plan on making an antenna with spreaders and run 4 wires on each leg of the dipole. I emulated this in EZNEC by simply making the wire thickness quite thick, ranging from 4 inches to a foot. The interesting thing was that as the thickness increased, the antenna length decreased for min VSWR. Is this a real thing? yes. It's often explained as "extra capacitance from the bigger size", but I think that's not what's really going on. The effect is clearly shown in Hallen's curves which appear in many of the standard text books (e.g. Jordan & Balmain, Electromagnetic waves and radiating systems). There's certainly a likelihood of greater shunt capacitance at the feed point, but if the limbs are made conical - radiating geometrically from the feedpoint - then this doesn't apply. In that case, Schelkunoff might be of some help. Chris |
Antenna design question
Jim Lux wrote:
"It`s often explained as "extra capacitance from the bigger size", but I think that`s not what`s really going on." Arnold B. Bailey in "TV and Other Receiving Antennas" agrees with Jim. Bailey writes on page 317: "We should expect such thin rods to be resonant when their physical length is slightly less than a free-space half-wave length. When the rod is thick, the effective velocity along the rod is considerably less than the free-space velocity, thus reducing the wavelength proportionally." The above may be grist for Arthur`s mill. Bailey produces emperical equations (equilibrium?), graphs, and worked examples for various cross sections. Best regards, Richard Harrison, KB5WZI |
Antenna design question
Richard Harrison wrote:
Jim Lux wrote: "It`s often explained as "extra capacitance from the bigger size", but I think that`s not what`s really going on." Arnold B. Bailey in "TV and Other Receiving Antennas" agrees with Jim. Bailey writes on page 317: "We should expect such thin rods to be resonant when their physical length is slightly less than a free-space half-wave length. When the rod is thick, the effective velocity along the rod is considerably less than the free-space velocity, thus reducing the wavelength proportionally." Some might argue, though, that the reason the effective velocity is less is because the sqrt(1/LC) term is smaller because C is bigger because of the increased surface area. And that might not be far from the truth for a restricted subset of antennas. All of this kind of confusion is trying to make one sort of model (a transmission line) fit something else (a radiator). Just like the things that treat the antenna as a lumped RLC. |
Antenna design question
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. The clarity to this confusion starts with the Biconical Dipole. S. A. Schelkunoff describes it as a "Linear" antenna and used transmission line math to build the mathematical model for the thin wire dipole in his classic publication "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) I'm inclined to allow the weight of his work stand until someone tips it - or can demonstrate I incorrectly read his thesis. Somehow given the weight of authorities (Ronold King being one) that cite him for this very reading (specific to the correlation) are abundant, I will await heavier lifting to tip the math. Accessible reference work can be found by searching the PTO with his patent number: 2235506. 73's Richard Clark, KB7QHC |
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 |
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 |
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! |
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 |
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 |
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 |
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! |
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 |
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. |
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..) |
Antenna design question
In message , Dave Platt
writes 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 :-) Yes, rule-of-thumb is more than good enough for me! I has a sneaky feeling that the feed impedance would end up at 377 ohms (impedance of free space). Many years ago, from some tables compiled by one of the many Wu's involved with antenna theory and design, I plotted Zin vs antenna length on a Smith chart. As the spiral progressively wound its way inwards with increasing antenna length, it seemed that it was heading for something between 200 and 600 ohms, so I thought to myself, "377 ohms?" Unfortunately, the table stopped when the antenna was about 5 wavelengths. I haven't seen similar tables since. -- Ian |
Antenna design question
On Oct 20, 5:57*pm, Jim Lux wrote:
.... Some might argue, though, that the reason the effective velocity is less is because the sqrt(1/LC) term is smaller because C is bigger because of the increased surface area. *And that might not be far from the truth for a restricted subset of antennas. On the other hand, the propagation velocity of coaxial cable of constant outer conductor ID is independent of the inner conductor diameter, even though the capacitance per unit length increases as the inner conductor diameter is increased. Clearly one must be careful about attributing the effect to a single cause like increased capacitance. I haven't noticed in this thread any reference to Ronold W. P. King's work. His writings should give more insight into the subject, if you can get deeply enough into them. It's discussed empirically in "Transmission Lines, Antennas and Waveguides," (with lots and lots of interesting graphs showing the effect from various viewpoints) but you can probably go deeper into the theory than you need in his other books and papers on linear antennas. Cheers, Tom |
Antenna design question
K7ITM wrote:
On Oct 20, 5:57 pm, Jim Lux wrote: ... Some might argue, though, that the reason the effective velocity is less is because the sqrt(1/LC) term is smaller because C is bigger because of the increased surface area. And that might not be far from the truth for a restricted subset of antennas. On the other hand, the propagation velocity of coaxial cable of constant outer conductor ID is independent of the inner conductor diameter, even though the capacitance per unit length increases as the inner conductor diameter is increased. Clearly one must be careful about attributing the effect to a single cause like increased capacitance. Which was the original intent of my comment. Fat radiators are shorter at resonance than thin ones, and the details of why are not simply explained by something like "capacitance effects", although such an explanation may sort of work over a limited range. |
Antenna design question
Jim Lux wrote:
K7ITM wrote: On Oct 20, 5:57 pm, Jim Lux wrote: ... Some might argue, though, that the reason the effective velocity is less is because the sqrt(1/LC) term is smaller because C is bigger because of the increased surface area. And that might not be far from the truth for a restricted subset of antennas. On the other hand, the propagation velocity of coaxial cable of constant outer conductor ID is independent of the inner conductor diameter, even though the capacitance per unit length increases as the inner conductor diameter is increased. Clearly one must be careful about attributing the effect to a single cause like increased capacitance. Which was the original intent of my comment. Fat radiators are shorter at resonance than thin ones, and the details of why are not simply explained by something like "capacitance effects", although such an explanation may sort of work over a limited range. Sorry, been away for a while, but I'm back. Certainly the capacitance may play some small part. But does added capacitance increase bandwidth to the extent - or at all - that is achieved by the cage or very thick dipole? Richard Harrison's reference to Baily regarding velocity is interesting. Why would the velocity be less at increased width? And would that increase the Bandwidth? - 73 de Mike N3LI - |
Antenna design question
Michael Coslo wrote:
Jim Lux wrote: K7ITM wrote: On Oct 20, 5:57 pm, Jim Lux wrote: ... Some might argue, though, that the reason the effective velocity is less is because the sqrt(1/LC) term is smaller because C is bigger because of the increased surface area. And that might not be far from the truth for a restricted subset of antennas. On the other hand, the propagation velocity of coaxial cable of constant outer conductor ID is independent of the inner conductor diameter, even though the capacitance per unit length increases as the inner conductor diameter is increased. Clearly one must be careful about attributing the effect to a single cause like increased capacitance. Which was the original intent of my comment. Fat radiators are shorter at resonance than thin ones, and the details of why are not simply explained by something like "capacitance effects", although such an explanation may sort of work over a limited range. Sorry, been away for a while, but I'm back. Certainly the capacitance may play some small part. But does added capacitance increase bandwidth to the extent - or at all - that is achieved by the cage or very thick dipole? Nope.. that's why "increased capacitance" is a bad model. Richard Harrison's reference to Baily regarding velocity is interesting. Why would the velocity be less at increased width? And would that increase the Bandwidth? larger C per unit length makes 1/sqrt(LC) smaller no for the BW |
Antenna design question
Mike, N3LI wrote:
"Why would the velocity be less at increased (antenna element) width?" Let B = the phase velocity on the antenna element, in radians per unit length. 2pi/B = wavelength on the element. Therefore, 2pi/B=velocity of phase propagation. Due to the behavior of of open-circuited transmission lines and open-circuited antennas: B=2pif times sq.rt. of LC radians / unit length. 2 pi f / B = velocity of propagation. It is intuitive that a fat antenna element has more L & C than a thin element and thus a lower velocity of propagation. Best regards, Richard Harrisob, KB5WZI |
Antenna design question
"Richard Harrison" wrote in message
... Mike, N3LI wrote: "Why would the velocity be less at increased (antenna element) width?" Let B = the phase velocity on the antenna element, in radians per unit length. 2pi/B = wavelength on the element. Therefore, 2pi/B=velocity of phase propagation. Due to the behavior of of open-circuited transmission lines and open-circuited antennas: B=2pif times sq.rt. of LC radians / unit length. 2 pi f / B = velocity of propagation. It is intuitive that a fat antenna element has more L & C than a thin element and thus a lower velocity of propagation. Best regards, Richard Harrisob, KB5WZI Hmmmm... my straight wire inductance equation from the ARRL handbook indicates smaller wire diameters have larger inductance. ??? 73, John |
Antenna design question
On Oct 22, 9:52*pm, "John KD5YI" wrote:
"Richard Harrison" wrote in message ... Mike, N3LI wrote: "Why would the velocity be less at increased (antenna element) width?" Let B = the phase velocity on the antenna element, in radians per unit length. 2pi/B = wavelength on the element. Therefore, 2pi/B=velocity of phase propagation. Due to the behavior of of open-circuited transmission lines and open-circuited antennas: B=2pif times sq.rt. of LC radians / unit length. 2 pi f / B = velocity of propagation. It is intuitive that a fat antenna element has more L & C than a thin element and thus a lower velocity of propagation. Best regards, Richard Harrisob, KB5WZI Hmmmm... my straight wire inductance equation from the ARRL handbook indicates smaller wire diameters have larger inductance. ??? 73, John Not surprisingly, that's what E&M texts say too--or leave as an exercise. With a larger diameter, there's less net magnetic field for a given current, so less energy stored, so less inductance. Cheers, Tom |
Antenna design question
In article , Richard Clark
wrote: Lest there be any confusion: an antenna IS a transmission line. Hello, and I think one would have to include two antennas and the intervening medium(s) for the above statement to make sense. In any event, the behavior of an antenna-medium-antenna as a passive 2-port device can be considered as a transmission line at a given frequency. The "loss" associated with this topology can be mitigated by keeping the two antennas within a near, rather than far, field separation. Over a range of frequencies the behavior of this 2-port can easily differ from that of a transmission line, though. In some electromagnetics textbooks an antenna is developed mathematically via the gradual unfolding of a twin-lead transmission line. And many hams know that a quick and dirty dipole can be created by simply folding the braid back on a length of coax so that the braid and the exposed center conductor become the radiating elements. A more correct statement might be that a transmission line can be an antenna. This can include unintended radiotion (e.g. RF flowing on the outside of caox due to imbalance and/or stray coupling) or intended such as Andrew's "Radiax" brand of leaky transmission line for installation in tunnels and elevator shafts as a convenient means to extend the reach of over-the-air broadcasts. Sincerely, and 73s from N4GGO, John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
Antenna design question
Richard Harrison wrote:
Mike, N3LI wrote: "Why would the velocity be less at increased (antenna element) width?" Let B = the phase velocity on the antenna element, in radians per unit length. 2pi/B = wavelength on the element. Therefore, 2pi/B=velocity of phase propagation. Due to the behavior of of open-circuited transmission lines and open-circuited antennas: B=2pif times sq.rt. of LC radians / unit length. 2 pi f / B = velocity of propagation. It is intuitive that a fat antenna element has more L & C than a thin element and thus a lower velocity of propagation. I thought that the inductance tends downward as the diameter of the wire increases. I can understand your calculation after the wavelength part, but don't quite get the increased inductance part. - 73 de Mike N3LI - |
Antenna design question
"K7ITM" wrote in message ... On Oct 22, 9:52 pm, "John KD5YI" wrote: "Richard Harrison" wrote in message ... Mike, N3LI wrote: "Why would the velocity be less at increased (antenna element) width?" Let B = the phase velocity on the antenna element, in radians per unit length. 2pi/B = wavelength on the element. Therefore, 2pi/B=velocity of phase propagation. Due to the behavior of of open-circuited transmission lines and open-circuited antennas: B=2pif times sq.rt. of LC radians / unit length. 2 pi f / B = velocity of propagation. It is intuitive that a fat antenna element has more L & C than a thin element and thus a lower velocity of propagation. Best regards, Richard Harrisob, KB5WZI Hmmmm... my straight wire inductance equation from the ARRL handbook indicates smaller wire diameters have larger inductance. ??? 73, John Not surprisingly, that's what E&M texts say too--or leave as an exercise. With a larger diameter, there's less net magnetic field for a given current, so less energy stored, so less inductance. Cheers, Tom So, then, it isn't intuitive (to me, at least) that a fat antenna has more inductance. Intuitive to me is that the reverse may be true. Cheers to you, too, Tom. John |
Antenna design question
John Wood, N4GGO wrote:
"---or intended such as Andrew`s "Radiax" brand of leaky transmission line for installation in tunnels and elevator shafts as a convenient means to extend the reach of over-the-air broadcasts." Similar results are obtained more cheaply by replacing the Radiax with 300-ohm twin-lead. Best regards, Richard Harrison, KB5WZI |
Antenna design question
Mike, N3LI wrote:
"I thought that the inductance tends donward as the diameter of the wire increases. I can understand your calculation after the wavelength part, but don`t quite get the increased inductance part." Good observation. Wire inductance decreases with the circumference increase as this effectively places more parallel inductors in place along the surface of the wire. Wire capacitance increases proportionally with the square of the circunference of the wire as it is proportional to the wire`s surface area. The fatter wire grows capacitance faster than it changes inductance. Reactance along a wire antenna element varies quickly near resonant and antiresonant points so is not uniformly distributed. This complicates calculations and requires average values for some. Bailey says of surge impedance: "Nevertheless, this variation in theoretical surge impedance shall not deter us from setting uup practical "average" values of surge impedance. Best regards, Richard Harrison, KB5WZI |
Antenna design question
"Richard Harrison" wrote in message ... Mike, N3LI wrote: "Why would the velocity be less at increased (antenna element) width?" Let B = the phase velocity on the antenna element, in radians per unit length. 2pi/B = wavelength on the element. Therefore, 2pi/B=velocity of phase propagation. Due to the behavior of of open-circuited transmission lines and open-circuited antennas: B=2pif times sq.rt. of LC radians / unit length. 2 pi f / B = velocity of propagation. It is intuitive that a fat antenna element has more L & C than a thin element and thus a lower velocity of propagation. Best regards, Richard Harrisob, KB5WZI If B is the 'phase velocity' then surely it's also the 'velocity of phase propagation' so that's not 2pi/B? The velocity of propagation of an electromagnetic wave, be it the phase or group velocity, is most-strongly dependent on the permittivity and permeability of the medium in which the wave is propagating. Another clue to resolution of this issue is that the terminal impedance of an open circuit stub has a slope with respect to frequency that depends on the characteristic impedance of the stub. For the case of a cylindrical dipole, the characteristic impedance is not constant but increases progressively along the lengths of the limbs, but the slope of the terminal impedance can be related to an 'average characteristic impedance' and the value of this parameter depends on the average distributed inductance and capacitance, as has been said here before. Z = sqrt(L/C) in general terms. The self-inductance of a conducting cylinder has been shown fairly rigorously by some (e.g. Rosa, and used by Terman) to be inversely proportional to its radial 'thickness' but this appears to be a contentious issue and can lead the unwary user into a paradox! Chris |
Antenna design question
|
Antenna design question
On Oct 23, 10:35*am, (Richard Harrison)
wrote: Mike, N3LI wrote: "I thought that the inductance tends donward as the diameter of the wire increases. I can understand your calculation after the wavelength part, but don`t quite get the increased inductance part." Good observation. Wire inductance decreases with the circumference increase as this effectively places more parallel inductors in place along the surface of the wire. Wire capacitance increases proportionally with the square of the circunference of the wire as it is proportional to the wire`s surface area. The fatter wire grows capacitance faster than it changes inductance. Reactance along a wire antenna element varies quickly near resonant and antiresonant points so is not uniformly distributed. This complicates calculations and requires average values for some. Bailey says of surge impedance: "Nevertheless, this variation in theoretical surge impedance shall not deter us from setting uup practical "average" values of surge impedance. * Best regards, Richard Harrison, KB5WZI I know we're talking about linear antennas here, but even in that case, it's surely not true that capacitance increases as the square of the wire diameter (or radius or circumference); nor inductance proportional to 1/diameter. Consider that if both those were true, doubling the wire diameter would quadruple the capacitance and halve the inductance, and the propagation velocity along that wire would be 1/sqrt(4*0.5) or about .707 times as great as with the thinner wire. Clearly things change much more gradually than that. In the controlled environment of a coaxial capacitor, the capacitance per unit length is proportional to 1/log(b/a), where a is the inner conductor diameter and b is the inside diameter of the outer conductor. If you change b/a from 10000 to 5000 (huge outer diameter, like a thin wire well away from ground), the capacitance increases by about 8 percent. Going from b/a = 100000 to 50000, the capacitance increases by a little over 6 percent. Similarly, inductance in coax is proportional to log(b/a), so in coax as you change the inner conductor diameter, the capacitance change offsets the inductance change exactly and the propagation velocity is unchanged. The environment of an antenna wire is different than that, but not so different that doubling the wire diameter has a drastic 30% effect on the resonant frequency. Cheers, Tom |
Antenna design question
In article , Richard Clark
wrote: On Thu, 23 Oct 2008 07:01:18 -0400, (J. B. Wood) wrote: In article , Richard Clark wrote: Lest there be any confusion: an antenna IS a transmission line. Hello, and I think one would have to include two antennas and the intervening medium(s) for the above statement to make sense. ... Over a range of frequencies the behavior of this 2-port can easily differ from that of a transmission line, though. It would appear your first sentence is contested by your last sentence in your reply. snip Hello, Richard, and all. And as I previously pointed out the 2-port model might not be the equivalent of a line in a broadband sense. Another way to put it would be that the 2-port could have the electrical characteristics (characteristic impedance, delay, loss) of a particular line at one frequency but of a different line at another frequency. Please excuse my snipping of the remainder of your comments but they sound more of philosophy than science and quite frankly I have no idea what you're talking about. You emphatically stated an antenna "IS" a transmission line without a few words on why this should be so. My take on a transmission line (or waveguide) is that it is a medium (ideally lossless) used to convey electromagnetic energy from one place to another. An antenna (or antenna array) is used to introduce or extract electromagnetic energy from a medium. Unlike the power available at the output of a low-loss transmission line, a receiving antenna operating at a far-field distance from a transmitter can only extract a macimum of 1/2 the power available from an incident electromagnetic wave. Now, if you meant that antennas and transmission lines share phenomena in common (e.g. standing waves) that would be a correct statement. And Maxwell's equations certainly apply to both. But I don't see an equivalency of a single antenna and a non-radiating (at least intended by design) transmission line and I don't recall any of my many electromagnetics texts making such a statement. Sincerely, What you are arguing is a failure of application, not a failure of the device. I've seen similar arguments that forced terms of transformer or transducer into the mix to show how they fail. I find the terms suitable in a casual discussion, but the new minted failures occur on the basis of forcing definitions when the casual applications worked just fine. One can, by a simple twist of the oscillator's frequency knob, find failure in all analogues of antennas, lumped circuits, and transmission lines. Those failures are not exotic perturbations in the 5th decimal place, but simple and utter refusals to conform to a general rule (such as my bald statement). For any attempt to refute my bald statement with "proven concepts" will reveal those challenging concepts built on a foundation of sand by a similar token of counter proof. 73's Richard Clark, KB7QHC John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 |
Antenna design question
K7ITM wrote:
On Oct 23, 10:35 am, (Richard Harrison) wrote: Mike, N3LI wrote: "I thought that the inductance tends donward as the diameter of the wire increases. I can understand your calculation after the wavelength part, but don`t quite get the increased inductance part." Good observation. Wire inductance decreases with the circumference increase as this effectively places more parallel inductors in place along the surface of the wire. Wire capacitance increases proportionally with the square of the circunference of the wire as it is proportional to the wire`s surface area. The fatter wire grows capacitance faster than it changes inductance. Reactance along a wire antenna element varies quickly near resonant and antiresonant points so is not uniformly distributed. This complicates calculations and requires average values for some. Bailey says of surge impedance: "Nevertheless, this variation in theoretical surge impedance shall not deter us from setting uup practical "average" values of surge impedance. Best regards, Richard Harrison, KB5WZI I know we're talking about linear antennas here, but even in that case, it's surely not true that capacitance increases as the square of the wire diameter (or radius or circumference); nor inductance proportional to 1/diameter. Consider that if both those were true, doubling the wire diameter would quadruple the capacitance and halve the inductance, and the propagation velocity along that wire would be 1/sqrt(4*0.5) or about .707 times as great as with the thinner wire. Clearly things change much more gradually than that. Trying to make a "readers Digest" version here.... If I'm following so far: The lowered frequency of resonance is due to changes in the velocity factor. The lowered vf is somewhat due to increased capacitance, and an increase in inductance - the latter part I'm still trying to grok. I think there is likely something more going on. I'm still left with the increased bandwidth phenomenon. None of the above would seem to account for this. I've been working with mobile antennas for the past several months, and I might be going astray, because I keep thinking about increased bandwidth as a partner of lowered efficiency. Not likely the case here. Thanks to everyone for the help, while I'm happy to accept the obvious real results, It is even better if I can understand what is going on. - 73 de Mike N3LI - |
Antenna design question
J. B. Wood wrote:
Hello, and I think one would have to include two antennas and the intervening medium(s) for the above statement to make sense. In any ... John Wood (Code 5550) e-mail: Naval Research Laboratory 4555 Overlook Avenue, SW Washington, DC 20375-5337 Absolutely, such as two antennas stuck into a superconducting media, which extends in all directions to the point of seemingly infinite distances. A rather unique way of viewing the phenomenon, and rarely presented in such terms. Yet, quite valid, nonetheless. Regards, JS |
Antenna design question
Tom, K7ITM wrote:
"The environment of an antenna wire is different from that, but not so different that doubling the diameter has a drastic 30% effect on the resonant frequency." Thanks to Tom for his observation. It makes sense to me. I was wrong. Best regards, Richard Harrison, KB5WZI |
| All times are GMT +1. The time now is 08:02 PM. |
|
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com