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Antenna wires and ferrite
Let's start a parallel thread on the effect of coating a 14-gauge
antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? ---- Reg |
Reg Edwards wrote: Let's start a parallel thread on the effect of coating a 14-gauge antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? a.yes b. don't know Perhaps you could look at it from the point of view of an increase in the inductance per unit length. Or else model it with a 3D E-M simulator. alan |
Reg Edwards wrote:
Let's start a parallel thread on the effect of coating a 14-gauge antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? Why not start with an insulating material like Teflon? That's what the previous discussion was about. Given the dielectric constant of Teflon and the thickness, EZNEC+ 4.0 will take the VF into account. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Reg Edwards wrote:
Let's start a parallel thread on the effect of coating a 14-gauge antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? Something easier for me to do was to take the 20m dipole, DIPTL.EZ, that came with EZNEC+ 4.0, remove the transmission line, and determine the resonant frequency for uninsulated wire and for wire insulated with 0.1 inch of neoprene with a dielectric constant of 6.7 (deliberately chosen to emphasize the differences). The resonant frequency for uninsulated wire was 14.42 MHz. The resonant frequency for neoprene insulated wire was 13.3 MHz. That is an abundant amount of insulation with a high dielectric constant and it lowered the resonant frequency by 7.8% according to EZNEC. Adding the insulation increased the feedpoint impedance from 57 ohms to 65 ohms which means the forward and reflected waves on the standing-wave antenna were attenuated more using insulated wire and sure enough, using that particular insulation reduced the EZNEC maximum gain by 0.12 dB. Many people have noticed shifts in resonant frequency when their antenna gets wet. Water has a dielectric constant around 80. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Reg Edwards wrote:
Let's start a parallel thread on the effect of coating a 14-gauge antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? ---- Reg In his book _Ferromagnetic Core Design & Application Handbook_ Doug DeMaw claimed to have put ferrite sleeves on a vhf dipole which reduced its size without introducing significant loss. He claimed to have cut the size of the dipole in half. You'll have to do the same thing yourself if you want to know whether or not he was right. 73, Tom Donaly, KA6RUH |
interesting.
if coating the antenna with ferrite can reduce its size, would ferrite sleeves over the ferrite sleeves reduce the size even further? we're always looking for ways of reducing the size of our dipoles. "Tom Donaly" wrote in message m... Reg Edwards wrote: Let's start a parallel thread on the effect of coating a 14-gauge antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? ---- Reg In his book _Ferromagnetic Core Design & Application Handbook_ Doug DeMaw claimed to have put ferrite sleeves on a vhf dipole which reduced its size without introducing significant loss. He claimed to have cut the size of the dipole in half. You'll have to do the same thing yourself if you want to know whether or not he was right. 73, Tom Donaly, KA6RUH |
Tom Donaly wrote: Reg Edwards wrote: Let's start a parallel thread on the effect of coating a 14-gauge antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? ---- Reg In his book _Ferromagnetic Core Design & Application Handbook_ Doug DeMaw claimed to have put ferrite sleeves on a vhf dipole which reduced its size without introducing significant loss. He claimed to have cut the size of the dipole in half. You'll have to do the same thing yourself if you want to know whether or not he was right. 73, Tom Donaly, KA6RUH Yea, but isn't that the same thing as winding a helix to increase the inductance per unit length to accomplish the same results. I don't know if Doug was right, cause I have not done either. May try it. Gary N4AST |
Hal Rosser wrote:
interesting. if coating the antenna with ferrite can reduce its size, would ferrite sleeves over the ferrite sleeves reduce the size even further? we're always looking for ways of reducing the size of our dipoles. "Tom Donaly" wrote in message m... Reg Edwards wrote: Let's start a parallel thread on the effect of coating a 14-gauge antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? ---- Reg In his book _Ferromagnetic Core Design & Application Handbook_ Doug DeMaw claimed to have put ferrite sleeves on a vhf dipole which reduced its size without introducing significant loss. He claimed to have cut the size of the dipole in half. You'll have to do the same thing yourself if you want to know whether or not he was right. 73, Tom Donaly, KA6RUH Balanis, in his book _Antenna Theory, Analysis and Design_, has a short section dealing with this. Define a parameter Q = (mu - 1)ln(b/a), where mu is complex permeability of the ferrite, a is the radius of the conducting wire, and b is the radius of the conducting wire plus the ferrite. According to Balanis, increasing the real part of Q "a. increases the peak input admittance b. increases the electrical length (lowers the resonant frequency c. narrows the bandwidth." In order to use this formula, you have to know the complex permeability of the ferrite coating. I don't know how you'd measure that. Maybe Richard Clark knows. It would be fun to try. I wouldn't pin any hopes on it being practical, though, since it doesn't seem to be in general use anywhere. 73, Tom Donaly, KA6RUH |
You can measure the complex impedance of a ferrite core quite easily and
with moderate accuracy using an antenna analyzer. From that reading and a low frequency impedance measurement, you could calculate the complex permeability. However, you can find graphs of the values for common ferrite types at http://www.conformity.com/040spotlight.pdf and other web sources. But it's not obvious to me why you'd need to calculate or measure the complex permeability -- all you need to do is measure the impedance of a short wire with the core slipped over it. When you slip the core over the antenna, it'll behave just as though an impedance of that value was inserted in series with the antenna wire at that point. Different types of ferrites are quite different at HF. Low frequency ferrites like the Fair-Rite 70 series are primarily resistive at HF, and would simply add loss to an antenna like adding a series resistor. High frequency types like the 60 series are inductive with reasonable Q through the HF range so would behave pretty much like a series inductor of moderate Q. Type 43, probably the most common type now available, has a Q on the order of 1 at HF, so it also would primarily just add loss to an antenna. But hey, if you use one of the lossy ferrites you'll end up with an antenna that's really broadband and quiet. That's what we all want, isn't it? Roy Lewallen, W7EL Tom Donaly wrote: Balanis, in his book _Antenna Theory, Analysis and Design_, has a short section dealing with this. Define a parameter Q = (mu - 1)ln(b/a), where mu is complex permeability of the ferrite, a is the radius of the conducting wire, and b is the radius of the conducting wire plus the ferrite. According to Balanis, increasing the real part of Q "a. increases the peak input admittance b. increases the electrical length (lowers the resonant frequency c. narrows the bandwidth." In order to use this formula, you have to know the complex permeability of the ferrite coating. I don't know how you'd measure that. Maybe Richard Clark knows. It would be fun to try. I wouldn't pin any hopes on it being practical, though, since it doesn't seem to be in general use anywhere. 73, Tom Donaly, KA6RUH |
On Sun, 03 Apr 2005 00:53:34 GMT, "Tom Donaly"
wrote: In order to use this formula, you have to know the complex permeability of the ferrite coating. I don't know how you'd measure that. Maybe Richard Clark knows. Hi Tom, I've measured a number of ferrites, but only in the HF region. They do show a range of values, with most of them not very reactive (in relation to the R). 73's Richard Clark, KB7QHC |
"Cecil Moore" bravely wrote to "All" (02 Apr 05 09:57:02)
--- on the heady topic of " Antenna wires and ferrite" CM From: Cecil Moore CM Xref: aeinews rec.radio.amateur.antenna:27806 [,,,] CM Adding the insulation increased the feedpoint impedance from CM 57 ohms to 65 ohms which means the forward and reflected CM waves on the standing-wave antenna were attenuated more CM using insulated wire and sure enough, using that particular CM insulation reduced the EZNEC maximum gain by 0.12 dB. CM Many people have noticed shifts in resonant frequency when CM their antenna gets wet. Water has a dielectric constant CM around 80. Speaking of odd antennas, how about using a long neon tube as an antenna and what would eznec give as values then? After all a plasma behaves like a conductor doesn't it? A*s*i*m*o*v .... Email returned to sender -- insufficient voltage. |
On Sat, 2 Apr 2005 17:15:03 -0500, "Hal Rosser"
wrote: interesting. if coating the antenna with ferrite can reduce its size, would ferrite sleeves over the ferrite sleeves reduce the size even further? we're always looking for ways of reducing the size of our dipoles. And conversely, judging by the number of email offers I receive, always looking for ways to *increase* the size of our monopoles. |
And conversely, judging by the number of email offers I receive,
always looking for ways to *increase* the size of our monopoles. ============================= A sort of a ferrite-viagra ointment? |
Asimov wrote:
Speaking of odd antennas, how about using a long neon tube as an antenna and what would eznec give as values then? Back in college, we used to use a florescent bulb to detect RF electric fields. What's the feedpoint impedance of a neon tube? -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
"Cecil Moore" bravely wrote to "All" (03 Apr 05 08:38:34)
--- on the heady topic of " Antenna wires and ferrite" CM From: Cecil Moore CM Xref: aeinews rec.radio.amateur.antenna:27837 CM Asimov wrote: Speaking of odd antennas, how about using a long neon tube as an antenna and what would eznec give as values then? CM Back in college, we used to use a florescent bulb CM to detect RF electric fields. What's the feedpoint CM impedance of a neon tube? I suppose the plasma looks like a DC resistance over a region of its characteristic and maybe even negative at some point. It depends of course if we are talking about a glow or an arc. With an arc the current discharge is somewhat infinite and the huge noise makes it impractical. I'm not sure about the noise with glow discharge on a long neon tube but I'm assuming it is very low judging from some brief measurements I made on a neon indicator bulb. BTW these make good UV detectors wrapped in aluminium and biased at the conduction threshold. I'm only guessing the long neon tube has a DC resistance of about 50 ohms per foot. Something 30 feet high would therefore be around 1.5K. What are typical running voltages and currents for neon signs? BTW don't know if running 1KW would it make a fabulous light show? Shades of Nicolai Tesla! A*s*i*m*o*v .... Speeding doesn't kill people... Stopping really fast does! |
My dear friend Cecil,
It's a waste of time mentioning things like DIP.TL.EZ and EZNEC4. Hardly anybody has ever heard of whatever they are. I certainly havn't. And the chances of obtaining them, even if legal, within the next 12 months is so remote, by then, everybody will have forgotten what it's all about and will have lost interest in the subject. So nobody ever takes any notice of references and switches to another more-interesting thread on the newsgroup. If you have any facts to say then say them. It's up to you to be convincing. If you think you need the support of Terman or Kraus then you lack self-confidence. Bibles are usually misquoted, or taken out of context anyway. Second-hand, plagiarised, information adds nothing to reliability. As usual, you gave only half of the information needed to make sense. In addition to a thick neoprene layer of 0.1 inches, with a high permittivity of 6.7, what was the antenna wire diameter and the approximate height above ground? Without such details your information is old-wives' waffle. As things are, your velocity factor reduction of 7.8% does not go out of the ball park value predicted by my formula. My formula takes a few milliseconds to calculate. Whereas your method requires a 4-weeks training course and several hours making the model. ---- Reg,G4FGQ |
Reg Edwards wrote:
It's a waste of time mentioning things like DIP.TL.EZ and EZNEC4. Hardly anybody has ever heard of whatever they are. I certainly havn't. And the chances of obtaining them, even if legal, within the next 12 months is so remote, by then, everybody will have forgotten what it's all about and will have lost interest in the subject. EZNEC is available in a free demo version from www.eznec.com You can enter dielectric constant and thickness of insulation. If you have any facts to say then say them. It's up to you to be convincing. I once replaced an uninsulated loop with insulated wire to try to reduce wind static/noise in AZ. The resonant frequency went down by about 200 kc on 40m. As things are, your velocity factor reduction of 7.8% does not go out of the ball park value predicted by my formula. Maybe I misunderstood. I thought you were saying insulation has no effect. My formula takes a few milliseconds to calculate. Whereas your method requires a 4-weeks training course and several hours making the model. Some training is worth it. My training using ELNEC and later EZNEC has been very valuable. I certainly wouldn't spend "several hours" on a model only to report the results in one thread on this newsgroup. That's not enough return on investment. It took me about seven minutes for that last report. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Oh my. It is time for a story. My students use superb oscilloscopes that
are actually computers with a D-to-A converter used to display waveforms. When measuring things such as the peak-to-peak size of a periodic waveform, students (being students) initially writedown the number exported by the oscilloscope. Whereupon, I ask the student if they wrote the computer code that exported the number and, if not, why did they believe the result. The student is sent back to note the max. and min. value, and to perform the reliable calculation of subtraction. NEC source code exists, is understandable, and has been verified many times by independent persons. I teach my students to avoid using tools that can not be verified at a fundamental level. Of course, rules-of-thumb are an important part of error checking, and somethings are only amiable of being approximated with a heuristic equation. Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Reg Edwards" snip My formula takes a few milliseconds to calculate. Whereas your method requires a 4-weeks training course and several hours making the model. ---- Reg,G4FGQ |
Reg Edwards wrote: Let's start a parallel thread on the effect of coating a 14-gauge antenna wire with a thick layer of ferrite. Say 1mm thick, permeability = 100. Would this have any effect on velocity factor? If so, by how much? ---- Reg The way it looks to me, the speed of propagation is pretty much the inverse of the squareroot of the product of mu and epsilon for the dielectric between conductors. ac6xg |
Jim Kelley wrote:
The way it looks to me, the speed of propagation is pretty much the inverse of the squareroot of the product of mu and epsilon for the dielectric between conductors. That's almost correct, but not quite. You need to modify it by changing "the dielectric between conductors" to "the medium containing the fields". Inside a coaxial cable, both are the same, so you can easily calculate the velocity factor from the dielectric constant (relative epsilon) of the dielectric. In the case of ladder line, TV twinlead, or microstrip line, though, part of the field is in the dielectric and part is in the air. So the velocity factor is a function of the dielectric constants of both. Often, an "effective" dielectric constant is calculated that fits the rule you mentioned(*). For the types of line I mentioned, it's between those of air and the dielectric material. It's not at all trivial to calculate, so it's usually determined by measurement or a field-solving computer program. In the case of an insulated antenna wire or one with a ferrite core on the outside, the "other conductor" is usually a very great distance away so the vast majority of the field is in the air. Also, the simple formula you refer to might not apply when the distance between conductors is a substantial fraction of a wavelength or more. If you take a piece of coax with solid polyethylene dielectric and measure its velocity factor, you'll find it to be around 0.66 (following the formula you mention). But if you strip off the shield and use the same center wire and insulation for an antenna, you'll find the insulation slows the wave on the antenna by only a few percent (almost certainly less than five). (*) In the case of microstrip line, the field distribution changes with frequency. This results in an effective dielectric constant, and hence velocity factor, which changes with frequency. With something like Teflon dielectric, which has a relatively low dielectric constant, this change isn't much. But it sure gave me grief when designing time-domain circuitry using microstrip lines on an alumina substrate (dielectric constant ~ 10), where the change was much greater. Roy Lewallen, W7EL |
Roy Lewallen wrote:
Jim Kelley wrote: The way it looks to me, the speed of propagation is pretty much the inverse of the squareroot of the product of mu and epsilon for the dielectric between conductors. That's almost correct, but not quite. You need to modify it by changing "the dielectric between conductors" to "the medium containing the fields". Inside a coaxial cable, both are the same, so you can easily calculate the velocity factor from the dielectric constant (relative epsilon) of the dielectric. In the case of ladder line, TV twinlead, or microstrip line, though, part of the field is in the dielectric and part is in the air. Yes, air is obviously also a dielectric. So the velocity factor is a function of the dielectric constants of both. Often, an "effective" dielectric constant is calculated that fits the rule you mentioned(*). For the types of line I mentioned, it's between those of air and the dielectric material. It's not at all trivial to calculate, so it's usually determined by measurement or a field-solving computer program. Exactly. That's why I chose to adhere as strictly as possible to absolute generalities. ;-) In the case of an insulated antenna wire or one with a ferrite core on the outside, the "other conductor" is usually a very great distance away so the vast majority of the field is in the air. Also, the simple formula you refer to might not apply when the distance between conductors is a substantial fraction of a wavelength or more. If you take a piece of coax with solid polyethylene dielectric and measure its velocity factor, you'll find it to be around 0.66 (following the formula you mention). But if you strip off the shield and use the same center wire and insulation for an antenna, you'll find the insulation slows the wave on the antenna by only a few percent (almost certainly less than five). I think in the case where the distance between conductors is much larger than the diameter of the conductor, a better form would probably be one over the squareroot of the product of inductance per unit length and capacitance per unit length. (*) In the case of microstrip line, the field distribution changes with frequency. This results in an effective dielectric constant, and hence velocity factor, which changes with frequency. With something like Teflon dielectric, which has a relatively low dielectric constant, this change isn't much. But it sure gave me grief when designing time-domain circuitry using microstrip lines on an alumina substrate (dielectric constant ~ 10), where the change was much greater. Roy Lewallen, W7EL Thanks for the excellent tutorial, Roy. 73, ac6xg |
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