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#1
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Richard Harrison wrote:
Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. Roy Lewallen, W7EL |
#2
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On Fri, 14 Mar 2008 11:57:41 -0700, Roy Lewallen wrote:
Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. If what I suspect is true, would not the coax also need to be scaled? - 73 de Mike N3LI - -- -73 de Mike N3LI - |
#3
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Mike Coslo wrote:
On Fri, 14 Mar 2008 11:57:41 -0700, Roy Lewallen wrote: Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. If what I suspect is true, would not the coax also need to be scaled? Dunno. What do you suspect? Roy Lewallen, W7EL |
#4
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Roy Lewallen wrote:
Mike Coslo wrote: On Fri, 14 Mar 2008 11:57:41 -0700, Roy Lewallen wrote: Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. If what I suspect is true, would not the coax also need to be scaled? Dunno. What do you suspect? I suspect that the antenna is a tuned circuit on top of coax, and it needs that coax to radiate effectively. So just scaling the antenna wouldn't translate to the same results? - 73 de Mike N3LI - |
#5
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Michael Coslo wrote:
Roy Lewallen wrote: Mike Coslo wrote: On Fri, 14 Mar 2008 11:57:41 -0700, Roy Lewallen wrote: Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. If what I suspect is true, would not the coax also need to be scaled? Dunno. What do you suspect? I suspect that the antenna is a tuned circuit on top of coax, and it needs that coax to radiate effectively. So just scaling the antenna wouldn't translate to the same results? Yes. If the coax is radiating, it's part of the antenna. To make an accurate scale model of the antenna, you have to scale the entire antenna (that is, every radiating conductor), not just some part of it which someone has declared to be "The Antenna". In this case, however, radiating coax isn't likely to be a major fraction of the total loss, so scaling it in a model probably wouldn't make much difference to the loss. Its diameter might have a noticeable effect on how much current it gets and therefore how much it radiates, though, which is an argument in favor of scaling it. Roy Lewallen, W7EL |
#6
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On Mar 18, 12:26 pm, Michael Coslo wrote:
Roy Lewallen wrote: Mike Coslo wrote: On Fri, 14 Mar 2008 11:57:41 -0700, Roy Lewallen wrote: Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. If what I suspect is true, would not the coax also need to be scaled? Dunno. What do you suspect? I suspect that the antenna is a tuned circuit on top of coax, and it needs that coax to radiate effectively. So just scaling the antenna wouldn't translate to the same results? - 73 de Mike N3LI - If you are familiar with computor programming then why not model it instead of repeating over and over again this transmission line radiation theory.? |
#7
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Art Unwin wrote:
On Mar 18, 12:26 pm, Michael Coslo wrote: Roy Lewallen wrote: Mike Coslo wrote: On Fri, 14 Mar 2008 11:57:41 -0700, Roy Lewallen wrote: Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. If what I suspect is true, would not the coax also need to be scaled? Dunno. What do you suspect? I suspect that the antenna is a tuned circuit on top of coax, and it needs that coax to radiate effectively. So just scaling the antenna wouldn't translate to the same results? - 73 de Mike N3LI - If you are familiar with computor programming then why not model it instead of repeating over and over again this transmission line radiation theory.? It comes up in the conversation Art, I only rinse and repeat as necessary. I'm still trying to wrap my mind around the antenna, I'm nowhere near ready to model it. So as to not make any ignorant mistakes, the antenna is counter-wound inductors, correct? and they are concurrently wound, as in they sort of weave against each other? And this is a full wave antenna? Do you use enameled wire, or what is the insulation? I'm assuming that this might be important in regards to VF. - 73 de Mike N3LI - |
#8
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On Mar 19, 7:27 am, Michael Coslo wrote:
Art Unwin wrote: On Mar 18, 12:26 pm, Michael Coslo wrote: Roy Lewallen wrote: Mike Coslo wrote: On Fri, 14 Mar 2008 11:57:41 -0700, Roy Lewallen wrote: Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. If what I suspect is true, would not the coax also need to be scaled? Dunno. What do you suspect? I suspect that the antenna is a tuned circuit on top of coax, and it needs that coax to radiate effectively. So just scaling the antenna wouldn't translate to the same results? - 73 de Mike N3LI - If you are familiar with computor programming then why not model it instead of repeating over and over again this transmission line radiation theory.? It comes up in the conversation Art, I only rinse and repeat as necessary. I'm still trying to wrap my mind around the antenna, I'm nowhere near ready to model it. So as to not make any ignorant mistakes, the antenna is counter-wound inductors, correct? and they are concurrently wound, as in they sort of weave against each other? And this is a full wave antenna? Do you use enameled wire, or what is the insulation? I'm assuming that this might be important in regards to VF. - 73 de Mike N3LI - I think you should forget the whole idea. We have another expert on line that can voutch for the fact that it is just a dummy load. He joins the majority and I am only one,.....who actually has one no less.! Art |
#9
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![]() "Roy Lewallen" wrote in message ... Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. Roy Lewallen, W7EL Hi Roy I'm curious to know why you didnt use stainless steel as an example rather than lead as your example. Jerry |
#10
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Jerry wrote:
"Roy Lewallen" wrote in message ... Richard Harrison wrote: Build a small scale model that can be tested indoors and report its characteristics. Antennas are scaleable. That's more easily said than done. One of the critical characteristics of a small antenna is loss. And to correctly replicate loss in a scaled antenna requires scaling the conductivity of the conductors as the square root of the frequency. To scale to a higher frequency requires that the conductivity be better than the original. Unless the original is made from lead and the scale factor moderate, this wouldn't be possible. Roy Lewallen, W7EL Hi Roy I'm curious to know why you didnt use stainless steel as an example rather than lead as your example. The main reason is that many years ago I was involved in using physical models scaled up in size in order to optimize microstrip transitions and other features used for time domain equipment having rise times on the order of 10 ps. At that time, I looked to see if it was possible to model the loss accurately, and found I'd need a semiconductor to do it. I recall that lead was about the least conductive common metal available. So I tossed that one out in my example. I don't remember investigating stainless or other steels, but that might indeed be a way to do it. To make a strictly accurate scale model, the permeability and permittivity (dielectric constant) stay fixed with frequency, so a non-magnetic stainless steel would be necessary. However, since the skin depth is inversely proportional to the square root of permeability, a magnetic material has the loss of a non-magnetic material having a conductivity lower by a factor equal to its relative permeability. So a magnetic material such as steel or magnetic stainless steel might be used to extend the range of possible loss values available for making larger physical models. There's a very large number of steel and stainless steel alloys, and good information on the required parameters can be hard or impossible to find. So samples would probably have to be measured. In the case in question, however, a smaller scale model was proposed, and we can probably assume that the original is made from copper. So it's not really possible to create a smaller scale model which accurately imitates the loss of the original. Roy Lewallen, W7EL |
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