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Locked Modeling inductively loaded loop (was Re: Re:)
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There has been some discussion about the difficulty
of modeling a helical loaded loop. In the case of a helically loaded vertical, we know that there is nothing magic about helical loading compared to lumped element inductor loading. IOW, a series of lumped coils will do much the same thing as helical loading. EZNEC, according to the manual, can model square loops down to 0.05 wavelength perimeter, with one segment per side. I modeled a .1 wavelength loop, putting a current source in one side and loads in the other three sides. If there were anything to the helical loading concept, I should be seeing similar effects in this simple model. I included a receiving antenna 10 wavelengths away consisting of a half wave dipole loaded with 1000 ohms. The wires were 0.001 wavelength diameter. Modeling was done in free space. I started with the loads set to zero, and the loop modeled as expected, 1.6 dBi gain, 0.013 ohms driving resistance. Then I tried setting the loads to various values of inductive reactance. As they were increased, the driving point resistance increased. For example, with three 100 ohm loads, it was 0.024 ohms. The gain with this loading decreased to -0.48 dBi. The power from the current source, of course, roughly doubled. However, the received power in the receiving antenna only went up about 20%. The loading coils decreased the efficiency. Going beyond 100 ohms results in the loop beaming broadside and the polarization flips. Have the helically loaded loops claimed to beam broadside? Now here is what I think the misunderstanding is: you might think that since the driving resistance nearly doubled, then the "radiation resistance" also doubled. Then the helical loaded loop would seem to make sense. However, this is based on a misunderstanding of the definition of radiation resistance. The effect of the loading acts more like using multiple turns on a loop to increase the driving resistance. That is the same as simply putting a transformer between the source and loop, which of course has no effect on efficiency. How do we know this? Because the EZNEC model shows a decrease in efficiency when the loading is added. I tried adding resistance to account for a coil Q of 1000. This additional resistance only served to decrease the efficiency by over 10 dB. Thus real world coils are either too low in inductance to do anything at all, or their resistance adds significant losses, even with a Q of 1000. We can be quite certain that helically wound strap does not have anything approaching this kind of Q. Rick N6RK |
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Rick, since I have seen differences between 'real life' and what NEC-2 based MOM modelers yield regarding loop antennas, I don't have quite the 'faith' you do in their infallibility. Were it not for the fact that a helical loop is not a simple planar (2-space) structure but occupies a non-trivial amount of volume in 3-space I would still like to see a basis-in-physics explanation outside of the reliance of the usual modeling programs.?
Yet to be explained as well is the phenom where Ae > physical size as it may apply to loops. Jim? ? |
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On 8/30/2014 6:12 AM, jwin95@... [loopantennas] wrote:
Rick, since I have seen differences between 'real life' and what NEC-2It would be very easy to build a loop and put in lumped element inductors to experimentally verify that magic happens or doesn't happen. But consider this thought experiment. If the loop radiation resistance is only 0.013 ohms, any coils you insert with more than a few dozen ohms of reactance add significant losses relative to 0.013 ohms. But a few dozen ohms of reactance has negligible effect on anything. A helically wound strap is not a free lunch. The total conductor path is longer and narrower, hence increasing losses. It has been experimentally verified countless times that helically loaded verticals have no special properties compared to conventional loading coils. Why should it be different with loops? What is it that hasn't been explained? Lots of antennas have Ae > physical size. That doesn't mean that helically wound loops have additional Ae. Where physical size comes into play is that you cannot simultaneously have small physical size and high efficiency without extremely narrow bandwidth and limited power handling due to the necessarily high loaded Q. The proof of this is independent of how the antenna is constructed. The article originally cited makes no mention of bandwidth. Rick N6RK |
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Rick,?
It has still not been explained how?Ae > physical_size on any antenna, let alone loops.? The only thing that has been 'explained' is that the phenom exists, and that is not an explanation in physics as to _why_ the phenom exists nor how it comes about. Just grand 'hand waves' exist as far as I have seen. Even the MIT online EM course video lectures do not address this issues (or maybe I missed that part of a course). And please, save the lecture about the trade-off for B-E-S (BW, EFF and SIZE; pick any two the the 3rd is a given) for the newbies. The only material remotely addressing this is far above the pay grade (and ed level) of most 'ham and egg' operators and lies in the field of actual physics, e.g. as this paper: ¡°Resonant energy absorption and the CTF hypothesis¡° Abstract Similarly, atoms can absorb energy from fields with energy densities so low that the atom must have an effective interaction cross-sectional diameter on the order of tens of microns. It appears that resonant energy absorption exhibits a sort of ¡°suction¡± effect by the absorbing dipole, or a ¡°pushing¡± effect by the field, or a combination of both. This allows the field energy to converge from a larger volume into a smaller region. We will argue that this effect may actually correspond to the field preferentially directing energy into such resonant systems, and discuss how this provides further evidence for the utility of our proposition of a universal, complex tension field (CTF). We have proposed that CTF can support propagating field gradients, like EM waves, as well as resonant, localized and self-looped oscillations representing various particles. Different gradients in the CTF, generated by different kinds of particle-oscillations, represent the various forces experienced by particles within each others¡¯ physical domain. Even time emerges as a secondary property. Jim |
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Perhaps it will help if you realize that the aperture is just a number calculated from the way the antenna picks up the signal.? It does not represent the real size of the antenna.? I believe it is very analogous to numerical aperture (NA) in optics which also is not truly the size of the lens but a measure of the amount of light it can pick up.? For example for some applications a lens is used with a liquid between the lens and the object being looked at.? The liquid does not increase the size of the lens but increases the NA.
Optics is not electronics so maybe this is not a great analogy, but perhaps this can help you. Rick (a different Rick) |
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Rick,?
It isn't 'help' I'm seeking. It's an explanation in physics I'm looking for to explain observed phenomenon. Perhaps this distinction has not been made clear. Now it has been.?The paper I referenced earlier is in the vein of what I am seeking (and obviously found), as opposed to continued 'grand hand waves' by most antenna experts and physicists even. Maybe it would help you to visualize what I'm getting at - If you have ever seen a waveguide to coax adapter ask yourself how it is that the little 'probe' is able to so efficiently 'channel' all the RF from out of the waveguide and into the coaxial element, and do so with low insertion loss and a respectable 'return loss' (SWR or 'match') as well. There is more going on with 'simple' antennas even (e.g. dipoles and verticals) than is commonly conveyed by articles and discussions on these subjects. At the 'ham and egg' operator level such detailed and nuanced understanding is obviously not necessary, but we have gone beyond that as we dissect various forms of tuned loops and understand what determines their theoretical limitations from the stand point of physics. Jim |
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In loopantennas@..., <richard@...> wrote :
> It has been experimentally verified countless times that helically >?loaded verticals have no special properties compared to conventional > loading coils. Why should it be different with loops? The detrimental effect on efficiency of helical winding of a small loop has finally been verified in practice by K4HKX with experimental data measured using the Reverse Beacon Network and carefully controlling for environmental parameters. The result: helical winding introduces loss, about 1-2 dB with K4HKX's loop, compared to a non-helically-wound loop of equivalent enclosed area. W8JI already pointed this out more than three years ago when the helically-wound loop was first proposed. Quote by W8JI: "Regretfully, all helcially winding the small loop does is increase inductance and loss resistance. It is of no benefit at all for radiation." (Reference:??) The measured objective data showing this conclusion can be seen here:??(see "Section 3: ?Magloop Comparisons vs My Reference 40M Dipole:" and "Section 9: ?Helically Wound Magloop". Simply winding the strap non-helically around the support frame would improve efficiency (see for example K4HKX's aluminum strap loop) by reducing the current path length and the loss resistance. Best regards, qrp.gaijin@... ---In loopantennas@..., <richard@...> wrote : On 8/30/2014 6:12 AM, jwin95@... [loopantennas] wrote: > Rick, since I have seen differences between 'real life' and what NEC-2It would be very easy to build a loop and put in lumped element inductors to experimentally verify that magic happens or doesn't happen. But consider this thought experiment. If the loop radiation resistance is only 0.013 ohms, any coils you insert with more than a few dozen ohms of reactance add significant losses relative to 0.013 ohms. But a few dozen ohms of reactance has negligible effect on anything. A helically wound strap is not a free lunch. The total conductor path is longer and narrower, hence increasing losses. It has been experimentally verified countless times that helically loaded verticals have no special properties compared to conventional loading coils. Why should it be different with loops? >What is it that hasn't been explained? Lots of antennas have Ae > physical size. That doesn't mean that helically wound loops have additional Ae. Where physical size comes into play is that you cannot simultaneously have small physical size and high efficiency without extremely narrow bandwidth and limited power handling due to the necessarily high loaded Q. The proof of this is independent of how the antenna is constructed. The article originally cited makes no mention of bandwidth. Rick N6RK ? |
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Folklore dies hard.
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At a recent ham convention some clown brought his helically loaded loop and put it on display. He proudly held court and explained to anyone who would listen about how great it was. I bet my 4 inch Hi-Q screwdriver with CB whip on top could beat that loop in a shoot out. Rick N6RK On 12/6/2014 1:48 AM, qrp.gaijin@... [loopantennas] wrote:
In loopantennas@..., <richard@...> wrote : |
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Let's cut to the chase, per the author:
?The helical loop (#3) is the least efficient, also as expected due to the extra length copper conductor, increased skin effect ?and current crowding losses. BTW, that was 12.5 mil copper tape he used, and not all quote "copper" unquote alloys posses the same conductivity. Overall, I'm surprised the correlation was within 1.5 dB given differences; I would have used 1) a larger diameter form and 2) greater spacing between turns and 3) constructed a reference loop using the same copper tape.? BTW, if you start letting W8JI write your physics handbooks you will set 'science' back about 100 years ... he's right when it comes to bread and butter mainstream 'hammy' issues but runs out gas on the fringes (IOW he is not a physicist.) Jim ?? |
Rick Collins
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