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#1
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Does this antenna have an upper limit on the frequency it'll operate at? I'd think some real life data on efficiency could be derived from enclosing anantenna in a 'big enough' styrofoam cooler type enclosure. If the parts that *I* suspect to be lossy might cause a significant temperature rise with several hundred watts fed to the antenna. Jerry There is no limit on the operating frequency of any antenna. If you can feed RF into it, it will radiate. Question is where, pattern is determined by the electrical length of the radiator. The real efficiency of shortened antenna should be indicated by the comparison with full size equivalent (or any other known type) and measured field strength, like it is done in mobile antenna shootouts. Any RF energy lost in heat is not radiated and will show up in lower signal levels. It is important to compare antennas with the same radiation pattern and ground system. Yuri, K3BU.us |
#2
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"Yuri Blanarovich" wrote in message ... Does this antenna have an upper limit on the frequency it'll operate at? I'd think some real life data on efficiency could be derived from enclosing anantenna in a 'big enough' styrofoam cooler type enclosure. If the parts that *I* suspect to be lossy might cause a significant temperature rise with several hundred watts fed to the antenna. Jerry There is no limit on the operating frequency of any antenna. If you can feed RF into it, it will radiate. Question is where, pattern is determined by the electrical length of the radiator. The real efficiency of shortened antenna should be indicated by the comparison with full size equivalent (or any other known type) and measured field strength, like it is done in mobile antenna shootouts. Any RF energy lost in heat is not radiated and will show up in lower signal levels. It is important to compare antennas with the same radiation pattern and ground system. Yuri, K3BU.us Yuri Youve gotten too refined. I mostly know about basic antenna theory and modeling. But building a VHF model of a 40 meter antenna with #12 copper wire wound around a 4 inch mandril might be impractical. But, if the "wonder antenna" is small enough to be enclosed (mostly) in something transparent to the RF but not to the thermal thats generated by any I^2* R losses, wouldnt the temperture rise inside the enclosure give a decent indication of efficiency? If this "wonder antenna" designer claims to be able to shape the radiation pattern with an antenna significantly shorter that a 1/4 wave stub, He really has something. I suspect that the something he has is mental illness. I've been following this info on the RI antenna and have considered it to be so 'snake oilish' that it would never see the light of an auditorium. Your post about attending the lecture made me wonder if anyone asked about *any* simple (approximations) measurements like the styrofoam radome to get some idea of the I^2*R power lost in the antenna. If he has an antenna, and a transmitter and enough room to build a foam igloo it seems that alot of data on efficiency could be obtained. If I had any interest in describing an antenna I'd built, I'd at least use an infared thermometer on the section of the antenna suspected as being the most lossy. Jerry |
#3
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There's a fundamental problem in making a scale model of an antenna to
determine loss -- the wire conductivity has to be scaled with frequency. Of course, the wire diameter can be changed from the normally scaled size in order to create the same loss, but then the antenna might behave differently. It's not clear to me how you could get quantitative data from measurements in a styrofoam container. Let's say you put 100 watts into the antenna for five minutes and the temperature rise (of the coil? helical winding? air?) was 10 degrees C. How would you calculate the loss or efficiency from that information? Roy Lewallen, W7EL Jerry Martes wrote: Youve gotten too refined. I mostly know about basic antenna theory and modeling. But building a VHF model of a 40 meter antenna with #12 copper wire wound around a 4 inch mandril might be impractical. But, if the "wonder antenna" is small enough to be enclosed (mostly) in something transparent to the RF but not to the thermal thats generated by any I^2* R losses, wouldnt the temperture rise inside the enclosure give a decent indication of efficiency? If this "wonder antenna" designer claims to be able to shape the radiation pattern with an antenna significantly shorter that a 1/4 wave stub, He really has something. I suspect that the something he has is mental illness. I've been following this info on the RI antenna and have considered it to be so 'snake oilish' that it would never see the light of an auditorium. Your post about attending the lecture made me wonder if anyone asked about *any* simple (approximations) measurements like the styrofoam radome to get some idea of the I^2*R power lost in the antenna. If he has an antenna, and a transmitter and enough room to build a foam igloo it seems that alot of data on efficiency could be obtained. If I had any interest in describing an antenna I'd built, I'd at least use an infared thermometer on the section of the antenna suspected as being the most lossy. Jerry |
#4
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Roy
I'm a real slopy builder. So, I do alot of half-assed things. I might begin with a 100 watt light bulb inside the "icebox" to establish a referance temperature. maybe even a 50 watt and even a 25 watt. And, if the antenna could be modeled at some frequency where I could build an ice box thats sufficiently transparent to RF, like 6 Meters, I'd build a full size antenna for use as a referance. I've never done something like this. It just sems very practical, since a 100% efficient antenna wouldnt introduce any heat. But , a 50% efficient one might produce about the same heat as a 50 watt light bulb. I'd not declare this "ice box" to be a Lab Standard. But, if there was need, I'd sure try to evaluate the concept. When people like you cast doubt on anything I tink up, I begin to doubt myself. Jerry "Roy Lewallen" wrote in message ... There's a fundamental problem in making a scale model of an antenna to determine loss -- the wire conductivity has to be scaled with frequency. Of course, the wire diameter can be changed from the normally scaled size in order to create the same loss, but then the antenna might behave differently. It's not clear to me how you could get quantitative data from measurements in a styrofoam container. Let's say you put 100 watts into the antenna for five minutes and the temperature rise (of the coil? helical winding? air?) was 10 degrees C. How would you calculate the loss or efficiency from that information? Roy Lewallen, W7EL Jerry Martes wrote: Youve gotten too refined. I mostly know about basic antenna theory and modeling. But building a VHF model of a 40 meter antenna with #12 copper wire wound around a 4 inch mandril might be impractical. But, if the "wonder antenna" is small enough to be enclosed (mostly) in something transparent to the RF but not to the thermal thats generated by any I^2* R losses, wouldnt the temperture rise inside the enclosure give a decent indication of efficiency? If this "wonder antenna" designer claims to be able to shape the radiation pattern with an antenna significantly shorter that a 1/4 wave stub, He really has something. I suspect that the something he has is mental illness. I've been following this info on the RI antenna and have considered it to be so 'snake oilish' that it would never see the light of an auditorium. Your post about attending the lecture made me wonder if anyone asked about *any* simple (approximations) measurements like the styrofoam radome to get some idea of the I^2*R power lost in the antenna. If he has an antenna, and a transmitter and enough room to build a foam igloo it seems that alot of data on efficiency could be obtained. If I had any interest in describing an antenna I'd built, I'd at least use an infared thermometer on the section of the antenna suspected as being the most lossy. Jerry |
#5
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Jerry Martes wrote:
Roy I'm a real slopy builder. So, I do alot of half-assed things. I might begin with a 100 watt light bulb inside the "icebox" to establish a referance temperature. maybe even a 50 watt and even a 25 watt. And, if the antenna could be modeled at some frequency where I could build an ice box thats sufficiently transparent to RF, like 6 Meters, I'd build a full size antenna for use as a referance. I've never done something like this. It just sems very practical, since a 100% efficient antenna wouldnt introduce any heat. But , a 50% efficient one might produce about the same heat as a 50 watt light bulb. It would if the thermal mass of the antenna were the same as the light bulb, and if the thermal loss through the wiring were the same for the antenna as for the light bulb. But otherwise, they could be vastly different. I'd not declare this "ice box" to be a Lab Standard. But, if there was need, I'd sure try to evaluate the concept. When people like you cast doubt on anything I tink up, I begin to doubt myself. Jerry Well, then I've done a good service. Only when you have doubts about your methodolgy will you have any incentive to take the time to try and understand what you're doing. Any conclusions you reach about the antenna's efficiency (except that it's not 100% efficient - duh) will be completely worthless unless you get at least some kind of handle on the relationship between the heat and the temperature. Roy Lewallen, W7EL |
#6
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"Roy Lewallen" wrote in message ... Jerry Martes wrote: Roy I'm a real slopy builder. So, I do alot of half-assed things. I might begin with a 100 watt light bulb inside the "icebox" to establish a referance temperature. maybe even a 50 watt and even a 25 watt. And, if the antenna could be modeled at some frequency where I could build an ice box thats sufficiently transparent to RF, like 6 Meters, I'd build a full size antenna for use as a referance. I've never done something like this. It just sems very practical, since a 100% efficient antenna wouldnt introduce any heat. But , a 50% efficient one might produce about the same heat as a 50 watt light bulb. It would if the thermal mass of the antenna were the same as the light bulb, and if the thermal loss through the wiring were the same for the antenna as for the light bulb. But otherwise, they could be vastly different. I'd not declare this "ice box" to be a Lab Standard. But, if there was need, I'd sure try to evaluate the concept. When people like you cast doubt on anything I tink up, I begin to doubt myself. Jerry Well, then I've done a good service. Only when you have doubts about your methodolgy will you have any incentive to take the time to try and understand what you're doing. Any conclusions you reach about the antenna's efficiency (except that it's not 100% efficient - duh) will be completely worthless unless you get at least some kind of handle on the relationship between the heat and the temperature. Roy Lewallen, W7EL Roy Since I consider antenna efficiency difficult to measure accurately, I'd be willing to try to measure it as carefully as *practical*. If that required that the referance heater had a similar "thermal mass" to that of the antenna under test, I could probably make a heater that did have a thermal mass equivalent to that of the antenna. But, I would propose that there is no easier way to actually measure an antenna's efficiency than to measure the amount of power it turns into heat. Thats a statement I cant back up, and I know it. What is a better way to measure an antenna's efficiency?? |
#7
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Jerry Martes wrote:
Roy Since I consider antenna efficiency difficult to measure accurately, I'd be willing to try to measure it as carefully as *practical*. If that required that the referance heater had a similar "thermal mass" to that of the antenna under test, I could probably make a heater that did have a thermal mass equivalent to that of the antenna. But, I would propose that there is no easier way to actually measure an antenna's efficiency than to measure the amount of power it turns into heat. Thats a statement I cant back up, and I know it. What is a better way to measure an antenna's efficiency?? I believe it would usually be easier and more accurate to measure the amount of power it turns into radiation. That is, measure the strength of the radiated field compared with an antenna of known efficiency and preferably having a similar pattern. Neither is easy, and a method that's "practical" from your point of view might well yield results which are so grossly inaccurate as to be useless. But go ahead, do a careful estimate of the accuracy you expect, do some measurements of various simple antennas whose loss is easy to calculate and compare the measured results, and see how well you can do. It's probably within the reach of a very careful amateur to make measurements which are accurate enough to be useful. It just isn't easy, and requires knowing the relationship between heat and temperature, where the heat is going, sources of error and the amount they can contribute, and a whole lot of care and attention to detail. And that's to get even crudely accurate results. For myself, I'd model it as Reg suggests (although I'd use a program rather than the back of an envelope as he would) and easily believe that the model results are more accurate than any measurement I'd be able to make. However, neither calculation or measurement results are likely to alter the claims made by manufacturers of miracle antennas and their fans. Roy Lewallen, W7EL |
#8
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On Mon, 16 Aug 2004 23:25:20 GMT, "Jerry Martes"
wrote: What is a better way to measure an antenna's efficiency?? Hi Jerry, I see you are suffering from answers out to three decimals again. Your idea is not so far fetched, it is the simple calorimetric bomb. With enough patience and references, yes, you could measure inefficiency. However, the inefficiency could easily be lost in the inaccuracy and patience is not a virtue where simpler methods prevail. You rightly note that a total integration of all field strengths would be required, and be far more cumbersome. This is the classic treatment, but when done once with a reference for comparison, it is unnecessary to apply to other antennas of simple characteristics. Others note that simple comparisons serve quite suitably. For a small antenna (that is, in relation to wavelength such as the DLM and others like CFA/eh/fractals purport to operate efficiently in) there is no hint that the radiation lobes are going to offer manifestly high gain so as to drive comparisons off the chart. Certainly inventors make such fantasy claims, and those claims characteristically remain unsubstantiated. The DLM is a classic example. Side by side comparisons of the DLM with known good antennas, or even known poor ones with similar lobe patters could easily reveal efficiency. I suppose these same inventors could crow about a razor thin 24dB gain lobe pointed at the horizon (if you only knew the tune-up procedures), but you are not going to find this from any antenna packed into a box with less than quarterwave dimensions on all sides. [readers: Examples proving this last sentence wrong are welcome.] 73's Richard Clark, KB7QHC |
#9
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The most intelligent, sensible and successful radio engineers make no attempt to measure antenna radiating efficiency. It never occurs to them to do so - they invariably calculate it. It takes no longer than a couple of minutes on the back of a used envelope, using a pocket calculator. It is more accurate and is absolutely certain to be enormously less expensive. But perhaps some old wife who cannot do simple arithmetic will find a fellow creature somewhere in Google who is reported as having attempted to measure it. The real problem lies in the western world's educational system. Arithmetic has been neglected for far too long. It will take two generations to return to normal and dis-arm children who take loaded guns, even automatic weapons, to school. Never mind about tower blocks. As a priority, teachers must be protected from acts of terrorism. But too late. Chinese and other Eastern countries' education, science, engineering and technology will have already acheived supremacy. But why make a battle out of it? I'm sure the peaceful, generous, Chinese will be quite willing to share the products of their educational system. ---- Reg, G4FGQ |
#10
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On Mon, 16 Aug 2004 11:42:19 -0700, Roy Lewallen
wrote: It's not clear to me how you could get quantitative data from measurements in a styrofoam container. Let's say you put 100 watts into the antenna for five minutes and the temperature rise (of the coil? helical winding? air?) was 10 degrees C. How would you calculate the loss or efficiency from that information? First of all, the test should not be run for 5 minutes, but until a thermal equilibrium inside the container has been established, possibly mixing the air inside the container with a fan. This solves the thermal mass problems when equilibrium has been established. Measure the power fed to the antenna. The temperature outside of the container should remain stable during the test. Then the temperature difference across the container wall is known, as well as the wall surface area and thickness and hopefully also the thermal conductivity for styrofoam (from the manufacturer). Now the thermal resistance of the container as well as the temperature difference is known and the power dissipation can be calculated. This is very similar to calculating the heatsink requirements for transistors. If the thermal conductivity for the styrofoam is not known, run a predefined amount of power to the antenna until equilibrium has been reached and measure the temperature. Inside the container, disconnect the antenna, attach the feedline to a dummy load (possibly attaching it to some large metallic part of the antenna to act as a heatsink). Close the container and feed RF-power into the dummy load at an adjustable power level, until the same stable temperature is reached as with the actual antenna. Measure the power fed into the dummy load. This will eliminate the feedline conductivity problem as well as offset errors in the power meter (but not linearity errors). This way the power dissipated by the dummy load is the same as the power dissipated in the antenna losses in the first place and thus, the efficiency can be calculated. To avoid any power meter linearity problems, reconnect the antenna again, feed in the original full power and verify that there is a thermal equilibrium at the original temperature and outside temperature. Insert a calibrated step attenuator ahead of the power meter and adjust it until it reads the same value as in the dummy load test. You can then read directly from the step attenuator, how many dBs the total full transmitter power was compared to the dummy load power which is also the antenna thermal dissipation value. A 10 dB setting would indicate that the thermal losses are only 10 % of the total power or 90 % antenna efficiency, a 3 dB reading would indicate that half of the power is radiated and the other half is dissipated in the antenna, thus 50 % efficiency. However, you would need a step attenuator with 0.1 steps to measure the worst antennas :-). Paul OH3LWR |
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