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The Texas Bugcatcher and capacity hats
On Thursday, February 13, 2014 4:09:53 AM UTC-6, gareth wrote:
My reasoning is that because the waves spread out in all directions in the hat, then there is no, or very little, radiation from the hat because of field cancellation, but when the waves all return simultaneously to the main element, they have incurred a phase change that you'd get had there been a capacitor in circuit? (Always willing to learn more, and to be corrected if my thought experiment has gone up a blind alley, or in this case, a capacity hat alley!) The use of the hat has two main purposes. #1, it will reduce the number of turns needed with the loading coil, assuming the whip is shorter than 1/4 wave including the hat. #2, and probably most important. It improves the current distribution through the length of the whip and makes the current distribution a good bit more linear from the base to the tip. I don't use one myself, because they catch a lot of wind, and they look ugly on a vehicle. :/ I compromise by mounting the loading coil as high as possible, which also helps current distribution. |
#2
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The Texas Bugcatcher and capacity hats
wrote in message
... On Thursday, February 13, 2014 4:09:53 AM UTC-6, gareth wrote: My reasoning is that because the waves spread out in all directions in the hat, then there is no, or very little, radiation from the hat because of field cancellation, but when the waves all return simultaneously to the main element, they have incurred a phase change that you'd get had there been a capacitor in circuit? (Always willing to learn more, and to be corrected if my thought experiment has gone up a blind alley, or in this case, a capacity hat alley!) The use of the hat has two main purposes. #1, it will reduce the number of turns needed with the loading coil, assuming the whip is shorter than 1/4 wave including the hat. #2, and probably most important. It improves the current distribution through the length of the whip and makes the current distribution a good bit more linear from the base to the tip. I don't use one myself, because they catch a lot of wind, and they look ugly on a vehicle. :/ I compromise by mounting the loading coil as high as possible, which also helps current distribution. Yes, all very well, and not disputed, but I was interested in a discussion of how it actually works at the physics level? |
#3
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The Texas Bugcatcher and capacity hats
On Thursday, February 13, 2014 4:44:04 AM UTC-6, gareth wrote:
I was interested in a discussion of how it actually works at the physics level? The capacitance to ground concept is necessary if the lumped circuit model is being used. When one starts talking about phase shifts, one needs to change over to the distributed network (wave reflection) model. An HF mobile antenna is usually 90 degrees long electrically. The following paper indicates that if phase is important, the distributed network model should be used for anything electrically longer than 15 degrees. http://hamwaves.com/antennas/inductance/corum.pdf Capacitance to ground and a phase shift due to reflections from the end of a top hat are incompatible concepts from two different math models. The distributed network model is closer to Maxwell's equations than is the lumped circuit model. |
#4
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The Texas Bugcatcher and capacity hats
On Thursday, February 13, 2014 4:44:04 AM UTC-6, gareth wrote:
wrote in message ... On Thursday, February 13, 2014 4:09:53 AM UTC-6, gareth wrote: My reasoning is that because the waves spread out in all directions in the hat, then there is no, or very little, radiation from the hat because of field cancellation, but when the waves all return simultaneously to the main element, they have incurred a phase change that you'd get had there been a capacitor in circuit? (Always willing to learn more, and to be corrected if my thought experiment has gone up a blind alley, or in this case, a capacity hat alley!) The use of the hat has two main purposes. #1, it will reduce the number of turns needed with the loading coil, assuming the whip is shorter than 1/4 wave including the hat. #2, and probably most important. It improves the current distribution through the length of the whip and makes the current distribution a good bit more linear from the base to the tip. I don't use one myself, because they catch a lot of wind, and they look ugly on a vehicle. :/ I compromise by mounting the loading coil as high as possible, which also helps current distribution. Yes, all very well, and not disputed, but I was interested in a discussion of how it actually works at the physics level? You are better off with the proper textbooks, than reading my jibber jabber. That's what I use when I want to brush up on how something works at the physics level. :| I read the various books. I don't trust too many on the interweb, as many will lead you astray with jibber jabber and gibberish. Then, if you have a problem with something in the various textbooks, you can come back and argue it on the interweb. |
#5
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The Texas Bugcatcher and capacity hats
wrote in message
... You are better off with the proper textbooks, than reading my jibber jabber. My attempt to re-awaken discussion on Usenet, if only because uk.radio.amateur is currently under threat of closure, strangely, because of the behaviour of those who want it closed. |
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