The Texas Bugcatcher and capacity hats
In another post, those of us from THIS side of the pond were
shown the Texas Bugcatcher antenna (easily googled), and its extensive website. It would seem to be a universal panacea for those with small gardens, subject to the availability of a ground plane. It discusses capacity hats. And my question is this, why is it called a capacity hat, because calculation of the capacity between the hat and the ground will only give, at a guess, a miniscule fraction of a puff. 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 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. |
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? |
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. |
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. :) |
The Texas Bugcatcher and capacity hats
"gareth" wrote in message ... In another post, those of us from THIS side of the pond were shown the Texas Bugcatcher antenna (easily googled), and its extensive website. And my question is this, why is it called a capacity hat, because calculation of the capacity between the hat and the ground will only give, at a guess, a miniscule fraction of a puff. Others have provided essential theory. In practice: http://en.wikipedia.org/wiki/KFI See paragraph entitled "Third Tower" and read second sentence. 75 feet of broadcast tower is a lot of steel and a fair amount of money. Pictures of the tower and hat are at http://www.oldradio.com/archives/warstories/640.htm and scroll down most of the way. |
The Texas Bugcatcher and capacity hats
In article ,
Sal salmonella@food poisoning.org wrote: "gareth" wrote in message ... In another post, those of us from THIS side of the pond were shown the Texas Bugcatcher antenna (easily googled), and its extensive website. And my question is this, why is it called a capacity hat, because calculation of the capacity between the hat and the ground will only give, at a guess, a miniscule fraction of a puff. Others have provided essential theory. In practice: http://en.wikipedia.org/wiki/KFI See paragraph entitled "Third Tower" and read second sentence. Another interesting "in practice" writeup can be found in the first 50 pages of Ed Laport's excellent "Radio Antenna Engineering", where he discusses the use of top-loading of low-frequency broadcast antennas. http://snulbug.mtview.ca.us/books/Ra...naEngineering/ |
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. |
The Texas Bugcatcher and capacity hats
"Sal" salmonella@food poisoning.org wrote in message
... "gareth" wrote in message ... In another post, those of us from THIS side of the pond were shown the Texas Bugcatcher antenna (easily googled), and its extensive website. And my question is this, why is it called a capacity hat, because calculation of the capacity between the hat and the ground will only give, at a guess, a miniscule fraction of a puff. Others have provided essential theory. In practice: http://en.wikipedia.org/wiki/KFI See paragraph entitled "Third Tower" and read second sentence. 75 feet of broadcast tower is a lot of steel and a fair amount of money. Pictures of the tower and hat are at http://www.oldradio.com/archives/warstories/640.htm and scroll down most of the way. Thanks, but I do not pursue URLS, my viewpoint being that if there is something worthwhile to discuss, then discuss it. |
The Texas Bugcatcher and capacity hats
On 14/02/2014 09:48, gareth wrote:
"Sal" salmonella@food poisoning.org wrote in message ... "gareth" wrote in message ... In another post, those of us from THIS side of the pond were shown the Texas Bugcatcher antenna (easily googled), and its extensive website. And my question is this, why is it called a capacity hat, because calculation of the capacity between the hat and the ground will only give, at a guess, a miniscule fraction of a puff. Others have provided essential theory. In practice: http://en.wikipedia.org/wiki/KFI See paragraph entitled "Third Tower" and read second sentence. 75 feet of broadcast tower is a lot of steel and a fair amount of money. Pictures of the tower and hat are at http://www.oldradio.com/archives/warstories/640.htm and scroll down most of the way. Thanks, but I do not pursue URLS, my viewpoint being that if there is something worthwhile to discuss, then discuss it. The latter link is a series of photographs. Are you saying you need somebody to describe them for you? |
The Texas Bugcatcher and capacity hats
"gareth" wrote in message ... Pictures of the tower and hat are at http://www.oldradio.com/archives/warstories/640.htm and scroll down most of the way. Thanks, but I do not pursue URLS, my viewpoint being that if there is something worthwhile to discuss, then discuss it. That's fine. "Sal" |
The Texas Bugcatcher and capacity hats
"Kafkaësque" wrote in message ... The latter link is a series of photographs. Are you saying you need somebody to describe them for you? Please don't pursue this. I would feel terrible if my post inspired yet another ****ing contest. Let it go, please. "Sal" (KD6VKW) |
The Texas Bugcatcher and capacity hats
On 14/02/2014 19:07, Sal wrote:
"Kafkaësque" wrote in message ... The latter link is a series of photographs. Are you saying you need somebody to describe them for you? Please don't pursue this. I would feel terrible if my post inspired yet another ****ing contest. Let it go, please. Of course. But thanks for posting it - fascinating that a circle of relatively small diameter, plus a few feet of wire, could save 75 feet of vertical tower. It seems that nobody has pointed out the third advantage of capacity hats; that they reduce the Q of the antenna, and therefore broaden the bandwidth. |
The Texas Bugcatcher and capacity hats
"Kafkaësque" wrote in message ... It seems that nobody has pointed out the third advantage of capacity hats; that they reduce the Q of the antenna, and therefore broaden the bandwidth. That makes sense when you say it but I wouldn't have thought it. I guess such a bandwidth broadening affect is akin to spreading out the element(s), as with a vertical cage or a fan-wire, but without adding full-lendth elements. Experiment time! (aka "play time") "Sal" |
The Texas Bugcatcher and capacity hats
On Friday, February 14, 2014 1:09:58 PM UTC-6, Kafkaësque wrote:
It seems that nobody has pointed out the third advantage of capacity hats; that they reduce the Q of the antenna, and therefore broaden the bandwidth. If we define bandwidth in terms of 3:1 SWR(50), a capacitive hat tends to decrease the bandwidth since it reduces the feedpoint impedance. For instance, the 3:1 SWR(50) bandwidth of a 1/4WL vertical is about 1.2 MHz while the 3:1 SWR(50) bandwidth of a 1/8WL vertical with horizontal top hat is about 0.5 MHz according to EZNEC. -- 73, Cecil, w5dxp.com |
The Texas Bugcatcher and capacity hats
It seems that nobody has pointed out the third advantage of capacity
hats; that they reduce the Q of the antenna, and therefore broaden the bandwidth. If we define bandwidth in terms of 3:1 SWR(50), a capacitive hat tends to decrease the bandwidth since it reduces the feedpoint impedance. For instance, the 3:1 SWR(50) bandwidth of a 1/4WL vertical is about 1.2 MHz while the 3:1 SWR(50) bandwidth of a 1/8WL vertical with horizontal top hat is about 0.5 MHz according to EZNEC. What would be the 3:1 SWR(50) bandwidth of the same 1/8WL vertical, with no horizontal top hat, under the same conditions? Actually I guess there would be two sets of conditions: - No base loading coil or matching network (in which case I'd guess it's outside the 3:1 range anyhow)? - A theoretical (but handy) zero-loss matching network at the base, giving it a 50-ohm feedpoint impedance at the desired center frequency? I think the latter is what was being referred to by the original poster. |
The Texas Bugcatcher and capacity hats
On Friday, February 14, 2014 7:25:29 PM UTC-6, David Platt wrote:
I think the latter is what was being referred to by the original poster. Sorry, I was looking at it from another angle - starting with a full-sized resonant antenna and adding a top hat for the purpose of decreasing its height. -- 73, Cecil, w5dxp.com |
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