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J pole question
Howdy:
In a j-pole the 1/4 wave element, and the bottom 1/4 of the 3/4 element should act as a transmission line, right? So... shouldn't these elements be as very close to each other to cancel one another out, closer than what I see on most j-poles? Or is this space have something to do with the impedance of the antenna? -- SeeYaa:) Harbin Osteen KG6URO This is YOUR futu http://halturnershow.com/aztlan_caps.wmv http://media.putfile.com/La-Gran-Marcha The Mexican Solution: http://www.centerforsecuritypolicy.o...s&code=06-D_18 - |
J pole question
On Thu, 4 May 2006 17:20:24 -0700, "Harbin"
wrote: Howdy: In a j-pole the 1/4 wave element, and the bottom 1/4 of the 3/4 element should act as a transmission line, right? So... shouldn't these elements be as very close to each other to cancel one another out, closer than what I see on most j-poles? Or is this space have something to do with the impedance of the antenna? Remember you are using wavelengths as your measurement. On 160-meters 6-foot separation would be okay. Danny In my many years I have come to a conclusion that one useless man is a shame, two is a law firm, and three or more is a congress. - John Adams email: k6mheatarrldotnet http://www.k6mhe.com/ |
J pole question
Hello Harbin
The bottom of the J-Pole antenna is a 1/4 wave length to transform the high impedance of the 1/2 wave length above into a lower impedance (close to 50 ohms). The impedance of that 1/4WL sould be Z = SQUARE ( Z coax X Z antenna) .... in that case Z antenna is the impedance of the 1/2 WL (usually several hundreds ohms) That's why the space between elements as well as there diameter is important. You certainly know the Topfkreis ... which is the same antenna ... 1/2 WL + 1/4 WL ... but with another technical realisation. Hoping it helps. Regards Jean-Marc F4DRH www.barbaxoops.com "Harbin" a écrit dans le message de news: ... Howdy: In a j-pole the 1/4 wave element, and the bottom 1/4 of the 3/4 element should act as a transmission line, right? So... shouldn't these elements be as very close to each other to cancel one another out, closer than what I see on most j-poles? Or is this space have something to do with the impedance of the antenna? -- SeeYaa:) Harbin Osteen KG6URO This is YOUR futu http://halturnershow.com/aztlan_caps.wmv http://media.putfile.com/La-Gran-Marcha The Mexican Solution: http://www.centerforsecuritypolicy.o...s&code=06-D_18 - |
J pole question
F4DRH wrote:
Hello Harbin The bottom of the J-Pole antenna is a 1/4 wave length to transform the high impedance of the 1/2 wave length above into a lower impedance (close to 50 ohms). The impedance of that 1/4WL sould be Z = SQUARE ( Z coax X Z antenna) .... in that case Z antenna is the impedance of the 1/2 WL (usually several hundreds ohms) That's why the space between elements as well as there diameter is important. You certainly know the Topfkreis ... which is the same antenna ... 1/2 WL + 1/4 WL ... but with another technical realisation. Hoping it helps. Regards Jean-Marc F4DRH Jean-Marc Thanks for mentioning the Topfkreis. It seems like it should work much better, and is so obvious once you see it. I will have to make one. tom K0TAR |
J pole question
On Sun, 07 May 2006 20:05:28 -0500, Tom Ring
wrote: Thanks for mentioning the Topfkreis. It seems like it should work much better, and is so obvious once you see it. I will have to make one. tom K0TAR OK, I did a search, unsuccessfully, on Topfkreis. What is it and where do I find it? thanks Buck -- 73 for now Buck N4PGW |
J pole question
writes:
If it's what I think it is the design is a coaxial feed section with a 1/2wl radiator. The difference being a coax fed rather than a balanced line feed. In either case it relies on an unterminated 1/4wl section reflecting a high impedence to match the end impedence of a 1/2wl section. That being something in the range of 3000-5000 ohms (at resonance). Here's a very simple end fed vertical dipole: http://www.la2t.org/otfgallery/album...plukk/1002.jpg http://f3wm.free.fr/radio/pocket.html It's simply a piece of coax. The braid is stripped off the last 1/4 wavelength. Another 1/4 wavelength from the end, there is a choke to isolate a 1/4 wavelength of braid from the rest of the cable. Presto, dipole! Support it with a fishing rod, hang it from a tree, whatever. 73 de LA4RT Jon |
J pole question
In article ,
Tom Ring wrote: OK, I did a search, unsuccessfully, on Topfkreis. What is it and where do I find it? I found these. Not in english, but it hardly matters. http://membres.lycos.fr/arrad38/bido.../topfkreis.htm http://f5jtz.club.fr/pjacquet/topkreis.htm Also try Googling for "sperrtopf" or "sperrtof". Same basic idea... a J-pole-like extended Zepp in which the matching section is a coaxial sleeve. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
J pole question
Harbin, You seem to understand the "J-Antenna". What you really seem to be asking is how far apart is "too far" for a balanced transmission line. That's a question I can't answer and I'm sure others can, but , in the common custom of adding obtuse references which sort of seem to be relevant... what about that old one wire transmission like the "G-Line"? [no joke] 73, Steve, K9DCI "Harbin" wrote in message ... Howdy: In a j-pole the 1/4 wave element, and the bottom 1/4 of the 3/4 element should act as a transmission line, right? So... shouldn't these elements be as very close to each other to cancel one another out, closer than what I see on most j-poles? Or is this space have something to do with the impedance of the antenna? -- SeeYaa:) Harbin Osteen KG6URO This is YOUR futu http://halturnershow.com/aztlan_caps.wmv http://media.putfile.com/La-Gran-Marcha The Mexican Solution: http://www.centerforsecuritypolicy.o...s&code=06-D_18 - |
J pole question
OOPS! Jon,
IF it is "end-fed" is isn't a "di-pole". It is just a half wave. I guess it is technically a monopole. 73, Steve, K9DCI "Jon KÃ¥re Hellan" wrote in message ... Here's a very simple end fed vertical dipole: http://www.la2t.org/otfgallery/album...plukk/1002.jpg http://f3wm.free.fr/radio/pocket.html It's simply a piece of coax. The braid is stripped off the last 1/4 wavelength. Another 1/4 wavelength from the end, there is a choke to isolate a 1/4 wavelength of braid from the rest of the cable. Presto, dipole! Support it with a fishing rod, hang it from a tree, whatever. 73 de LA4RT Jon |
J pole question
"Steve N." wrote in message ...
Harbin, You seem to understand the "J-Antenna". What you really seem to be asking is how far apart is "too far" for a balanced transmission line. That's a question I can't answer and I'm sure others can, but , in the common custom of adding obtuse references which sort of seem to be relevant... what about that old one wire transmission like the "G-Line"? [no joke] 73, Steve, K9DCI Hi Steve: I have read about the G-Line, which is very low loss (so claimed). Have you heard of anybody that has used this line? |
J pole question
"Steve N." writes:
OOPS! Jon, IF it is "end-fed" is isn't a "di-pole". It is just a half wave. I guess it is technically a monopole. Sort of, sort of. The feed point is actually in the middle of the half wave. Traditionally, it's called coaxial sleeve antenna. Except originally, the sleeve was outside the coax braid. In this variation, due to LA1IC Rolf Brevig, the sleeve is the outside of the coax, and the end of the sleeve is a choke. He's written about it in English in CQ, 1999, Aug pg 22. The version in Norwegian is on the web at http://www.la2t.org/teknikk/vertikal2m.html. I found an article from Microwave Journal about a very similar concept: http://www.ansoft.com/news/articles/04.05_MWJ.pdf. They use different kinds of choke, since they're at much higher frequencies. They report performance very similar to a dipole. 73 de LA4RT Jon |
J pole question
"Jon KÃ¥re Hellan" wrote in message ... "Steve N." writes: OOPS! Jon, IF it is "end-fed" is isn't a "di-pole". It is just a half wave. I guess it is technically a monopole. Sort of, sort of. The feed point is actually in the middle of the half wave. Traditionally, it's called coaxial sleeve antenna. OK, perhaps I missed the full reference. I read "end fed dipole" and went from that. The sleeve dipole is still a dipole (I have one for 2M) and is still center fed. The lower half (lower, hollow, quarter wave with the t-line passing through it) is viewed as a "choke" by some as well. You can do some mental games with your models and take the sleve and spread it out into a cone, then a plane and say that the sleeve dipole is sort of a configuration of a 1/4 wave ground plane of a different color. Various analogies are possible depending on your particular way of understanding the basics. This is what I call "mental models". That Microwave article is showing different concepts, but along the same line of the "non-sleve" antenna. you can take a coax, strip off 1/4 wave of outer to leave 1/4 wave on inner projecting out the end, then do various things with gaps in the the shield (whthout adding a classical sleeve) and get radiation and minimize coax radiation ( except were you want it, or perhaps more correctly, where it helps it to be a better antenna (for whatever charasteristic you want in the first place).). Rolf Brevig's choke is something I don't think I've seen before, but I understand the concept. I can't tell, but I suspect it is common "1/4 wave stub", no? 73, Steve, K9DCI Except originally, the sleeve was outside the coax braid. In this variation, due to LA1IC Rolf Brevig, the sleeve is the outside of the coax, and the end of the sleeve is a choke. He's written about it in English in CQ, 1999, Aug pg 22. The version in Norwegian is on the web at http://www.la2t.org/teknikk/vertikal2m.html. I found an article from Microwave Journal about a very similar concept: http://www.ansoft.com/news/articles/04.05_MWJ.pdf. They use different kinds of choke, since they're at much higher frequencies. They report performance very similar to a dipole. 73 de LA4RT Jon |
J pole question
"Steve N." wrote in message
... "Jon KÃ¥re Hellan" wrote in message ... "Steve N." writes: OOPS! Jon, IF it is "end-fed" is isn't a "di-pole". It is just a half wave. I guess it is technically a monopole. Sort of, sort of. The feed point is actually in the middle of the half wave. Traditionally, it's called coaxial sleeve antenna. OK, perhaps I missed the full reference. I read "end fed dipole" and went from that. The sleeve dipole is still a dipole (I have one for 2M) and is still center fed. The lower half (lower, hollow, quarter wave with the t-line passing through it) is viewed as a "choke" by some as well. You can do some mental games with your models and take the sleve and spread it out into a cone, then a plane and say that the sleeve dipole is sort of a configuration of a 1/4 wave ground plane of a different color. Various analogies are possible depending on your particular way of understanding the basics. This is what I call "mental models". That Microwave article is showing different concepts, but along the same line of the "non-sleve" antenna. you can take a coax, strip off 1/4 wave of outer to leave 1/4 wave on inner projecting out the end, then do various things with gaps in the the shield (whthout adding a classical sleeve) and get radiation and minimize coax radiation ( except were you want it, or perhaps more correctly, where it helps it to be a better antenna (for whatever charasteristic you want in the first place).). Rolf Brevig's choke is something I don't think I've seen before, but I understand the concept. I can't tell, but I suspect it is common "1/4 wave stub", no? 73, Steve, K9DCI Jon, I also just realized that the choke is very similar to the Microwave article's concept of simply opening the shield. I think that was your point. 73, Steve, K9DCI Except originally, the sleeve was outside the coax braid. In this variation, due to LA1IC Rolf Brevig, the sleeve is the outside of the coax, and the end of the sleeve is a choke. He's written about it in English in CQ, 1999, Aug pg 22. The version in Norwegian is on the web at http://www.la2t.org/teknikk/vertikal2m.html. I found an article from Microwave Journal about a very similar concept: http://www.ansoft.com/news/articles/04.05_MWJ.pdf. They use different kinds of choke, since they're at much higher frequencies. They report performance very similar to a dipole. 73 de LA4RT Jon |
J pole question
"Steve N." writes:
"Steve N." wrote in message ... Rolf Brevig's choke is something I don't think I've seen before, but I understand the concept. I can't tell, but I suspect it is common "1/4 wave stub", no? A 1/4 wave stub would work better, but be less simple to fabricate. This design is trivial. It's just a coiled length of coax. I also just realized that the choke is very similar to the Microwave article's concept of simply opening the shield. I think that was your point. That was my point. But I now see that we're putting it at a high impedence point, so the isolation is going to be far from perfect. Anyway, my *real* point was that this was a useful antenna. Carry one rolled up in your pocket and hang it from a tree to extend the range of your HT. Cut one for 156.8 MHz and keep on your sailboat in case you're dismasted. Etc. 73 LA4RT Jon |
J pole question
Jon Kåre Hellan wrote:
. . . Anyway, my *real* point was that this was a useful antenna. Carry one rolled up in your pocket and hang it from a tree to extend the range of your HT. . . I did a bit of experimenting along this line and found that any fixed antenna hung from a tree was a poor way to extend range. As you probably know, multipath propagation causes dead spots every few cm -- the cause of "picket fencing" when mobile -- when you're in a marginal area where a better antenna would help. Any fixed antenna is fairly likely to end up in one of those dead spots. And additional gain due to height is likely to be largely canceled by feedline loss unless you carry along some large diameter coax. What I've found to be the best VHF antenna for portable use in marginal conditions is a telescoping half wave antenna like the AEA "Hot Rod" or various imitators like the MFJ. They radiate as well as a J Pole, but have the advantage that you can move them around to find a "sweet spot". Roy Lewallen, W7EL |
J pole question
Nope. Only read about it and, perhaps, understand the theory.
73, Steve, K9DCI "Harbin" wrote in message ... "Steve N." wrote in message ... Harbin, You seem to understand the "J-Antenna". What you really seem to be asking is how far apart is "too far" for a balanced transmission line. That's a question I can't answer and I'm sure others can, but , in the common custom of adding obtuse references which sort of seem to be relevant... what about that old one wire transmission like the "G-Line"? [no joke] 73, Steve, K9DCI Hi Steve: I have read about the G-Line, which is very low loss (so claimed). Have you heard of anybody that has used this line? |
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