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J pole/coax radition
I was wondering 2 things: While on hf i have a current balun, thyre pretty common to help elmin any bad signals on my coax shield is their such a thing for uhf/vhf?? if so who specifically sells them second I have a arrow 2/440 j pole works great---- it dosn't have any radials hanging off naturally, so since basically just the center conductor is connected would you suspect my coax shield is 'part of the antenna' radiating alot of stuff?? (course the shield does attach to the bottom mounting plate of the antenna but i wasn't counting that tnx |
ml wrote:
second I have a arrow 2/440 j pole works great---- it dosn't have any radials hanging off naturally, so since basically just the center conductor is connected would you suspect my coax shield is 'part of the antenna' radiating alot of stuff?? (course the shield does attach to the bottom mounting plate of the antenna but i wasn't counting that In the typical J-pole antenna, the shield connects to the long element and the center to the stub. I don't know your antenna in particular, but I would suspect this is the case and don't believe it is considered a radiating element. |
Modeing shows that radiating current can occur on a J-Pole feedline due
to mutual coupling from the antenna. The amount depends heavily on the length and orientation of the feedline. The main effect of the current on the pattern is to distort the main lobe, reducing the amount of signal directed horizontally. I suspect that the widely varying reports on the effectiveness of a J-Pole are due, in part, at least, to different feedline lengths and orientations and therefore different amounts of feedline radiation. If there is significant current on the feedline, it might take a couple of current (choke) baluns, placed about a quarter wave apart, to reduce it to a small value. Most people don't realize that ground plane antennas are subject to the same phenomenon. Roy Lewallen, W7EL Larry wrote: ml wrote: second I have a arrow 2/440 j pole works great---- it dosn't have any radials hanging off naturally, so since basically just the center conductor is connected would you suspect my coax shield is 'part of the antenna' radiating alot of stuff?? (course the shield does attach to the bottom mounting plate of the antenna but i wasn't counting that In the typical J-pole antenna, the shield connects to the long element and the center to the stub. I don't know your antenna in particular, but I would suspect this is the case and don't believe it is considered a radiating element. |
Roy:
My "bumbling goof-abouts" with antennas has proven to me you are once again right on the money... I have gotten to where I don't even worry about the loss of a well constructed choking--balun/unun in the circuit--better to be safe than sorry... John "Roy Lewallen" wrote in message ... Modeing shows that radiating current can occur on a J-Pole feedline due to mutual coupling from the antenna. The amount depends heavily on the length and orientation of the feedline. The main effect of the current on the pattern is to distort the main lobe, reducing the amount of signal directed horizontally. I suspect that the widely varying reports on the effectiveness of a J-Pole are due, in part, at least, to different feedline lengths and orientations and therefore different amounts of feedline radiation. If there is significant current on the feedline, it might take a couple of current (choke) baluns, placed about a quarter wave apart, to reduce it to a small value. Most people don't realize that ground plane antennas are subject to the same phenomenon. Roy Lewallen, W7EL Larry wrote: ml wrote: second I have a arrow 2/440 j pole works great---- it dosn't have any radials hanging off naturally, so since basically just the center conductor is connected would you suspect my coax shield is 'part of the antenna' radiating alot of stuff?? (course the shield does attach to the bottom mounting plate of the antenna but i wasn't counting that In the typical J-pole antenna, the shield connects to the long element and the center to the stub. I don't know your antenna in particular, but I would suspect this is the case and don't believe it is considered a radiating element. |
I have gotten to where I don't even worry about the loss of a well
constructed choking--balun/unun in the circuit--better to be safe than sorry... If the decoupling is designed into the antenna, using cones, bells, sleeves, etc, there is little if any loss. Being as the feed radiation skews the pattern up off the horizon, adding decoupling usually ends up with a "gain" at that low angle. Also, I've heard different types of "J" feed methods, naturally decouple better than others..This also applies to the ringo's, etc. The gamma loop matching device on a ringo does a halfway job of decoupling the line, but it can always be improved by a 2nd decoupling section. I don't use conventional ferrite chokes when decoupling VHF/UHF antennas. IE: If I were to add additional decoupling to a ground plane, I would use a 2nd set of radials, or a sleeve, vs a ferrite choke. MK |
Hello, Roy & the Group.
One think you may have missed, the original post was about a Arrow Antenna J-Pole. This is an Open Stub type J-Pole, Not a Closed Stub type like the copper pipe ones. The OSJ does not need a choke, it does not have a problem with feedline radiation or a problem with Common Mode Currents. In the typical J-pole antenna, the shield connects to the long element and the center to the stub. =A0I don't know your antenna in particular, but I would suspect this is the case and don't believe it is considered a radiating element. The OSJ is not a typical J-pole. 73 Al Lowe N0IMW Arrow Antenna |
Hm, why is that? How does an open stub prevent coupling from the antenna
to the feedline? Roy Lewallen, W7EL Al wrote: Hello, Roy & the Group. One think you may have missed, the original post was about a Arrow Antenna J-Pole. This is an Open Stub type J-Pole, Not a Closed Stub type like the copper pipe ones. The OSJ does not need a choke, it does not have a problem with feedline radiation or a problem with Common Mode Currents. In the typical J-pole antenna, the shield connects to the long element and the center to the stub. �I don't know your antenna in particular, but I would suspect this is the case and don't believe it is considered a radiating element. The OSJ is not a typical J-pole. 73 Al Lowe N0IMW Arrow Antenna |
On 12 Jun 2005 14:15:52 -0700, "Al" wrote:
Hello, Roy & the Group. One think you may have missed, the original post was about a Arrow Antenna J-Pole. This is an Open Stub type J-Pole, Not a Closed Stub type like the copper pipe ones. The OSJ does not need a choke, it does not have a problem with feedline radiation or a problem with Common Mode Currents. In the typical J-pole antenna, the shield connects to the long element and the center to the stub. I've read, in this group, it doesn't matter which side the center connects to. That's not true? don't know your antenna in particular, but I would suspect this is the case and don't believe it is considered a radiating element. The OSJ is not a typical J-pole. 73 Al Lowe N0IMW Arrow Antenna By open stub, you mean there is no loop at the bottom of the "J"? bob k5qwg |
Hm, why is that? How does an open stub prevent coupling from the antenna
to the feedline? Roy Lewallen, W7EL I was hoping we would not have to go through this AGAIN. The typical J-Pole (Closed Stub) was designed to feed with open feed line. Because Hams insist on feeding it with coax, a Band-Aid is needed to choke off feed line radiation & common mode currents. The Open Stub J-Pole was designed to feed with coax. So it don't need the Band Aid. The difference between the two is discussed in several antenna books. Just ask anyone the owns one. (http://www.eham.net/reviews/detail/1613) Besides your the antenna expert, tell us why it would. 73 Al Lowe N0IMW |
I was hoping we would not have to go through this AGAIN. The typical J-Pole (Closed Stub) was designed to feed with open feed line. Because Hams insist on feeding it with coax, a Band-Aid is needed to choke off feed line radiation & common mode currents. The Open Stub J-Pole was designed to feed with coax. So it don't need the Band Aid. The difference between the two is discussed in several antenna books. Just ask anyone the owns one. (http://www.eham.net/reviews/detail/1613) Besides your the antenna expert, tell us why it would. 73 Al Lowe N0IMW For exactly the same reason that you need a choke on a ground plane or any other antenna feed with coax. Depending upon the length of the feed line common mode current can be substantial. This, in turn, increases higher angle radiation. Now if you are most interested in talking to airplanes that would be a good thing, but if you prefer maximum signal towards the horizon then put a choke on that sucker! Danny, K6MHE |
By open stub, you mean there is no loop at the bottom of the "J"?
bob k5qwg No, Open Stub means the 1/4 wave element and the 3/4 wave element are not connected. In the typical (closed stub) J-Pole I have seen the center of the coax connected to ether side of the antenna, seems to work equally poor ether way. In the case of the Open Stub J-Pole I have found that a wider bandwidth can be obtained connecting the center to the 3/4 wave element. (about 12 MHz.) Connecting to the 1/4 wave element, bandwidth is only about 6 MHz. The 2 meter band is only 4 MHz. so it really don't matter too much. Try that with the typical copper pipe J-Pole. 73 Al Lowe N0IMW |
Dan:
Some of these arguments carry on into the realm where I lose interest... but right on when you say, "...put a choke on that sucker!" This may be due to the fact that I am not an "antenna guru" and can't be certain when one is best, and when not... so what? Make fun of me then!!! tongue-sticking-out-grin Warmest regards, John "Dan Richardson arrl net" k6mheatdot wrote in message ... I was hoping we would not have to go through this AGAIN. The typical J-Pole (Closed Stub) was designed to feed with open feed line. Because Hams insist on feeding it with coax, a Band-Aid is needed to choke off feed line radiation & common mode currents. The Open Stub J-Pole was designed to feed with coax. So it don't need the Band Aid. The difference between the two is discussed in several antenna books. Just ask anyone the owns one. (http://www.eham.net/reviews/detail/1613) Besides your the antenna expert, tell us why it would. 73 Al Lowe N0IMW For exactly the same reason that you need a choke on a ground plane or any other antenna feed with coax. Depending upon the length of the feed line common mode current can be substantial. This, in turn, increases higher angle radiation. Now if you are most interested in talking to airplanes that would be a good thing, but if you prefer maximum signal towards the horizon then put a choke on that sucker! Danny, K6MHE |
Al wrote:
Hm, why is that? How does an open stub prevent coupling from the antenna to the feedline? Roy Lewallen, W7EL I was hoping we would not have to go through this AGAIN. The typical J-Pole (Closed Stub) was designed to feed with open feed line. Because Hams insist on feeding it with coax, a Band-Aid is needed to choke off feed line radiation & common mode currents. The Open Stub J-Pole was designed to feed with coax. So it don't need the Band Aid. The difference between the two is discussed in several antenna books. Just ask anyone the owns one. (http://www.eham.net/reviews/detail/1613) Besides your the antenna expert, tell us why it would. 73 Al Lowe N0IMW Sorry, I didn't realize we'd been through it before. If you'll tell me the date, I'll look up the postings and review them. There are two things which can cause current on the feedline. One is by conduction. That mechanism is illustrated in Fig 21-1 at http://www.w2du/r2ch21.pdf, and elaborated on in http://eznec.com/Amateur/Articles/Baluns.pdf. Some types of feed systems can minimize this, and it looks like both the open and shorted stub feed systems are in this category. But it's the second mechanism I was talking about -- mutual coupling, or induction. Even if you have a perfect current balun (common mode choke) at the feedpoint or any sort of feed system you want, current can still be coupled to the feedline. In a center fed dipole, it won't occur if you first make sure there's no conducted current, and place the feedline symmetrically relative to the dipole to make the coupling from the two dipole halves cancel. But you can't do this with a ground plane, J-Pole, or similar antenna -- there will always be coupling from the antenna to the feedline. Fiddling with the feedpoint won't prevent it, although it might change the current distribution on the feedline. Changing the feedline length and/or orientation and/or using a couple of current baluns spaced about a quarter wavelength apart are two ways it can be minimized. I'd be interested in seeing any book which explains how a feedpoint modification can prevent current being coupled to the feedline (as opposed to being conducted). Roy Lewallen, W7EL |
Al wrote:
One think you may have missed, the original post was about a Arrow Antenna J-Pole. This is an Open Stub type J-Pole, Not a Closed Stub type like the copper pipe ones. The OSJ does not need a choke, it does not have a problem with feedline radiation or a problem with Common Mode Currents. Al, have you modeled it with EZNEC and a path to ground? -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
For exactly the same reason that you need a choke on a ground plane or
any other antenna feed with coax. sniped Danny, K6MHE I agree with the choke needed on a ground plane. But all antennas ? Sure you can put a choke on a OSJ won't hurt, won't help ether. Been there, done that. 73 Al Lowe N0IMW |
Bob Miller wrote:
By open stub, you mean there is no loop at the bottom of the "J"? He means the bottom of the stub is the feedpoint, like this: +------------------------------------------------------------ | FP | +----------------- And that looks like an unbalanced feedpoint so it should result in common-mode currents, sorta like an off center-fed dipole. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Sorry, I didn't realize we'd been through it before. If you'll tell me
the date, I'll look up the postings and review them. There are two things which can cause current on the feedline. sniped I don't remember if you were involved or not Roy. It's been about 10 years ago. One I remember is Tom Raunch (hope I spelled that right). We argued for weeks about the OSJ until I sent him one. Some tried modeling it, some even claimed it would not work at all. The OSJ has been around for a long time, I did not invent it, I just build it better. I guess all antennas have feed line radiation and/or common mode currents to some degree. I suppose with the right equipment you can measure anything. But I think most would find a big difference between an OSJ & a CSJ Just because two antennas look similar, don't mean they work the same. Ok, that was fun, But I have wasted enough time, I got to get back to building Antennas. 73 Al Lowe N0IMW |
He means the bottom of the stub is the feedpoint, like this:
+----------------------------- ------------------------------ - | FP | +----------------- And that looks like an unbalanced feedpoint so it should result in common-mode currents, sorta like an off center-fed dipole. Not quite. Look closer - http://www.arrowantennas.com/j-pole.html You assume it should result in common-mode currents. And you know what happens when you Ass u me. 73 Al Lowe N0IMW |
On 12 Jun 2005 16:18:35 -0700, "Al" wrote:
For exactly the same reason that you need a choke on a ground plane or any other antenna feed with coax. sniped Danny, K6MHE I agree with the choke needed on a ground plane. But all antennas ? Sure you can put a choke on a OSJ won't hurt, won't help ether. Been there, done that. 73 Al Lowe N0IMW Allow me to repeat: *ANY* antenna feed with coax should employee a choke. It is just good engineering practice. If your beloved open base j-pole is feed with coax that is any multiple of a 1/2-wavelength you ARE GOING TO HAVE HIGH COMMON MODE CURRENT! So just what makes you beleive that your open base j-pole has some magical properties that is self-decouples itself from the feed line? 73 Danny, K6MHE |
Al wrote:
He means the bottom of the stub is the feedpoint, like this: +-----------------x------------------------------------------- | FP | +-----------------y And that looks like an unbalanced feedpoint so it should result in common-mode currents, sorta like an off center-fed dipole. Not quite. Look closer - http://www.arrowantennas.com/j-pole.html You assume it should result in common-mode currents. And you know what happens when you Ass u me. Would you agree this is a 2m Zepp? Zepps are known to have common mode currents. We know the current at point 'x' is not the same as the current at point 'y'. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Not quite. Look closer - http://www.arrowantennas.com/j-pole.html
You assume it should result in common-mode currents. And you know what happens when you Ass u me. But....Common mode current on the feedline can't be avoided no matter how the antenna is fed. I agree that the antenna is quite usable without decoupling, and I've found that the 1/2 WL ringo's usual "gamma loop" feed is also fairly good as far as that. But there is no way to totally avoid all currents, without some form of decoupling. If you believe you can, try some tests adding decoupling sections. If you don't see an increase in performance, I would be *very* surprised. In some cases, it's possible for the currents to add, and produce a gain at low angles, but it's fairly unlikely. Most of the time, the pattern is skewed upwards. It takes at least two decoupling sections to fully decouple an elevated vertical. To me, you have none, although maybe you could argue that your feed of feeding qualifies as a first decoupling section...Lets say you win, and this is the case...You still need one more, if you want to be fully decoupled... If you believe you are now, I'd be willing to bet you are deluding yourself. Thats not to say the antenna doesn't work ok as is...I'm sure it probably does, except the most retentive of users...I've used loads of halfwaves with no decoupling...But those were all on 10m, where it's not so critical. BTW...The results of adding decoupling sections will vary from user to user, depending on their lines. But if the antenna is totally decoupled, the amount of current will be the appx same for any user, no matter feed length. When I say total, I mean as much as practical. I don't believe you can totally eliminate all current. There will always be a small amount. I doubt I would change the antenna....Would make it cost more... I'd consider a super deluxe version for a higher price, if they wanted full 2 section decoupling...:/ And if I went that far, I'd use a dual 5/8 design to get more gain. You'd be reinventing the isopole...:/ MK |
On 12 Jun 2005 15:44:18 -0700, "Al" wrote:
Hm, why is that? How does an open stub prevent coupling from the antenna to the feedline? Roy Lewallen, W7EL I was hoping we would not have to go through this AGAIN. The typical J-Pole (Closed Stub) was designed to feed with open feed line. Because Hams insist on feeding it with coax, a Band-Aid is needed to choke off feed line radiation & common mode currents. The Open Stub J-Pole was designed to feed with coax. So it don't need the Band Aid. The difference between the two is discussed in several antenna books. Just ask anyone the owns one. (http://www.eham.net/reviews/detail/1613) Besides your (sic) the antenna expert, tell us why it would. Careful Roy, you're goring a commercial ox. Al, you have some really ingenious machinery and your products look to be well made. But you should probably stick to machining and hire two more folks: one to do antenna design and another to write copy and manage your web site. With just a cursory look I see that you sell "beems" instead of "beams" and a claim that corner reflectors work better on packet than do Yagis is pure snake oil. Simply modeling your super J-pole and adding a third element to represent the transmission line would show that the line radiates like crazy and affects the feedpoint Z. |
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With just a cursory look I see that you sell "beems" instead of
"beams" Look a little more and you will find more typo's I am sure. and a claim that corner reflectors work better on packet than do Yagis is pure snake oil. I think I said long Yagi's, something to do with phase distortion. I am just going by what customers have told me that have used both. Simply modeling your super J-pole and adding a third element to represent the transmission line would show that the line radiates like crazy and affects the feedpoint Z. I never said anything about a "super J-Pole" . Other than maybe it is super simple. If you simply modeled the OSJ and it showed the feed line radiates like crazy, Maybe you didn't do it right. Maybe you need to step out into the real world. By the way, it has a 3rd element, makes it a dual J-Pole not just a dual band. 73 Al Lowe N0IMW |
They who discuss "power radiated from the feedline", yet are unable to measure (in watts) or calculate (in watts) the MAGNITUDE of the effect, belong to a set of waffling old wives. How is it possible to decide whether or not a choke or balun is needed, and where to locate it, unless the magnitude of what one MIGHT wish to prevent is known. Do we always, without valid reason, just copy the practical construction details from ARRL handbooks, on the grounds that if it once worked OK for somebody else it might work for me. But of course, you'll never get to know, having copied a very simple system which contains a choke, unless you remove the choke and observe what happens to system performance. Or alternatively, add a choke if the ARRL handbook implies that you don't need one, and then make more observations. My guess is that in many cases hardly anything will happen or be noticed. Simply because the MAGNITUDE of the effects due to line radiation is too small to be of consequence or detected even. If you can't measure or calculate the MAGNITUDE of the effects, stop worrying about it. The frequency at which "radiation from the line" is mentioned in this newsgroup is out of all proportion to its importance. There's a tendency to drag it into the discussion because it is the last of the few remaining technical topics available to argue about. You have at least heard about the subject in the magazines. The very last, of course, will be SWR. Because there is a meter which supposedly measures it but doesn't. And it is difficult to argue against meter users, such as Bird, suffering from delusions of accuracy, who are invariably convinced they are right. ---- Reg, G4FGQ |
Al:
In college courses I have pointed out the typos in handouts/papers my professors have provided me. Strange, but from the reactions I received, you would almost think the professors would have rather not had it brought to their attention (I am quite sure I am mistaken on this point)--I did quickly discover it had a direct effect on my course grades, at this point I stopped being so "helpful." I see others went ahead and developed this "science" of "being helpful" into an art... grin Warmest regards, John "Al" wrote in message oups.com... With just a cursory look I see that you sell "beems" instead of "beams" Look a little more and you will find more typo's I am sure. and a claim that corner reflectors work better on packet than do Yagis is pure snake oil. I think I said long Yagi's, something to do with phase distortion. I am just going by what customers have told me that have used both. Simply modeling your super J-pole and adding a third element to represent the transmission line would show that the line radiates like crazy and affects the feedpoint Z. I never said anything about a "super J-Pole" . Other than maybe it is super simple. If you simply modeled the OSJ and it showed the feed line radiates like crazy, Maybe you didn't do it right. Maybe you need to step out into the real world. By the way, it has a 3rd element, makes it a dual J-Pole not just a dual band. 73 Al Lowe N0IMW |
Al wrote:
If you simply modeled the OSJ and it showed the feed line radiates like crazy, Maybe you didn't do it right. Maybe you need to step out into the real world. Al, you could get a relative measure of common-mode current by slipping a #68 toroidal pickup coil over the coax. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Reg Edwards wrote:
If you can't measure or calculate the MAGNITUDE of the effects, stop worrying about it. Does RF burns on my lip count as a measurement? :-) -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
Sniped
I don't believe you can totally eliminate all current. There will always b= e a small amount. Agreed, but it is small enough I don't think most people could see a difference in performance if it had a choke or not. All I am saying, to say the antenna won't work with out a choke is just wrong. I doubt I would change the antenna....Would make it cost more... I'd consider a super deluxe version for a higher price, if they wanted full 2 section decoupling...:/ =A0And if I went that far, I'd use a dual 5/8 design to get more gain. =A0You'd be reinventing the isopole...:/ MK I have an "Isopole" here in the shop, it works good, maybe one of the best 5/8 wave antennas I have ever tested. I think it could be built a little better. I have seen a lot of them that were broke. But that is comparing apples to oranges The OSJ is a simple 1/2 wave antenna that has 0 dBd gain. No magic. Again, I just think it is wrong to tell someone the antenna won't work with out a choke. 73 Al Lowe N0IMW |
Reg Edwards wrote:
They who discuss "power radiated from the feedline", yet are unable to measure (in watts) or calculate (in watts) the MAGNITUDE of the effect, belong to a set of waffling old wives. Those who take no precautions to prevent their feedline from becoming part of the antenna, belong to a set of people who don't even know what their antenna IS. There's a part you call "the antenna", and another part you call "the feedline". Wishful thinking will not stop RF current from flowing directly from one to the other. How is it possible to decide whether or not a choke or balun is needed, and where to locate it, unless the magnitude of what one MIGHT wish to prevent is known. Try a clamp-on RF current meter, a little modeling... or even a little common sense. There's a place called "the feedpoint" where the antenna and the feedline are connected directly together. Might that be a good location for a choke to keep them separate? Yes, it almost certainly would. Chokes may also be needed at other locations, but it's hard to justify anywhere else as your *first* choice. (The exception is the Carolina Windom and similar antennas where part of the feedline is intended to radiate. But even there, they put a good choke at the point where they want RF currents on the feedline to stop.) Queen Elizabeth 1 of England had the good sense to take a bath every six months "whether I need it or not". If you don't know whether your feedline smells of RF, then follow her excellent advice and use a choke. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
In article ,
Cecil Moore wrote: By open stub, you mean there is no loop at the bottom of the "J"? He means the bottom of the stub is the feedpoint, like this: +------------------------------------------------------------ | FP | +----------------- And that looks like an unbalanced feedpoint so it should result in common-mode currents, sorta like an off center-fed dipole. If I recall correctly, Cebik has some discussion and modelling of such open-sleeve J-pole designs in his "Tales and Technicals" section. Start at http://www.cebik.com/vhf/jp1.html and go down about half-way. The version of the "non-standard J-pole" which Cebik models is fed at bottom center, in a dimensionally-symmetrical way. His current plot seems to show equal currents at this feedpoint, and this would seem to make the use of a choke or balun on the feedline somewhat less significant than with a standard J-pole. Cebik's model would need to be modified somewhat apply to the Arrow open-sleeve J-pole, as the Arrow is fed at the very base of the one element rather than halfway between the two. Anybody care to model up the Arrow, with all three elements included, with and without a separate wire representing the outside of the coax? -- 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! |
On 13 Jun 2005 08:20:08 -0700, "Al" wrote:
With just a cursory look I see that you sell "beems" instead of "beams" Look a little more and you will find more typo's I am sure. Let me tell you a little war story and see what you think. About 20 years ago my boss, a newly minted PhD, got a job teaching a Senior/Graduate level course on Microwave Measurements Theory and Practice at the U of AZ. At the time the U's equipment was abysmal so he got permission to bring the kids into my lab at Hughes for the lab portion of the class. I got the job of devising the experiments, setting up the gear and grading the lab notebooks. The kids could absolutely not see straight when I graded their grammar. Their cry was, "This is an engineering course, not an English course." My reply was that they were prepping themselves for a job in industry (actually a few of them already worked at Hughes) and I wrote memos and reports every day, that if poorly written would reflect badly on me. Your web site is your place of business...fill it up with typos, bad grammar and bogus claims and your business can suffer. On second thought, considering your client base, maybe they'll never notice. and a claim that corner reflectors work better on packet than do Yagis is pure snake oil. I think I said long Yagi's, something to do with phase distortion. I am just going by what customers have told me that have used both. Aha. So all I have to do is buy one of your antennas and write and tell you how awful it works and you'll put that on your web site. Simply modeling your super J-pole and adding a third element to represent the transmission line would show that the line radiates like crazy and affects the feedpoint Z. I never said anything about a "super J-Pole" . Sorry, a facetious remark that escaped you. Other than maybe it is super simple. If you simply modeled the OSJ and it showed the feed line radiates like crazy, Maybe you didn't do it right. Maybe you need to step out into the real world. By the way, it has a 3rd element, makes it a dual J-Pole not just a dual band. No, I modeled the OSJVHF as depicted he http://www.arrowantennas.com/inst/igif/ijmurs1.gif You claim 150-162 MHz with 1.5:1 SWR. I modeled it as follows. I saw the SWR vary from a low of 1.4:1 at 150 MHz to a high of almost 7:1 at 162 MHz. Here's the file: ************************************************** ********************* EZNEC+ ver. 4.0 Created from MultiNEC 6/13/2005 11:25:41 AM --------------- ANTENNA DESCRIPTION --------------- Frequency = 150 MHz Wire Loss: Aluminum (6061-T6) -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1 --------------- WIRES --------------- No. End 1 Coord. (in) End 2 Coord. (in) Dia (in) Segs Insulation Conn. X Y Z Conn. X Y Z Diel C Thk(in) 1 W4E1 0, 0, 0 W2E1 0, 0, 18 0.375 13 1 0 2 W1E2 0, 0, 18 0, 0, 54 0.375 26 1 0 3 W4E2 0, 5, 0 0, 5, 18 0.375 13 1 0 4 W1E1 0, 0, 0 W3E1 0, 5, 0 0.375 4 1 0 Total Segments: 56 -------------- SOURCES -------------- No. Specified Pos. Actual Pos. Amplitude Phase Type Wire # % From E1 % From E1 Seg (V/A) (deg.) 1 1 0.00 3.85 1 1 0 SI No loads specified No transmission lines specified Ground type is Free Space ************************************************** ********************* Where did I go wrong? |
Reg Edwards wrote: If you can't measure or calculate the MAGNITUDE of the effects, stop worrying about it. Does RF burns on my lip count as a measurement? :-) ============================= Not unless your yelp indicated how may watts the microphone was radiating at the time. ;o) --- Reg |
Dave Platt wrote:
By open stub, you mean there is no loop at the bottom of the "J"? He means the bottom of the stub is the feedpoint, like this: +------------------------------------------------------------ FP +----------------- And that looks like an unbalanced feedpoint so it should result in common-mode currents, sorta like an off center-fed dipole. If I recall correctly, Cebik has some discussion and modelling of such open-sleeve J-pole designs in his "Tales and Technicals" section. Start at http://www.cebik.com/vhf/jp1.html and go down about half-way. The version of the "non-standard J-pole" which Cebik models is fed at bottom center, in a dimensionally-symmetrical way. His current plot seems to show equal currents at this feedpoint, and this would seem to make the use of a choke or balun on the feedline somewhat less significant than with a standard J-pole. An antenna model without a feedline will *force* equal and opposite currents at the feedpoint - it is always fed through the perfect balun! Add an un-choked feedline to the model (another thick wire, representing the coax shield) at either side of the feedpoint, and see where the current goes now. If you don't use a choke on a real-life antenna, there's nothing to stop the current going wherever it likes. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
Aha. =A0So all I have to do is buy one of your antennas and write and
tell you how awful it works and you'll put that on your web site. Sure, or post it yourself on e-ham.net for the whole world to see. But Please base your test on the real antenna, not a computer model. And when your done send back the antenna and I will refund your money. Just like it says on the web page. 73 Al Lowe N0IMW Arrow Antenna |
In article ,
Ian White GM3SEK wrote: The version of the "non-standard J-pole" which Cebik models is fed at bottom center, in a dimensionally-symmetrical way. His current plot seems to show equal currents at this feedpoint, and this would seem to make the use of a choke or balun on the feedline somewhat less significant than with a standard J-pole. An antenna model without a feedline will *force* equal and opposite currents at the feedpoint - it is always fed through the perfect balun! Cebik's plot of currents in the "standard" J-pole shows unequal currents all the way down to the bottom, and he notes this in his text and asserts the need for a choke. He doesn't say whether his models do or do not include a feedline. Add an un-choked feedline to the model (another thick wire, representing the coax shield) at either side of the feedpoint, and see where the current goes now. If you don't use a choke on a real-life antenna, there's nothing to stop the current going wherever it likes. Agreed. The results are likely to be quite variable depending on the feedline distance to the nearest ground. Seems to me that the worst case would result from a small integral multiple of 1/2 wavelength, no? My guess is that in most simple J-pole installations, the feedline radiation and the resulting disturbance of the antenna's omni pattern are probably not going to be worth worrying about too much. Nearby buildings, trees, etc. are likely to result in larger differences in the far-field pattern than any quirks in the antenna's own pattern. As an alternative to using a choke on the feedline, what sort of results might one get with a standard J-pole by using a half-wave coaxial balun and tapping up a bit further on the elements? An arrangement of this sort might solve two problems at once. One of the problems I've noticed with making the usual plumber's-delight copper J-pole for 440, is that the 50-ohm matching points are down quite close to the bottom shorting-bar. If one makes attachment clamps out of copper-pipe support straps (as is suggested in many of the sets of plans I've seen), and if the J-pole is made from 1/2" copper pipe with elbow and T fittings, it's difficult or impossible to achieve a good match - the attachment clamps can't be slid far enough down on the pipes to reach the match point, before they hit the elbow fittings. Using a coaxial 4:1 balun would allow the attachment points to be a bit further up. -- 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! |
Al wrote:
And when your done send back the antenna and I will refund your money. Just like it says on the web page. Hey Al, if I send the antenna back to you, will you refund my money? What will the bank do with a $00.00 refund? Your antenna certainly works as advertised, and like the Zepp after which it was patterned. But I don't know of any real-world antenna that is immune to common-mode problems. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
Hey Al,
Don't let the nay-sayers in this NG bother you, and don't waste your time arguing with them. Some of them can't stand the thought that any antenna which differs from their personal pet(s) might be good. Others can't grasp the idea that when model predictions and actual results differ, it's the results that count. Still others can't seem to understand that effects too small to measure usually do not matter in the real world. Your antenna is beautifully made, very reasonably priced, and you are (apparently) making a successful small business with it. Your customers are delighted, and refer their friends. You are starting to attract imitators. How many of these accomplishments can the nay-sayers claim? Keep up the good work... the XYL and I love our three OSJs! And they are now pretty much standard in our ARES/RACES organization. NO complaints heard to date. 73, Ed, W6LOL "Al" wrote in message oups.com... Sniped I don't believe you can totally eliminate all current. There will always be a small amount. Agreed, but it is small enough I don't think most people could see a difference in performance if it had a choke or not. All I am saying, to say the antenna won't work with out a choke is just wrong. I doubt I would change the antenna....Would make it cost more... I'd consider a super deluxe version for a higher price, if they wanted full 2 section decoupling...:/ And if I went that far, I'd use a dual 5/8 design to get more gain. You'd be reinventing the isopole...:/ MK I have an "Isopole" here in the shop, it works good, maybe one of the best 5/8 wave antennas I have ever tested. I think it could be built a little better. I have seen a lot of them that were broke. But that is comparing apples to oranges The OSJ is a simple 1/2 wave antenna that has 0 dBd gain. No magic. Again, I just think it is wrong to tell someone the antenna won't work with out a choke. 73 Al Lowe N0IMW |
Hey Al,
Don't let the nay-sayers in this NG bother you, and don't waste your time arguing with them. Some of them can't stand the thought that any antenna which differs from their personal pet(s) might be good. =A0Others can't grasp the idea that when model predictions and actual results differ, it's the results that count. =A0Still others can't seem to understand that effects too small to measure usually do not matter in the real world. Your antenna is beautifully made, very reasonably priced, and you are (apparently) making a successful small business with it. Your customers are delighted, and refer their friends. =A0You are starting to attract imitators. =A0How many of these accomplishments can the nay-sayers claim? Keep up the good work... the XYL and I love our three OSJs! And they are now pretty much standard in our ARES/RACES organization. =A0NO complaints heard to date. Thanks, Ed, W6LOL You are absolutely right. There is an old saying "You can't please all the people all the time". That's why I put the Money Back Guarantee on everything I sell. The return rate is less than 1 in 1,000 units sold. That's good enough for me. That's it, I am done. Thanks for all the fun. 73 Al Lowe N0IMW |
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