|
J pole question
I am thinking of making a 2 meter J-Pole from 3/4 or 5/8 copper pipe.
Most plans that I can find on the web call for a 58 inch main leg and a 20 inch support below, making the antenna 78 inches or so in height. I want to install it in my attic where I have 66 inches of clear space. Can I cut the lower support leg down to 8 inches without a major impact on the performance of the antenna? |
J pole question
"ai8o" wrote in message ... I am thinking of making a 2 meter J-Pole from 3/4 or 5/8 copper pipe. Most plans that I can find on the web call for a 58 inch main leg and a 20 inch support below, making the antenna 78 inches or so in height. I want to install it in my attic where I have 66 inches of clear space. Can I cut the lower support leg down to 8 inches without a major impact on the performance of the antenna? course you can, make it from 300 ohm ribbon feeder & hang it via the top, I did that with a slim jim which may be better than a j pole, just google slim jim for 2 m if you don' recognise it |
J pole question
|
J pole question
On Mar 16, 5:57*pm, ai8o wrote:
I am thinking of making a 2 meter J-Pole from 3/4 or 5/8 copper pipe. Most plans that I can find on the web call for a 58 inch main leg and a 20 inch support below, making the antenna 78 inches or so in height. I want to install it in my attic where I have 66 inches of clear space. Can I cut the lower support leg down to 8 inches without a major impact on the performance of the antenna? Sure leave it off altogether and hang it with fishing line. Jimmie |
J pole question
In message , Owen Duffy
writes ai8o wrote in news:1bcefed7-a087-4fda-976d- : I am thinking of making a 2 meter J-Pole from 3/4 or 5/8 copper pipe. Most plans that I can find on the web call for a 58 inch main leg and a 20 inch support below, making the antenna 78 inches or so in height. I want to install it in my attic where I have 66 inches of clear space. Can I cut the lower support leg down to 8 inches without a major impact on the performance of the antenna? End fed antennas are an implementation risk. What's an 'implementation risk'? Have you considered a vertical dipole hung from the roof structure, and taking the feedline away horizontally for a couple of wavelengths, then down. You could make it a folded dipole (TV ribbon?) and use a half wave coax balun. Are you suggesting that a centre-fed dipole is better than a J-Pole? There is absolutely no problem with the J-Pole / Slim Jim type of antenna, or with feeding it. -- Ian |
J pole question
On 3/16/2010 5:57 PM, ai8o wrote:
I am thinking of making a 2 meter J-Pole from 3/4 or 5/8 copper pipe. Most plans that I can find on the web call for a 58 inch main leg and a 20 inch support below, making the antenna 78 inches or so in height. I want to install it in my attic where I have 66 inches of clear space. Can I cut the lower support leg down to 8 inches without a major impact on the performance of the antenna? The support leg is just that a leg. You can make it any length that works for you. John W3JXP |
J pole question
On Wed, 17 Mar 2010 08:22:52 +0000, Ian Jackson
wrote: End fed antennas are an implementation risk. What's an 'implementation risk'? Having it come out mismatched, a very much higher probability. Have you considered a vertical dipole hung from the roof structure, and taking the feedline away horizontally for a couple of wavelengths, then down. You could make it a folded dipole (TV ribbon?) and use a half wave coax balun. Are you suggesting that a centre-fed dipole is better than a J-Pole? A center fed dipole is simpler to tune and keep tuned. There is absolutely no problem with the J-Pole / Slim Jim type of antenna, or with feeding it. The majority of users who come here for solutions to theirs would suggest otherwise. 73's Richard Clark, KB7QHC |
J pole question
John Passaneau wrote:
The support leg is just that a leg. You can make it any length that works for you. John W3JXP A J pole, or ground plane for that matter, couples into the support structure and outside of the feedline, making those conductors part of the radiating system. This alters the radiation pattern and can result in further coupling to anything near the support structure or feedline. The amount of coupling can be anything from insignificantly small to very large -- with the support and/or feedline radiating more than the "antenna" --, depending on the length and orientation of the entire conductive path to Earth ground. Roy Lewallen, W7EL |
J pole question
Ian Jackson wrote in
: In message , Owen Duffy writes .... End fed antennas are an implementation risk. What's an 'implementation risk'? We will see if the OP, like so many who implement J poles in the roof space, becomes aware of problems and comes back for advice on how to solve them. The commonest problem is that VSWR doesn't meet expectations, but that it is the most mentioned problem speaks more of the visibility of VSWR that that it is the greatest problem with the J Pole. Like many if not most end fed antennas, the J Pole does little to deal with the common mode current that must flow on the feed system / support structure. Explanations of how they work usually deny the issue. But for all their faults, they spray RF around, and people would foolish to deny that they 'work' to some extent. When you look at the strengths and weaknesses of a J Pole, they are not a good choice IMHO for in-roof-space deployment. Owen |
J pole question
John Passaneau wrote in news:hnqnnl$kk5a$1
@tr22n12.aset.psu.edu: Can I cut the lower support leg down to 8 inches without a major impact on the performance of the antenna? The support leg is just that a leg. You can make it any length that works for you. The 'support leg' as it is being discussed in not just a physical support for the half wave. If the upper half wave section carries current, there must be a non zero current at the lower end of it. That current MUST exist as a common mode current on the upper part of the 'support leg' as the OP terms it and you concur. That common mode current is a standing wave, and the distribution of it on the 'support leg', feedline, and any conducting supporting structure makes all of these elements part of the radiating system to some extent, depending on the actual geometry and their coupling to other conductors in close proximity. So to represent that U shaped section as merely supporting structure is to deny how the thing really works... but then that is the stuff of J Poles. Owen |
J pole question
Owen Duffy wrote:
So to represent that U shaped section as merely supporting structure is to deny how the thing really works... but then that is the stuff of J Poles. Are you sure, Owen? I read it as the bottom part of the j-pole. I made a j-pole once, the top was copper, and the support was thick walled PVC. Obviously that wasn't an active part. The "u" shaped portion was all above the support leg. The antenna matched up well with moving the clips back and forth. Still works okay. I don't look at them as any better or worse thatn a 1/4 wave gp, or vertical dipole, both of which I had before. Did not measure for feedline radiation, but I have people at home who let me know of such problems. - 73 de Mike N3LI - |
J pole question
Michael Coslo wrote in
: .... Are you sure, Owen? I read it as the bottom part of the j-pole. I made a j-pole once, the top was copper, and the support was thick walled PVC. Obviously that wasn't an active part. The "u" shaped portion was all above the support leg. The antenna matched up well with moving the clips back and forth. Still works okay. I don't look at them as any better or worse thatn a 1/4 wave gp, or vertical dipole, both of which I had before. Did not measure for feedline radiation, but I have people at home who let me know of such problems. Mike, Even if you put a J Pole on an insulating support, or build it with flexible wire inside a rigid PVC tube, you can't pretend the the common mode current path on the outside of the coax feedline does not exist. If you take extreme care with symmetry of the J Pole, then common mode current is lower, and with some further decoupling might be very acceptable. I doubt that common constructions achieve that goal, enquirers here and elsewhere often complain of a sensitivity of VSWR of proximity of their body and other things to the feedline... a sign of feedline common mode current. If by a vertical dipole, you mean a coaxial dipole (though I note people are now using 'coaxial dipole' to refer to the Double Bazooka), they are also notoriously bad for feedline decoupling and need further sleeves (Bazooka Balun) or radials to effectively decouple the feedline. The J Pole (and many other end fed antennas such as the Ringo) have a matching arrangement that makes VSWR very sensitive to the antenna's local environment. One of the most common questions is "I tuned this up real good in the workshop, and when I put it in the attic, the VSWR is 3:1". Sure, the attic is a challenging place to put an antenna, but a narrowband matching network exacerbates the problem. Why is the attic so bad? You have to ask is the roof conductive or variabile permittivity (under any weather) (steel, shingles, concrete tiles etc), does it have sarking or other conductive insulation products (such as foil aircell blankets) under it, or on the ceiling, does the ceiling have foil backed insulation on it, what other conductors are in the roof space (water, gas, flues, HVAC ducts, structural steel, framing braces, wiring...etc). What works for one installation might not work for another because the installer has no idea of the installation. An experienced eye sees things that an inexperience eye doesn't. A well executed ground plane antenna is an antenna that a person with little knowledge and experience can implement with a high level of confidence that it is reasonably efficient and effective. I didn't really want to can the J Pole, but to say to the OP if he REALLY MUST put an antenna in the roof space, he ought consider a folded dipole with half wave coax balun and dresss the feedline away horizontally for a wavelength or two for minimal effects on pattern (though that is not well controlled in the roof space). BTW, a question that reveals the understanding of a J Pole by its devotees is "should the braid go to one side or the other". All three answers (yes, three) are offered in similar proportion, but I have never seen anyone support them with measurement. It seems to me that the preferred method from purely an electrical point of view is none of those three. If you establish the tapping point, drill a hole in either tube at that point, and another at the bottom of the U, then pass the coax up through the bottom hole inside the tube (bonding the shield to that point), exit through the other hole bonding the coax shield to the tube and connecting the inner conductor to the opposite side, you have built an integral balun which helps to reduce common mode current. A further ferrite balun below the tube somewhat using say #61 would give improved suppression at 144MHz. Owen |
J pole question
Owen Duffy wrote:
If by a vertical dipole, you mean a coaxial dipole (though I note people are now using 'coaxial dipole' to refer to the Double Bazooka), they are also notoriously bad for feedline decoupling and need further sleeves (Bazooka Balun) or radials to effectively decouple the feedline. I assumed when he said a vertical dipole, he meant exactly that. Two 19 inch wires center fed. It's cheap, versatile, and does a lot better than your average rubber ducky. You mount it verticaly for FM, horizontaly for ssb/cw and at an angle for satellite work. Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or understanding, as in he has a sub-wikipedia understanding of the situation. i.e possessing less facts or information than can be found in the Wikipedia. |
J pole question
"Geoffrey S. Mendelson" wrote in
: .... I assumed when he said a vertical dipole, he meant exactly that. Two 19 inch wires center fed. It's cheap, versatile, and does a lot better than your average rubber ducky. I suspect you probably haven't read the thread. That was my original suggestion with the addition of a balun and some detail on feedline routing. Owen |
J pole question
In article ,
Owen Duffy wrote: A well executed ground plane antenna is an antenna that a person with little knowledge and experience can implement with a high level of confidence that it is reasonably efficient and effective. That would certainly be another workable alternative for this situation. An SO-239, a few feet of 10-gauge solid wire, and a bit of soldering, and you end up with a ground-plane antenna with a hanging loop at the top, two or four ground radials drooping at around a 45-degree angle, and a near-ideal match to 50-ohm coax. Cheap and quick to make. Stick a ferrite or two on the feedline just below it, and feedline radiation shouldn't be a problem. If I recall correctly, this very design appeared in the "quick tips" freebie blurb I got from the ARRL after I first got my ticket. It seems to me that the preferred method from purely an electrical point of view is none of those three. If you establish the tapping point, drill a hole in either tube at that point, and another at the bottom of the U, then pass the coax up through the bottom hole inside the tube (bonding the shield to that point), exit through the other hole bonding the coax shield to the tube and connecting the inner conductor to the opposite side, you have built an integral balun which helps to reduce common mode current. A further ferrite balun below the tube somewhat using say #61 would give improved suppression at 144MHz. I used that very design to build a two-arm "Copper Cactus", with separate stubs and feedlines for 2 meters and 440. It seemed to work well (although I never actuually measured the feedline currents). One gotcha to this approach - you have to be *very* careful to waterproof the point where the coax exits from the upper hole and has its shield bonded to the pipe! If you don't get a thoroughly waterproof coating of silicone sealant (or something similar) here, the exposed braid will wick up water during every rainstorm which comes along, and your coax will turn into a hose. The SWR on mine went sky-high after a winter up in the California rains. When I disconnected the N connectors down at the base of the antenna, I found them full of water and algae(!) and had a devil of a time getting things dried out again. -- Dave Platt AE6EO Friends of 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
Owen Duffy wrote:
I suspect you probably haven't read the thread. That was my original suggestion with the addition of a balun and some detail on feedline routing. Some people have more agressive killfiles than others. :-) Geoff. -- Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or understanding, as in he has a sub-wikipedia understanding of the situation. i.e possessing less facts or information than can be found in the Wikipedia. |
J pole question
On Mar 17, 5:36*pm, (Dave Platt) wrote:
In article , Owen Duffy wrote: A well executed ground plane antenna is an antenna that a person with little knowledge and experience can implement with a high level of confidence that it is reasonably efficient and effective. That would certainly be another workable alternative for this situation. *An SO-239, a few feet of 10-gauge solid wire, and a bit of soldering, and you end up with a ground-plane antenna with a hanging loop at the top, two or four ground radials drooping at around a 45-degree angle, and a near-ideal match to 50-ohm coax. *Cheap and quick to make. *Stick a ferrite or two on the feedline just below it, and feedline radiation shouldn't be a problem. *If I recall correctly, this very design appeared in the "quick tips" freebie blurb I got from the ARRL after I first got my ticket. That is my choice for a vertical antenna hung from rafters. I've got one in my attic. Quarter wave ground plane with 8 radials. Hung from the rafters with fishing line. I'm lucky that my roof is fairly non conductive. All the ducts etc are well below the antenna. I've never really like J poles for all the reasons already mentioned. The radiator is 19 inches, and ditto for the radials, except being as they slope at about a 45 degree angle, they require less height than if the lower half of the antenna went straight down from the feed point. BTW, if you model them, the usual difference between a 1/4 wave GP and a 1/2 wave vertical usually amounts to about .3 db... Not enough to worry about, and not enough to make using the taller antenna really worth the extra trouble. Not to mention the GP is much easier to decouple from the line. And decoupling from the line is much more critical than any extra gain from a longer radiator when used for VHF/UHF and the very low angles most local contacts will use. In many cases, the GP will outperform the J pole due to it's better decoupled feed line. |
J pole question
Owen Duffy wrote:
Michael Coslo wrote in : ... Are you sure, Owen? I read it as the bottom part of the j-pole. I made a j-pole once, the top was copper, and the support was thick walled PVC. Obviously that wasn't an active part. The "u" shaped portion was all above the support leg. The antenna matched up well with moving the clips back and forth. Still works okay. I don't look at them as any better or worse thatn a 1/4 wave gp, or vertical dipole, both of which I had before. Did not measure for feedline radiation, but I have people at home who let me know of such problems. Mike, Even if you put a J Pole on an insulating support, or build it with flexible wire inside a rigid PVC tube, you can't pretend the the common mode current path on the outside of the coax feedline does not exist. No argument there. If you take extreme care with symmetry of the J Pole, then common mode current is lower, and with some further decoupling might be very acceptable. I doubt that common constructions achieve that goal, enquirers here and elsewhere often complain of a sensitivity of VSWR of proximity of their body and other things to the feedline... a sign of feedline common mode current. Yes, I'll note that my installation was not particularly sensitive to proximity. If by a vertical dipole, you mean a coaxial dipole (though I note people are now using 'coaxial dipole' to refer to the Double Bazooka), they are also notoriously bad for feedline decoupling and need further sleeves (Bazooka Balun) or radials to effectively decouple the feedline. No, this is just a dipole hung vertically, mainly because it will be used for repeater work. Why is the attic so bad? You have to ask is the roof conductive or variabile permittivity (under any weather) (steel, shingles, concrete tiles etc), does it have sarking or other conductive insulation products (such as foil aircell blankets) under it, or on the ceiling, does the ceiling have foil backed insulation on it, what other conductors are in the roof space (water, gas, flues, HVAC ducts, structural steel, framing braces, wiring...etc). What works for one installation might not work for another because the installer has no idea of the installation. Agreed, for me at least, putting an antenna in an attic is a last resort. I came into this thread sideways, hence some of the confusion, but if I needed stealth, I'd probably make a ground plane out of pipe that fit nicely over one of the vent stacks above the bathrooms. Inside, I would go for a dipole of some sort, not a J-pole. we're pretty much on the same page here, any issues are there because of the point at which I jumped into the conversation. I may ask some questions about the stack antenna in a new thread. - 73 de Mike N3LI - |
| All times are GMT +1. The time now is 02:20 AM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com