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Stub J-pole for six meters
Can anyone direct me to the dimensions to build a six meter J-pole
that is patterned after the Arrow Antenna two meter design that had the feed point at the bottom of one of the elements rather than at a point part way up the height of the matching stub. The reason that I would like to use that design is that I would like to use Rigid Aluminum Conduit to build the antenna. There not being a lot of simple inexpensive fittings for Conduit the way there is for copper pipe a mechanically simpler design would be easier to build. Replies are welcome on or off list. Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. -- Tom Horne, W3TDH |
Stub J-pole for six meters
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Stub J-pole for six meters
On Sat, 26 Nov 2011 14:22:52 -0800 (PST), Tom Horne
wrote: Can anyone direct me to the dimensions to build a six meter J-pole that is patterned after the Arrow Antenna two meter design that had the feed point at the bottom of one of the elements rather than at a point part way up the height of the matching stub. The 144/440 antenna can be found at: http://www.arrowantennas.com/inst/OSJ146440.pdf with dimensions included. You probably won't need the short element in the middle which seems to do something for 440Mhz operation. Two ways to do this: 1. Plug the dimensions (in wavelengths) into an EZNec or 4NEC2 antenna model, and scale it for 52MHz. 2. Scale the antenna dimensions linearly from 146 to 52MHz and optimize with cut-n-try. The driven element is 1/4 wavelength long, and the parasitic element is 3/4 wavelength long. The reason that I would like to use that design is that I would like to use Rigid Aluminum Conduit to build the antenna. There not being a lot of simple inexpensive fittings for Conduit the way there is for copper pipe a mechanically simpler design would be easier to build. Maybe, but the 3/4 wavelength element is going to end up about 15ft long, which is rather long for tubing without a top support. The "rigid" aluminum conduit is made to be bent to shape. I suspect this 15ft long tube might deform in the wind. Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. Ok. I'll just ignore my experience with J-poles and pretend that they're suitable for your unstated purpose. That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. Approach for what application and mounting? If you want an antenna with a rather high takeoff angle, an end fed antenna, such as the J-pole will send most of your RF towards the birds instead of the horizon. If you want an antenna with a radiating coax cable (by lacking a balun), the J-pole is quite appropriate. If you want an antenna that produces impressive cloverleaf horizontal gain patterns when side mounted on a tower, the J-pole is the hot ticket. The J-pole does have one big advantage. It was originally called a Zepp antenna, after the Zepplin airship, where the antenna was suspended downward. Because the low voltage point is at the base of the antenna, there would be no sparks to ignite the hydrogen gas. However, if you don't plan on flying your j-pole on a Zepplin, this advantage might not be very useful. http://en.wikipedia.org/wiki/J-pole_antenna (first paragraph) Please feel free to offer undying praise and monetary renumeration for the advice. I'm always open to being paid. There are probably better designs but since you've already decided on a J-pole, it's probably futile to suggest alternatives. Besides, it might precipitate a debate or arguement. Oddly, I learn more from such debates and arguments, than from pontifications. 5/8 wave 6m ground plane: http://802.11junk.com/jeffl/antennas/gnd-plane-02/index.html 5/8 + 5/8 wave 6m ground plane: http://802.11junk.com/jeffl/antennas/gnd-plane-05/index.html I would normally offer to build you a NEC2 model of your proposed antenna, but since I don't know what diameter aluminum tubing you have in mind, and since it's 1:30AM, I'll pass for tonite. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Stub J-pole for six meters
"Tom Horne" wrote in message ... Can anyone direct me to the dimensions to build a six meter J-pole that is patterned after the Arrow Antenna two meter design that had the feed point at the bottom of one of the elements rather than at a point part way up the height of the matching stub. The reason that I would like to use that design is that I would like to use Rigid Aluminum Conduit to build the antenna. There not being a lot of simple inexpensive fittings for Conduit the way there is for copper pipe a mechanically simpler design would be easier to build. Replies are welcome on or off list. Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. -- Tom Horne, W3TDH Certainly not a rant but a question about the question: Is your reason for this approach mainly your access to beaucoup aluminum conduit? I ask because I'm a j-pole fan and I've built dozens of them, including three for 6m using copper pipe and one 6m from window line. The feed is less than 7 inches from the bottom of the active portion of the 6m version. Using your conduit, you might employ the "standard" j-pole design and bolt the segments together, placing wooden dowels inside your conduit ends to resist crushing forces. Am I striking a chord, yet? A brace near the top of the stub section would likely be required to prevent rotation where the bottom of the "j" attaches to the main section. You might be well-served to wrap the bottom of the "j" with a tight spiral of mesh sleeve (like the braid-only of some unneeded coax) and clamp the ends. This would help mitigate the electrical consequences of a bad joint or two. Alternatively, bridge (jumper) the individual joints. But see http://www.wagnercompanies.com/cast_aluminum.aspx , where a single 3/4 - inch tee or elbow is about $7.00. Do-able? Note, my 6m copper j-poles are all 3/4 - inch material except the top three feet of of the long section. I tried using all 1/2 - inch copper once but that build was so wobbly that I immediately took it apart. (It's about 13 feet tall.) Some of my j-poles have benefitted from having a capacitor from the feed connector center pin over to the the long side, in lieu of a straight piece of wire. I use one whenever the VSWR is mid-band but above 1:1. No science to it; I just tried a few around 100pF and SHAZAM! I was inspired by the capacitive feed in a gamma match. Happy brewing. "Sal" (KD6VKW) |
Stub J-pole for six meters
On Sat, 26 Nov 2011 14:22:52 -0800 (PST), Tom Horne wrote:
Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. Are you so clueless that you don't see the above as throwing down the gauntlet to every prospective troll within 50 groups of here? Good luck. |
Stub J-pole for six meters
On Nov 27, 7:07*pm, Andiour wrote:
On Sat, 26 Nov 2011 14:22:52 -0800 (PST), Tom Horne wrote: Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. *That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. Are you so clueless that you don't see the above as throwing down the gauntlet to every prospective troll within 50 groups of here? Good luck. OK I'm clueless because I asked to be spared tirades and rants. Than I guess I'm clueless but not so rude as you. -- Tom Horne, W3TDH |
Stub J-pole for six meters
On Nov 27, 2:53*am, Owen Duffy wrote:
Tom Horne wrote in news:8b22e1d5-907a-4ee6-ab40- : ... Replies are welcome on or off list. *Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. *That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. It is a sad statement of the state of affairs when you feel a need to compose a help request with such conditions, 40% of your post is setting conditions on your would be helpers responses. This is not unusualy, the posts can often can be paraphrased as "I have got this really good idea, and I don't want to hear from anyone that it is less than a really good idea... over". Your 40% conditions is by no means a record, higher percentages have been observed on QRZ and eHam, such high percentage as to make the whole thing look like a troll rather than a genuine request for help. My own view is that it helps to be a little humble when seeking help. Owen Owen I apologize for coming across as arrogant. I didn't mean to. Perhaps it would help to know that I was looking at the J-pole as a transportable antenna that would be used when the operating position is down in between buildings, ridges, mountains, etc. I did look at the online patterns for a J-pole. When I used a diamond ground plane under those conditions the results were less than satisfactory. The intent is to get the signals up out of the hole. The aluminum conduit was because it is much lighter than copper pipe and far less expensive at present pricing. I may want to replicate the antenna several times if it happens to overcome the problem for the purpose of reliable communications outside of simplex two meter range without limiting the operator pool to general and above licensees. Also there are several places were we could use six meter APRS to collect data from, for just example, a temporary stream gauge by installing the gauge in the time between a flash flood watch and any subsequent flash flood warning. Obviously the temporary gauging point may well contribute to the decision as to whether and when to issue the warning. I was thinking to use rigid aluminum conduit because of the availability of both compression and threaded couplings. I intended to test which coupling technique would stand the most bending force on the upper portion of the antenna. I already guessed that the antenna might need to be guyed but I wanted to try whether using a one inch or even one and a quarter inch nominal size would avoid the need for that. Since the Arrow Antenna design is fed at the bottom of the shorter matching stub I thought it might be easier to transport and deploy without damaging it. As for keeping the stub in line with the radiator I had envisioned a short piece of 5/8" fiberglass U channel with two stainless steel pipe clamps as being robust enough to withstand repeated handling. The three quarter wavelength portion of the antenna would be fashioned into three nearly equal sized sections for ease of transport. I have constructed a dual half wave collinear J-pole using copper pipe. It had given me much better empirical performance than the borrowed ground plane that I had used before it. If I could get the needed range out of the simple J-pole I was next going to work on the physical challenge of building another collinear dual half wave for six meters were the second have wave could be added readily for use when the antenna would not be located below local horizon. -- Tom Horne, W3TDH |
Stub J-pole for six meters
"Tom Horne" wrote in message ... Can anyone direct me to the dimensions to build a six meter J-pole that is patterned after the Arrow Antenna two meter design that had the feed point at the bottom of one of the elements rather than at a point part way up the height of the matching stub. The reason that I would like to use that design is that I would like to use Rigid Aluminum Conduit to build the antenna. There not being a lot of simple inexpensive fittings for Conduit the way there is for copper pipe a mechanically simpler design would be easier to build. Replies are welcome on or off list. Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. -- Tom Horne, W3TDH If you have not found anything by now, you may go here for a 2 meter version. Then divide the 150 mhz frequency by the 50 mhz frequency and get a number near 3. Then multiply ALL the dimensions by that number. That is the lengths, diameter of the elements and spacing. http://www.w4cll.com/ijpole.html |
Stub J-pole for six meters
On Nov 27, 4:44*am, Jeff Liebermann wrote:
On Sat, 26 Nov 2011 14:22:52 -0800 (PST), Tom Horne wrote: Can anyone direct me to the dimensions to build a six meter J-pole that is patterned after the Arrow Antenna two meter design that had the feed point at the bottom of one of the elements rather than at a point part way up the height of the matching stub. The 144/440 antenna can be found at: http://www.arrowantennas.com/inst/OSJ146440.pdf with dimensions included. *You probably won't need the short element in the middle which seems to do something for 440Mhz operation. Two ways to do this: 1. *Plug the dimensions (in wavelengths) into an EZNec or 4NEC2 antenna model, and scale it for 52MHz. 2. *Scale the antenna dimensions linearly from 146 to 52MHz and optimize with cut-n-try. *The driven element is 1/4 wavelength long, and the parasitic element is 3/4 wavelength long. The reason that I would like to use that design is that I would like to use Rigid Aluminum Conduit to build the antenna. *There not being a lot of simple inexpensive fittings for Conduit the way there is for copper pipe a mechanically simpler design would be easier to build. Maybe, but the 3/4 wavelength element is going to end up about 15ft long, which is rather long for tubing without a top support. *The "rigid" aluminum conduit is made to be bent to shape. *I suspect this 15ft long tube might deform in the wind. Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. Ok. *I'll just ignore my experience with J-poles and pretend that they're suitable for your unstated purpose. That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. Approach for what application and mounting? *If you want an antenna with a rather high takeoff angle, an end fed antenna, such as the J-pole will send most of your RF towards the birds instead of the horizon. *If you want an antenna with a radiating coax cable (by lacking a balun), the J-pole is quite appropriate. *If you want an antenna that produces impressive cloverleaf horizontal gain patterns when side mounted on a tower, the J-pole is the hot ticket. *The J-pole does have one big advantage. *It was originally called a Zepp antenna, after the Zepplin airship, where the antenna was suspended downward. *Because the low voltage point is at the base of the antenna, there would be no sparks to ignite the hydrogen gas. However, if you don't plan on flying your j-pole on a Zepplin, this advantage might not be very useful. http://en.wikipedia.org/wiki/J-pole_antenna (first paragraph) Please feel free to offer undying praise and monetary renumeration for the advice. *I'm always open to being paid. *There are probably better designs but since you've already decided on a J-pole, it's probably futile to suggest alternatives. *Besides, it might precipitate a debate or arguement. *Oddly, I learn more from such debates and arguments, than from pontifications. 5/8 wave 6m ground plane: http://802.11junk.com/jeffl/antennas/gnd-plane-02/index.html 5/8 + 5/8 wave 6m ground plane: http://802.11junk.com/jeffl/antennas/gnd-plane-05/index.html I would normally offer to build you a NEC2 model of your proposed antenna, but since I don't know what diameter aluminum tubing you have in mind, and since it's 1:30AM, I'll pass for tonite. -- Jeff Liebermann * * 150 Felker St #D * *http://www.LearnByDestroying.com Santa Cruz CA 95060http://802.11junk.com Skype: JeffLiebermann * * AE6KS * *831-336-2558 All hands Let me start over. I want a transportable, six meter, vertically polarized antenna. The parts have to be readily assembled & disassembled. I would like it to have a somewhat higher than normally attempted radiation pattern for use between buildings, hills, ridges... [I had thought that a J-pole might be a good compromise instead of antennas with a more horizontal gain pattern.] It would be really golden if the adding of additional parts would change the pattern back towards the horizontal to use the antenna when it is located above local horizon. Parts should be short enough to fit in the foot well of the back seat of an ordinary sedan. It's a stretch but If it did not need guying that would contribute to ease of deployment especially in an urban operating location were the anchoring possibilities for guy lines may be limited. [I'm trying to devise an X frame tripod. It would be made out of fiberglass structural channel in order to lower the weight of the tripod assembly but that is not an antenna design problem.] I may need the antenna to be elevated when operating near pedestrian traffic to reduce the risk of RF contact or excessive exposure. Shielding the feed point from physical damage; such as by having it inside a piece of Box channel would be quite desirable. I'll leave it there and see what folks can think of that might work. -- Tom Horne, W3TDH |
Stub J-pole for six meters
On 11/28/2011 10:40 AM, Ralph Mowery wrote:
"Tom wrote in message ... Can anyone direct me to the dimensions to build a six meter J-pole that is patterned after the Arrow Antenna two meter design that had the feed point at the bottom of one of the elements rather than at a point part way up the height of the matching stub. The reason that I would like to use that design is that I would like to use Rigid Aluminum Conduit to build the antenna. There not being a lot of simple inexpensive fittings for Conduit the way there is for copper pipe a mechanically simpler design would be easier to build. Replies are welcome on or off list. Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. -- Tom Horne, W3TDH If you have not found anything by now, you may go here for a 2 meter version. Then divide the 150 mhz frequency by the 50 mhz frequency and get a number near 3. Then multiply ALL the dimensions by that number. That is the lengths, diameter of the elements and spacing. http://www.w4cll.com/ijpole.html Tom, how concerned are you about the "match"? What length of coax are you feeding it with? Do you care if the coax radiates? What bandwidth do you need? (fat elements have broader bandwidth, for instance). How rugged does it need to be? You're looking at something with an overall length around 3 meters/10 feet? If the j-pole is being done so you have a mechanical support, there are other ways to approach the problem. I assume you want something that is "feedline comes out at the bottom", so why not something like a dipole with the feedline going up inside the bottom half of the dipole? You'd need something mechanically strong enough to hold the two pieces of aluminum together that's an insulator. Hose clamps would work to both make the wiring connection and hold the tubing to the insulator. (I'd look at a piece of fiberglass or plastic rod of appropriate diameter.) Since you want to use conduit kinds of things, what about using standard plastic electrical things. Perhaps a plastic conduit outlet body? (Cantex PVC conduit body, type C, 1/2" hub, Grainger part #4FYP5, about $5) Just screw the threaded end of the conduit into the body and you're done. (If you get the stuff with a removable coupling, especially easy.. 10ft stick, saw it in half, and you're almost done) Do you have test equipment for tuning? Or are you looking for a cookbook design that you can just build and know it will work as long as the dimensions match the recipe? |
Stub J-pole for six meters
Tom,
The clever thing with the Arrow implementation is its dual band version where it is mechanically elegant. Does it lend itself to scaling up by near three times? It isn't clear that you wanted a dual band version, though there are hints. If you are looking for a single band antenna as implied in the subject... As you have noted, the total height of the J pole for 6m is a potential issue. You could look at it as essentially an end fed half wave with a rather large impedance matching device (~1.5m). There are other more compact ways to impedance match an end fed half wave. The question for you is whether you can devise a robust implementation. Is there advantage in using aluminium tube over wire? Well, obviously the wire needs support, but perhaps a telescoping fibreglass pole and wire with a matching network in a PVC box is another robust option. Keep in mind that end feeding a half wave reduces the common mode current flowing to the feedline, the feed point voltage is highest at exactly a half wave. You might find a slightly shorter radiator to deliver most of the 'ground independent' feed, but with slightly less demands on the voltage rating of the matching network. It may be prudent to make your matching network adjustable to provide for locations where proximity to other things changes the tuning. I said fibreglass (meaning literally glass reinforced plastic), and that doesn't include graphite or carbon fibre. A lot of these fishing poles are now made from graphite, and that introduces potential for loss. BTW, I am not sure that your 'design' of the pattern is all that sound, however real antennas near lossy soil just don't have very low angle major lobes... so what you want happens for probably any low makeshift antenna. Owen |
Stub J-pole for six meters
On Nov 28, 10:31*am, Tom Horne wrote:
On Nov 27, 2:53*am, Owen Duffy wrote: Tom Horne wrote in news:8b22e1d5-907a-4ee6-ab40- : ... Replies are welcome on or off list. *Let me say that I would rather not be subjected to any tirades about the inferiority of the J-pole approach or the superiority of some other antenna design. *That does not mean that I am not open to suggestions for a better approach as long as it is civilly stated and I am spared the application of Rant Mode. It is a sad statement of the state of affairs when you feel a need to compose a help request with such conditions, 40% of your post is setting conditions on your would be helpers responses. This is not unusualy, the posts can often can be paraphrased as "I have got this really good idea, and I don't want to hear from anyone that it is less than a really good idea... over". Your 40% conditions is by no means a record, higher percentages have been observed on QRZ and eHam, such high percentage as to make the whole thing look like a troll rather than a genuine request for help. My own view is that it helps to be a little humble when seeking help. Owen Owen I apologize for coming across as arrogant. *I didn't mean to. Perhaps it would help to know that I was looking at the J-pole as a transportable antenna that would be used when the operating position is down in between buildings, ridges, mountains, etc. *I did look at the online patterns for a J-pole. *When I used a diamond ground plane under those conditions the results were less than satisfactory. *The intent is to get the signals up out of the hole. *The aluminum conduit was because it is much lighter than copper pipe and far less expensive at present pricing. *I may want to replicate the antenna several times if it happens to overcome the problem for the purpose of reliable communications outside of simplex two meter range without limiting the operator pool to general and above licensees. *Also there are several places were we could use six meter APRS to collect data from, for just example, a temporary stream gauge by installing the gauge in the time between a flash flood watch and any subsequent flash flood warning. Obviously the temporary gauging point may well contribute to the decision as to whether and when to issue the warning. *I was thinking to use rigid aluminum conduit because of the availability of both compression and threaded couplings. *I intended to test which coupling technique would stand the most bending force on the upper portion of the antenna. *I already guessed that the antenna might need to be guyed but I wanted to try whether using a one inch or even one and a quarter inch nominal size would avoid the need for that. *Since the Arrow Antenna design is fed at the bottom of the shorter matching stub I thought it might be easier to transport and deploy without damaging it. *As for keeping the stub in line with the radiator I had envisioned a short piece of 5/8" fiberglass U channel with two stainless steel pipe clamps as being robust enough to withstand repeated handling. *The three quarter wavelength portion of the antenna would be fashioned into three nearly equal sized sections for ease of transport. I have constructed a dual half wave collinear J-pole using copper pipe. *It had given me much better empirical performance than the borrowed ground plane that I had used before it. *If I could get the needed range out of the simple J-pole I was next going to work on the physical challenge of building another collinear dual half wave for six meters were the second have wave could be added readily for use when the antenna would not be located below local horizon. -- Tom Horne, W3TDH The love-hate relationship with J-poles has been pretty well documented over the years here on r.r.a.a. My feeling is that you can resolve that pretty well by decoupling the antenna from the feedline, and also from other nearby conductors. That's something you want to do with any antenna, at least where you want to have consistent, reliable performance in the face of differing feedline and antenna- support installations. One of the implications here is that you'll have a non-conductive support for the antenna. You should be able to get pretty reliable feedline decoupling with a choke in the feedline where it drops below the antenna, and another a quarter wave below that. For narrow-band operation (say operation in a band less than 1MHz wide at 50MHz), I'm partial to tuned chokes: a small coil of coax bridged by a capacitor to resonate it at the operating frequency. That's obviously not the only way to do it, but done right, it can give you quite a high impedance in series with the line while adding negligible additional loss due to the slightly longer feedline. Since a J-pole is nominally 3/4 wave long, at 6 meters wavelength that implies something roughly 4.5 meters, or about 14 feet, long. I guess you can come up with some arrangement of pipes that will fit across the back seat of your car that you can put together to get what you want, maybe four nominally 1/4 wave sections. I wonder, though, if it wouldn't be easier to use telescoping sections of fiberglass (or similar reasonably stiff mast material) that would take you up to where the top of the antenna needs to be, and then just use a wire antenna. Then if there happens to be a natural way to hang the antenna (e.g. from a line tossed over a tree limb), you don't need to bother with setting up any mast + support. As far as exact dimensions of the antenna itself, given all the good antenna analysis programs out there, I consider that the easy part, but dependent on just how you elect to make it: wire or sections of conductive tubing, and just how you will feed the antenna (tapped up the shorted quarter wave bottom section, or connected to the open bottom of the quarter wave). Pick a mechanical arrangement that works for you, then use a program to come close to the required feed arrangement and details of the element lengths and spacing, then build/ measure/trim to get what you want -- if you accept that anything below a 1.5:1 SWR works well, then you will probably have little if any trimming to do, assuming you modeled it accurately. Cheers, Tom |
Stub J-pole for six meters
Owen
My replies are in line. On Nov 28, 2:56*pm, Owen Duffy wrote: Tom, The clever thing with the Arrow implementation is its dual band version where it is mechanically elegant. Does it lend itself to scaling up by near three times? It isn't clear that you wanted a dual band version, though there are hints. If you are looking for a single band antenna as implied in the subject... Yes. I just want it to for six meters. As you have noted, the total height of the J pole for 6m is a potential issue. You could look at it as essentially an end fed half wave with a rather large impedance matching device (~1.5m). There are other more compact ways to impedance match an end fed half wave.. The question for you is whether you can devise a robust implementation. I'd love to hear about those other compact impedance matches. Two different online articles claim that the matching stub can be at a right angle to the radiating element without causing degradation in performance. Can anyone give some authoritative reference that can confirm or debunk that claim? Is there advantage in using aluminium tube over wire? Well, obviously the wire needs support, but perhaps a telescoping fibreglass pole and wire with a matching network in a PVC box is another robust option. Keep in mind that end feeding a half wave reduces the common mode current flowing to the feedline, the feed point voltage is highest at exactly a half wave. You might find a slightly shorter radiator to deliver most of the 'ground independent' feed, but with slightly less demands on the voltage rating of the matching network. It may be prudent to make your matching network adjustable to provide for locations where proximity to other things changes the tuning. I said fibreglass (meaning literally glass reinforced plastic), and that doesn't include graphite or carbon fibre. A lot of these fishing poles are now made from graphite, and that introduces potential for loss. BTW, I am not sure that your 'design' of the pattern is all that sound, however real antennas near lossy soil just don't have very low angle major lobes... so what you want happens for probably any low makeshift antenna. You lost me on this one. Are you saying that the take off angles of the half wave J-pole are not an effective way to get the signal up out of the holes? Owen -- Tom Horne, W3TDH |
Stub J-pole for six meters
Tom,
The clever thing with the Arrow implementation is its dual band version where it is mechanically elegant. Does it lend itself to scaling up by near three times? It isn't clear that you wanted a dual band version, though there are hints. If you are looking for a single band antenna as implied in the subject... Yes I am only concerned with the six meter band. As you have noted, the total height of the J pole for 6m is a potential issue. You could look at it as essentially an end fed half wave with a rather large impedance matching device (~1.5m). I am told that the matching section could be at ninety degrees to the radiating element without any loss of function. Can anyone confirm that this is true. There are other more compact ways to impedance match an end fed half wave. The question for you is whether you can devise a robust implementation. OK You have my attention. I look forward to hearing about them. Is there advantage in using aluminum tube over wire? Well, obviously the wire needs support, but perhaps a telescoping fibreglass pole and wire with a matching network in a PVC box is another robust option. Since it might have to be deployed when nigh winds are expected I wanted to stay with a self supporting design. Keep in mind that end feeding a half wave reduces the common mode current flowing to the feedline, the feed point voltage is highest at exactly a half wave. You might find a slightly shorter radiator to deliver most of the 'ground independent' feed, but with slightly less demands on the voltage rating of the matching network. It may be prudent to make your matching network adjustable to provide for locations where proximity to other things changes the tuning. I said fibreglass (meaning literally glass reinforced plastic), and that doesn't include graphite or carbon fibre. A lot of these fishing poles are now made from graphite, and that introduces potential for loss. BTW, I am not sure that your 'design' of the pattern is all that sound, however real antennas near lossy soil just don't have very low angle major lobes... so what you want happens for probably any low makeshift antenna. Are you saying that the J-poles pattern is inappropriate to the task of getting the signals up out of the hole I would really like to hear why that is true Owen -- Tom Horne |
Stub J-pole for six meters
All hands Let me start over. I want a transportable, six meter, vertically polarized antenna. The parts have to be readily assembled& disassembled. I would like it to have a somewhat higher than normally attempted radiation pattern for use between buildings, hills, ridges... [I had thought that a J-pole might be a good compromise instead of antennas with a more horizontal gain pattern.] It would be really golden if the adding of additional parts would change the pattern back towards the horizontal to use the antenna when it is located above local horizon. Parts should be short enough to fit in the foot well of the back seat of an ordinary sedan. It's a stretch but If it did not need guying that would contribute to ease of deployment especially in an urban operating location were the anchoring possibilities for guy lines may be limited. [I'm trying to devise an X frame tripod. It would be made out of fiberglass structural channel in order to lower the weight of the tripod assembly but that is not an antenna design problem.] I may need the antenna to be elevated when operating near pedestrian traffic to reduce the risk of RF contact or excessive exposure. Shielding the feed point from physical damage; such as by having it inside a piece of Box channel would be quite desirable. I'll leave it there and see what folks can think of that might work. -- Tom Horne, W3TDH Regarding the higher than normal angle request, either 1) make it fairly short so as to approximate an isotropic radiator 2) make it a significant amount longer than a normal J pole so you get lobes above the horizon. You will need to model this for each concrete or natural canyon you expect to be in. And then think a lot more. Possibly a different solution is better. Different band to start with. I don't know, just trying to stir up your thinking. tom K0TAR |
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