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What's the Dealio with the J-pole?
I decided to install a 2m FM rig in my garage to stay in touch with the
local repeaters while working on the car. I want a quick and dirty homebrew antenna at the roof line. I know the J-pole is a popular 2m antenna, but why??? The J-pole is basically a half wave antenna fed at the end, right? At 2m you are only 19 inches away from the 50 ohm feed point (a classic dipole), so why not just loop the coax into the middle and forget the matching stub? Am I missing something? If I do this, would a clip on ferrite at the feed point make a good choke-balun? -- Bob D. ND9B |
What's the Dealio with the J-pole?
"Bob D." wrote in message
... I decided to install a 2m FM rig in my garage to stay in touch with the local repeaters while working on the car. I want a quick and dirty homebrew antenna at the roof line. I know the J-pole is a popular 2m antenna, but why??? The J-pole is basically a half wave antenna fed at the end, right? At 2m you are only 19 inches away from the 50 ohm feed point (a classic dipole), so why not just loop the coax into the middle and forget the matching stub? Am I missing something? If I do this, would a clip on ferrite at the feed point make a good choke-balun? -- Bob D. ND9B The coax must depart perpendicular from the 75 ohm feed point by .2 to .5 wavelength or greater for the Vertical dipole you describe. This makes it a great tower side mount antenna with gain like a 2 element beam. When mounted this way, the coax gets dressed down the tower leg and all is well without ferrites. J-pole is simple to build with 300 ohm TV twinnlead that was once cheap and available anywhere. Also the J-pole includes a matching section to 50 ohms .. Hang up one by a string, and roll it up for travel. Light weight and slick. Vertical dipole needs something like a gamma match to get to 50 ohms, but 75 ohms isn't far off and the antenna is fairly broad too. A variation is the coaxial dipole, that feeds the coax up a 1/2 wave mounting pipe to the feedpoint and a 1/4 wave whip and larger 1/4 wave ground skirt. |
What's the Dealio with the J-pole?
On Thu, 28 May 2009 11:15:52 -0400, Bob D. wrote:
I decided to install a 2m FM rig in my garage to stay in touch with the local repeaters while working on the car. I want a quick and dirty homebrew antenna at the roof line. I know the J-pole is a popular 2m antenna, but why??? The J-pole is basically a half wave antenna fed at the end, right? At 2m you are only 19 inches away from the 50 ohm feed point (a classic dipole), so why not just loop the coax into the middle and forget the matching stub? Am I missing something? If I do this, would a clip on ferrite at the feed point make a good choke-balun? There's not much performance difference between a J-pole, vertical dipole, and a quarter-wave ground-plane vertical for working "local" repeaters. A J-pole for outdoor use can be easy to build, easy to mount, and very rugged. For indoor use, you might want to settle for a simple twin-lead J-pole, which is even easier to build and can be easy to mount. A vertical dipole will tend to have a screwy radiation pattern if you just run the coax up from below and connect to the middle. But probably not enough for you to care. I'd go with whatever seems easiest to make and easiest to mount in your particular situation. .... Rich W2RG |
What's the Dealio with the J-pole?
"Bob D." wrote in message ... I decided to install a 2m FM rig in my garage to stay in touch with the local repeaters while working on the car. I want a quick and dirty homebrew antenna at the roof line. I know the J-pole is a popular 2m antenna, but why??? The J-pole is basically a half wave antenna fed at the end, right? At 2m you are only 19 inches away from the 50 ohm feed point (a classic dipole), so why not just loop the coax into the middle and forget the matching stub? Am I missing something? If I do this, would a clip on ferrite at the feed point make a good choke-balun? -- Bob D. ND9B Ho Bib I have used the ferrite clip-on beads for 2 meter antennas. They work well for me. You will be able to test them by assembling your simple antenna and see how warm they get after transmitting. Keep the coax perpendicular to the dipole, dont "loop" it back parallel to the dipole. Jerry KD6JDJ |
What's the Dealio with the J-pole?
On Thu, 28 May 2009 11:15:52 -0400, "Bob D."
wrote: I know the J-pole is a popular 2m antenna, but why??? Hi Bob, Mostly because it serves vanity more than needed gain. If a lossy rubber ducky can hit a repeater with full quietening, a J-pole has no claim for need. If I do this, would a clip on ferrite at the feed point make a good choke-balun? Go with your instincts there. 73's Richard Clark, KB7QHC |
What's the Dealio with the J-pole?
In message , Richard Clark
writes On Thu, 28 May 2009 11:15:52 -0400, "Bob D." wrote: I know the J-pole is a popular 2m antenna, but why??? Hi Bob, Mostly because it serves vanity more than needed gain. If a lossy rubber ducky can hit a repeater with full quietening, a J-pole has no claim for need. If I do this, would a clip on ferrite at the feed point make a good choke-balun? Go with your instincts there. What 'vanity' does a J-pole serve? It's a simple, efficient, endfed halfwave (essentially a 'Zepp'), low angle radiation, easy to construct, tune up, and mount on the top of a pole. I see no 'vanity'. -- Ian |
What's the Dealio with the J-pole?
On Thu, 28 May 2009 22:30:20 +0100, Ian Jackson
wrote: What 'vanity' does a J-pole serve? It's a simple, Simple compared to something even more elaborate (11 out of 10), I suppose, which would be more vainglorious. efficient, More efficient than a rubber ducky was allowed (the need for that efficiency has been skirted, however), so the claim of efficiency appeals to vanity when the difference was already noted. More efficient than a rubber ducky giving full quietening? What does that matter if not to suit vanity? endfed halfwave (essentially a 'Zepp') Now there's vanity in its full glory with the trappings of provenance (the hushed heritage of the era of the romantic Zeppelins). I bet tagging it with the name Zepp adds 5dBd gain alone! , low angle radiation, Any lower angle than a rubber ducky? It is vanity to sneer at the ducky, especially when it gets the job done without all this pomp and circumstance. easy to construct, I presume this the vain form of "simple." tune up, and mount on the top of a pole. You can mount a rubber ducky on the top of a pole too, but suffer the humiliation. No vanity boost in doing that, of course; so guilt demands a J-pole. I see no 'vanity'. I can't either - not here in the basement where I can hit my buddy's repeater a dozen miles away with a 1/2W HT driving a 6" whip. Note I say that I "can," but I don't for the shame of not having a J-pole mounted on a 20 foot mast. I only kerchunk it knowing full well my buddies won't talk to me on my whip through their own J-Poles. Of course this is ironic where the repeater is using a quarterwave because it is: 1. Much simpler, 2. Vastly more efficient; 3. Has an immensely lower angle of radiation; 4. Is superior to tune up; 5. and is mounted on a really, really tall tower (not a pole). 73's Richard Clark, KB7QHC |
What's the Dealio with the J-pole?
Richard Clark wrote:
I can't either - not here in the basement where I can hit my buddy's repeater a dozen miles away with a 1/2W HT driving a 6" whip. Note I say that I "can," but I don't for the shame of not having a J-pole mounted on a 20 foot mast. I only kerchunk it knowing full well my buddies won't talk to me on my whip through their own J-Poles. Of course this is ironic where the repeater is using a quarterwave because it is: 1. Much simpler, 2. Vastly more efficient; 3. Has an immensely lower angle of radiation; 4. Is superior to tune up; 5. and is mounted on a really, really tall tower (not a pole). Oh dear, this sounds like another case of some poor innocent beaten with a J-Pole when they were a child, leaving a lifelong hatred of the infernal things. I too was a victim of J-poling, those of us who are identified by the strange double welts and scars, one large and one small. With years of therapy, I can now stand to be in the same neighborhood with the evil "Devil's Cane". Ever notice that they resemble the dreaded Wouff Hong? The elders were on to something. Seriously though, they are just another antenna, and usually used for a pretty simple purpose, that of hitting the local repeater. I've used quarter wave, dipole and J-Pole, and yup, they hit the repeater. No majick, just something to mess with. Right now I am using a J-pole for a more important reason. My wife likes the look better than a ground plane. - 73 de Mike N3LI - |
What's the Dealio with the J-pole?
"Richard Clark" wrote in message
... On Thu, 28 May 2009 22:30:20 +0100, Ian Jackson wrote: What 'vanity' does a J-pole serve? It's a simple, Simple compared to something even more elaborate (11 out of 10), I suppose, which would be more vainglorious. efficient, More efficient than a rubber ducky was allowed (the need for that efficiency has been skirted, however), so the claim of efficiency appeals to vanity when the difference was already noted. More efficient than a rubber ducky giving full quietening? What does that matter if not to suit vanity? endfed halfwave (essentially a 'Zepp') Now there's vanity in its full glory with the trappings of provenance (the hushed heritage of the era of the romantic Zeppelins). I bet tagging it with the name Zepp adds 5dBd gain alone! , low angle radiation, Any lower angle than a rubber ducky? It is vanity to sneer at the ducky, especially when it gets the job done without all this pomp and circumstance. easy to construct, I presume this the vain form of "simple." tune up, and mount on the top of a pole. You can mount a rubber ducky on the top of a pole too, but suffer the humiliation. No vanity boost in doing that, of course; so guilt demands a J-pole. I see no 'vanity'. I can't either - not here in the basement where I can hit my buddy's repeater a dozen miles away with a 1/2W HT driving a 6" whip. Note I say that I "can," but I don't for the shame of not having a J-pole mounted on a 20 foot mast. I only kerchunk it knowing full well my buddies won't talk to me on my whip through their own J-Poles. Of course this is ironic where the repeater is using a quarterwave because it is: 1. Much simpler, 2. Vastly more efficient; 3. Has an immensely lower angle of radiation; 4. Is superior to tune up; 5. and is mounted on a really, really tall tower (not a pole). 73's Richard Clark, KB7QHC The 1/4 wave ground plane has a useless pattern. Main lobe at about 30 degrees above horizon, but broad enough to be usable and simple to build. I have one as a receive antenna for duplex operation, shielded from the transmit antennas by an air conditioner and vertical separation, feeding several receivers with cavities. It is literally thrown on the roof and coax is routed through the ductwork. Hasn't moved for 12 years. Hint: if you use an SO-239, seal the center conductor so water wont run down the rod and into the connector. Tape the coax connector tightly. For frequencies below 220, it is best to put up an outside antenna. Above those frequencies, RF gets out with much less shielding from the building. J-poles, vertical dipoles have a more useful pattern below horizon. They provide useful performance without being too large. The typical 2 meter rubber ducky has anywhere from 6 to 20 db of loss. The typical 2m 5/8 and 5db 440 dual band mobile antenna will be noticeably better. What's with all the EMO girl chatter? |
What's the Dealio with the J-pole?
"JB" wrote in message
... The 1/4 wave ground plane has a useless pattern. Main lobe at about 30 degrees above horizon, but broad enough to be usable and simple to build. The quarter-wave antenna's height above ground has much more to do with the elevation angle than the fact that the antenna is a quarter-wave ground plane. Using EZNEC, I see that a quarter wave antenna situated 3 wavelengths above real/high accuracy ground of medium characteristics has a main lobe 4 degrees above the horizon. At that angle, the gain is 5.1 dBi. You can confirm this if you have a copy of EZNEC. John |
What's the Dealio with the J-pole?
John KD5YI wrote:
"JB" wrote in message ... The 1/4 wave ground plane has a useless pattern. Main lobe at about 30 degrees above horizon, but broad enough to be usable and simple to build. The quarter-wave antenna's height above ground has much more to do with the elevation angle than the fact that the antenna is a quarter-wave ground plane. Using EZNEC, I see that a quarter wave antenna situated 3 wavelengths above real/high accuracy ground of medium characteristics has a main lobe 4 degrees above the horizon. At that angle, the gain is 5.1 dBi. You can confirm this if you have a copy of EZNEC. John If you're just looking for a portable vertical antenna, just a wire on the end of the coax, and hanging it up, works pretty well. The shield of the coax (outside surface) serves as the other half of the dipole. Heck, it will have a bizarre pattern, and couple RF everywhere, but you're talking about an antenna you're hanging out of a hotel window or something.. You're not doing earth-venus-earth radar tests... you're just getting the antenna away from where you are sitting to "outside".. |
What's the Dealio with the J-pole?
JB wrote:
The 1/4 wave ground plane has a useless pattern. Main lobe at about 30 degrees above horizon, but broad enough to be usable and simple to build. . . J-poles, vertical dipoles have a more useful pattern below horizon. They provide useful performance without being too large. . . A quarter wave ground plane (with or without sloping radials), a J-pole, and a vertical dipole all have virtually identical patterns provided that the outside of the feedline can be adequately decoupled. In free space, the maximum is directed to the horizon. In a real installation, height above ground and reflections from other objects will modify the elevation pattern, in the same manner for all those antenna types. The patterns of all these antennas can be affected by current conducted to or induced on outside of the feedline. Those with EZNEC or other modeling program might find it interesting to attach a vertical wire to the "ground" side of the feedpoint and extending downward to represent the outside of a feedline. An effective current (choke) balun can be simulated with a 1k ohm resistive load inserted in the wire. You'll find considerable current can occur on the wire when it has particular lengths, the lengths depending on whether the bottom end is grounded or open, and placement of "baluns". When the current on the wire is high, considerable pattern distortion can result. I've always supposed that this is the cause of widely differing reports of the effectiveness of a J-Pole -- some people get luckier than others with feedline length. A pair of current baluns, one at or near the feedpoint and another about a quarter wavelength below, are usually enough to suppress the current on the outside of the feedline to a low enough value to prevent pattern distortion. Roy Lewallen, W7EL |
What's the Dealio with the J-pole?
On May 29, 9:31*am, "JB" wrote:
The 1/4 wave ground plane has a useless pattern. *Main lobe at about 30 degrees above horizon, but broad enough to be usable and simple to build. The 1/4 GP with sloping radials and the 1/2 wave are so close in performance to almost be unnoticeable if both are mounted in the air say at 30 ft. The two antennas will have almost the same pattern at almost the same exact angle. When modeling both at 30 ft, the GP's maximum is at 2.7 degrees, vs 2.6 degrees for the 1/2 wave. "145 MHZ". The GP's performance will vary a bit due to the number of radials, but in general the difference between the two antennas will amount to about .3 to .5 DB. Not much. This does not take into account common mode currents. In some cases, I bet it's possible for a 1/4 GP to outperform a J-pole if the J pole has no decoupling from the line. Myself, I've never used a J-pole. I prefer a gamma loop type match if I build a base fed 1/2 wave. I don't like the matching device to be parallel with the radiating element. Most of my "simple" 2m verticals are 1/4 wave GP's.. :/ I have one in the attic hung from the rafters. It has either 6 or 8 radials.. I forgot which.. Been a long time since I've been up there. If you use more than 3-4 radials which is the norm, you will see an increase in performance. I think more due to better decoupling of the line rather than less ground loss. At 30 ft, ground loss is not much of an issue as long as the antenna is complete. Just one radial will make for a 1/2 wave vertical dipole of sorts, and ground loss should not be much of an issue at several wavelengths in the air. So... I think the increase in performance is more due to better decoupling from the line. Either type can use further decoupling techniques for improved performance. The GP will usually use a 2nd set of radials 1/4 wave below the main radial set. The same scheme can be used for the 1/2 wave if common mode currents are a problem and skewing the pattern up off the horizon. In many cases, decoupling of the line is more important to gain at a low angle than element length. Does no good to use a longer element if line currents skew the pattern upwards. |
What's the Dealio with the J-pole?
Richard Clark wrote:
More efficient than a rubber ducky was allowed (the need for that efficiency has been skirted, however), so the claim of efficiency appeals to vanity when the difference was already noted. More efficient than a rubber ducky giving full quietening? What does that matter if not to suit vanity? Is it vanity to think that since the repeater is full quieting on their end with their rubber ducky, then their signal must be full quieting into the repeater, or vanity to wish such people would use a j-pole? ac6xg |
What's the Dealio with the J-pole?
"Rich Griffiths" wrote in message communications... On Thu, 28 May 2009 11:15:52 -0400, Bob D. wrote: I decided to install a 2m FM rig in my garage to stay in touch with the local repeaters while working on the car. snip A J-pole for outdoor use can be easy to build, easy to mount, and very rugged. For indoor use, you might want to settle for a simple twin-lead J-pole, which is even easier to build and can be easy to mount. snip I'd go with whatever seems easiest to make and easiest to mount in your particular situation. Amen. I have long been a fan of the copper pipe J-pole.. I've built about two dozen of them over the years, 6m, 2m, 220 and 440. We have three of mine on the local fire station for RACES. We go up on the roof and retape the connectors every few years -- that's it. My only deviation from the many Internet plans has been the inclusion of a 100 pF cap inline with the center conductor at the feed point if the VSWR minimum wasn't 1:1 somewhere in the band after the first couple of tries. Yes, you should choke the feedline, if not with ferrites, then with a few tight turns of the coax. The copper pipe j-pole can mount to almost anything; there's no significant RF current in the lowest pipe section (below the J). "Sal" |
What's the Dealio with the J-pole?
"Jim Lux" wrote in message ... John KD5YI wrote: "JB" wrote in message ... The 1/4 wave ground plane has a useless pattern. Main lobe at about 30 degrees above horizon, but broad enough to be usable and simple to build. The quarter-wave antenna's height above ground has much more to do with the elevation angle than the fact that the antenna is a quarter-wave ground plane. Using EZNEC, I see that a quarter wave antenna situated 3 wavelengths above real/high accuracy ground of medium characteristics has a main lobe 4 degrees above the horizon. At that angle, the gain is 5.1 dBi. You can confirm this if you have a copy of EZNEC. John If you're just looking for a portable vertical antenna, just a wire on the end of the coax, and hanging it up, works pretty well. The shield of the coax (outside surface) serves as the other half of the dipole. Heck, it will have a bizarre pattern, and couple RF everywhere, but you're talking about an antenna you're hanging out of a hotel window or something.. You're not doing earth-venus-earth radar tests... you're just getting the antenna away from where you are sitting to "outside".. If you are using such construction, why not just build a coax type of colinear? It isn't so difficult to add a section of outer shield for decoupling. Have you priced PA hybrids lately? BTW any degree uptilt is usually worthless unless you are deliberately limiting coverage to the horizon. and anything significantly higher in elevation is line of sight anyway. For ground level base station or repeater that covers a localized area, a J-pole is attractive because of its broad pattern that allows numerous reflections through and between the buildings. I've been told that an upside down ground plane ought to work better, but it doesn't |
What's the Dealio with the J-pole?
"Jim Lux" wrote in message
... John KD5YI wrote: "JB" wrote in message ... The 1/4 wave ground plane has a useless pattern. Main lobe at about 30 degrees above horizon, but broad enough to be usable and simple to build. The quarter-wave antenna's height above ground has much more to do with the elevation angle than the fact that the antenna is a quarter-wave ground plane. Using EZNEC, I see that a quarter wave antenna situated 3 wavelengths above real/high accuracy ground of medium characteristics has a main lobe 4 degrees above the horizon. At that angle, the gain is 5.1 dBi. You can confirm this if you have a copy of EZNEC. John If you're just looking for a portable vertical antenna, just a wire on the end of the coax, and hanging it up, works pretty well. The shield of the coax (outside surface) serves as the other half of the dipole. Heck, it will have a bizarre pattern, and couple RF everywhere, but you're talking about an antenna you're hanging out of a hotel window or something.. You're not doing earth-venus-earth radar tests... you're just getting the antenna away from where you are sitting to "outside".. Bob D. - Please don't take my comments as being negative about a J-Pole. I happen to like them for other reasons. They can be made very rugged. The few I made started out as a 30-foot mast. 3/4 wavelengths down from the top I mounted a horizontal metal bracket. To that I mounted a vertical 1/4 wave tube. I then used automotive hose clamps to connect a 1/2 wavelength piece of coax near the bottom just above the horizontal bracket. I adjusted the connection point of the coax to find the lowest SWR point and then replaced the 1/2 wave piece of coax with my longer lead-in. I put some weatherproofing on the coax. I ran 10 gauge wire from my nearby ground rod over to the bottom of the mast. What I like about this arrangement is that the entire assembly is grounded. I once had a nearby lightning strike cause my IC2AT to block signals for several seconds while I was listening to a local repeater with a home-made ground plane. Charge buildup, I guess. That never happened with my J-Pole. That thing worked flawlessly for several years until I replaced it with a commercial dual-band (expensive) antenna. Have fun. 73, John |
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