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On Sat, 15 Feb 2014 13:27:15 -0800, "Sal" salmonella@food
poisoning.org wrote: I had a question about his j-pole analysis. He says "A J-pole, like ANY end fed antenna, needs radials, a counterpoise or ground plane to work properly." I've never seen this anywhere else, even as a suggestion or hint. I can envision places to fit one but what would it do? As built, both the pattern and the impedance already seem to be what I want. Thoughts? I agree with the author (Terry Graves K7FE). The rule-of-thumb is that an end fed antenna requires a shield ground to act as a counterpoise and to keep the coax from radiating. The J-pole (and Zepp) antennas are not exactly end fed, but are close enough. A possible exception to the rule is a 1/2 wave end fed antenna, which allegedly does not require a ground plane. I'm undecided on that point. However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven element is a 1/4 wavelength long, and therefore DOES require a ground plane. This article covers the point (and more): http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm Quoting: Summary End-feds Without Grounds ANY END-FED ANTENNA REQUIRES A LARGE GROUNDPLANE OR OTHER EXTRAORDINARY ISOLATION METHOD OR METHODS TO PREVENT FEEDLINE OR MAST COMMON MODE CURRENTS! This is true for 5/8th waves, Zepp antennas, R7's, R5's, or even common J-poles. End-feeding antennas is bad news unless you have a large well-established ground at the feedpoint. Even 1/4wl groundplanes have common mode problems. When I designed a commercial 1/4 wave groundplane with four 1/4 wave long radials, I had to insulate the radials from the mast and isolate the coax shield from the mast and radials with a 1/4 wave stub that formed a choke balun. Without the decoupling, I could change SWR simply by changing mast or feedline grounding. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#2
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On Sat, 15 Feb 2014 19:58:10 -0800, Jeff Liebermann
wrote: http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm Not really a criticism, but more of an oddity. Starting on Pg 10, the author shows that a J-Pole driving the LONG element has 2.37dBi gain, while the same antenna driving the SHORT element has -3.17dBi gain. I've never really looked as which way is the correct way to connect the coax cable. I also don't have any J-Poles around the house. Skimming the available photos: https://www.google.com/search?tbm=isch&q=j-pole I find a mix of methods. Most seem to do it the right way, but there are plenty doing it wrong. http://forums.radioreference.com/scanner-receiver-antennas/208290-j-pole-discrepancy.html http://wmarc.wildmidwest.org/slide_shows/J-Pole_Antenna_Build/images/GEN_3764.jpg This one is interesting because the two drawings show the correct wiring, but the photograph shows it built backwards. http://www.iw5edi.com/technical-articles/144-430-dual-band-jpole-antenna Sigh. I hate to admit it, but I think I've built them backwards over the years. Maybe that's why J-poles have such a lousy reputation and why I think they suck? -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#3
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In article ,
Jeff Liebermann wrote: I hate to admit it, but I think I've built them backwards over the years. Maybe that's why J-poles have such a lousy reputation and why I think they suck? Jeff- Please pardon my ignorance. I understood that a J-Pole is a half wave antenna connected to a quarter wave stub. The transmission line is connected to a low impedance point (50 Ohms?) on the stub. In the literature, it is usually fed as an unbalanced antenna, but it is not. Therefore, it should not matter which side has the center conductor or shield - they are both wrong! Fred K4DII |
#4
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![]() "Fred McKenzie" wrote in message ... In article , Jeff Liebermann wrote: I hate to admit it, but I think I've built them backwards over the years. Maybe that's why J-poles have such a lousy reputation and why I think they suck? Jeff- Please pardon my ignorance. I understood that a J-Pole is a half wave antenna connected to a quarter wave stub. The transmission line is connected to a low impedance point (50 Ohms?) on the stub. In the literature, it is usually fed as an unbalanced antenna, but it is not. Therefore, it should not matter which side has the center conductor or shield - they are both wrong! Fred K4DII I acknowledge the split opinion on the feed. I've often thought it shouldn't matter, since the idea is to excite the stub (which is, itself, a half-wave if you count both sides). I follow the crowd, frankly. How would you change the feed method? I've had generally good performance from my J-poles but I'll gladly improve what I do, if you have some ideas. 73, "Sal" (KD6VKW) |
#5
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In article ,
Sal salmonella@food poisoning.org wrote: How would you change the feed method? I've had generally good performance from my J-poles but I'll gladly improve what I do, if you have some ideas. One of the older ARRL guides or antenna books shows a balanced method of feeding a J-pole. A standard half-wave coaxial balun is used. The two balanced outputs of the balun are tapped onto the two sides of the J-pole matching section, some distance above the usual "50-ohms-or- thereabouts" attachment point. My understanding is that the impedances "seen" on the two sides of the matching section won't be identical; the short side ends at an open-circuit point and the other side "ends" at the beginning of the half-wave section, where the impedance is high but not quite an open circuit. Hence, you won't achieve complete balance this way - there will probably be some current flow on the outside of the halfwave coax balun section. |
#6
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![]() "David Platt" wrote in message ... In article , Sal salmonella@food poisoning.org wrote: How would you change the feed method? I've had generally good performance from my J-poles but I'll gladly improve what I do, if you have some ideas. One of the older ARRL guides or antenna books shows a balanced method of feeding a J-pole. A standard half-wave coaxial balun is used. The two balanced outputs of the balun are tapped onto the two sides of the J-pole matching section, some distance above the usual "50-ohms-or- thereabouts" attachment point. My understanding is that the impedances "seen" on the two sides of the matching section won't be identical; the short side ends at an open-circuit point and the other side "ends" at the beginning of the half-wave section, where the impedance is high but not quite an open circuit. Hence, you won't achieve complete balance this way - there will probably be some current flow on the outside of the halfwave coax balun section. I don't know which one would be the best but I have seen 3 methods of feeding the J-pole. If you insulate the bottom then you hook the feedline to the bottom with the center of the coax to the long side. If you do not insulate the bottom you tap up the matching segment so that you get a 50 ohm (if that is the coax used) match with the center of the coax connected to the long leg. Then there is the balun made out of coax that is hooked up to the matching segment so that a low swr is obtained. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
#7
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#8
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On Tue, 18 Feb 2014 18:10:39 -0500, "Ralph Mowery"
wrote: I don't know which one would be the best but I have seen 3 methods of feeding the J-pole. If you insulate the bottom then you hook the feedline to the bottom with the center of the coax to the long side. If you do not insulate the bottom you tap up the matching segment so that you get a 50 ohm (if that is the coax used) match with the center of the coax connected to the long leg. Then there is the balun made out of coax that is hooked up to the matching segment so that a low swr is obtained. There's also the American Legion J-Pole or the Silicon Valley Emergency Communications Systems J-Pole: https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396072666399154 https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396111364421106 This design does one thing right that none of the other J-Pole mutations seem to consider. The length of the wire between the coax connector center conductor and the driven element is an inductor. In order to tune out this inductance, one needs a series capacitor, with the inductor and capacitor tuned to the operating frequency. In other words, a gamma match. The series capacitor is formed by the insulated turns of electrical wire wrapped around the driven element. Another thing this design does right is use the zero current point at the bottom of the antenna as a ground. The problem is that it also extends the length of the center wire, which makes using a gamma match all the more important. I think putting the 50 ohm feed point and the corresponding ground close to each other were either to reduce the inductance of the connecting wire, or some manner of mutation from when it was fed by a balance line. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#9
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On Mon, 17 Feb 2014 18:04:16 -0500, Fred McKenzie
wrote: In article , Jeff Liebermann wrote: I hate to admit it, but I think I've built them backwards over the years. Maybe that's why J-poles have such a lousy reputation and why I think they suck? Please pardon my ignorance. I understood that a J-Pole is a half wave antenna connected to a quarter wave stub. The transmission line is connected to a low impedance point (50 Ohms?) on the stub. Yep, something like that. In the literature, it is usually fed as an unbalanced antenna, but it is not. Therefore, it should not matter which side has the center conductor or shield - they are both wrong! Fred K4DII Well, if it really were a balanced feed, I guess(tm) there should be no difference in the pattern, gain, VSWR, etc between the two unbalanced methods of feeding the antenna. Yet the author of the original article shows large differences in the antenna models. See Pg 10 thru 13: http://www.cvarc.org/tech/antenna_myths/antenna_myths.pdf -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#10
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![]() "Jeff Liebermann" wrote in message ... On Sat, 15 Feb 2014 13:27:15 -0800, "Sal" salmonella@food poisoning.org wrote: I had a question about his j-pole analysis. snip I agree with the author (Terry Graves K7FE). snip However, a J-pole (or Zepp) is not a 1/2 wave antenna. The driven element is a 1/4 wavelength long, and therefore DOES require a ground plane. This article covers the point (and more): http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm Quoting: Summary End-feds Without Grounds ANY END-FED ANTENNA REQUIRES A LARGE GROUNDPLANE OR OTHER EXTRAORDINARY ISOLATION METHOD OR METHODS TO PREVENT FEEDLINE OR MAST COMMON MODE CURRENTS! Hi, Jeff, I can agree with the need for preventing feedline radiation but one thing you and Terry say may be erroneous. I believe the radiating element of a J-pole to be a half wavelength long, not a quarter-wave. I looked at Terry's EZNEC wires list and observed the long side (the radiator or driven element) of the J is 57 inches and the short side (the stub) is 19 inches. (These dimensions agree with my idea of a 2m J-pole. I've made a few.) As I understand the action of the J-pole, net radiation is low or nil from currents in the lower third of the antenna (bottom third of the radiator and the adjacent stub). The desired radiation comes from the top two-thirds (38 inches) of the radiator, which is very nearly a half wave at 2m. Allowing for so-called "end effect," it's almost exactly cut to 146 MHz. Please check my reasoning and math Your comments are welcome. Thanks. 73, "Sal" (KD6VKW) |
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