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Antenna article
Just found this; looks interesting; no connection with the author;
all in the spirit of free dissemination of knowledge to fellow radio amateurs ... http://www.cvarc.org/tech/antenna_my...enna_myths.pdf |
Antenna article
On Friday, February 14, 2014 1:59:15 PM UTC-6, gareth wrote:
Just found this; looks interesting; no connection with the author; all in the spirit of free dissemination of knowledge to fellow radio amateurs ... http://www.cvarc.org/tech/antenna_my...enna_myths.pdf Several parts of it are pure unadulterated horse doo-doo.. :( I find it to be in very poor taste to post a page claiming to expose and list myths and mysteries, only to replace them with other myths and mysteries. :+ That is akin to chasing one's tail while barking at the moon. |
Antenna article
wrote:
On Friday, February 14, 2014 1:59:15 PM UTC-6, gareth wrote: Just found this; looks interesting; no connection with the author; all in the spirit of free dissemination of knowledge to fellow radio amateurs ... http://www.cvarc.org/tech/antenna_my...enna_myths.pdf Several parts of it are pure unadulterated horse doo-doo.. :( I find it to be in very poor taste to post a page claiming to expose and list myths and mysteries, only to replace them with other myths and mysteries. :+ That is akin to chasing one's tail while barking at the moon. I confess to only have 'scan read' the reference and I thought it was still pretty good. What in particular do you have an issue with, please? -- 73 Brian G8OSN/W8OSN |
Antenna article
On Friday, February 14, 2014 11:21:01 PM UTC-6, Brian Reay wrote:
wrote: On Friday, February 14, 2014 1:59:15 PM UTC-6, gareth wrote: Just found this; looks interesting; no connection with the author; all in the spirit of free dissemination of knowledge to fellow radio amateurs ... http://www.cvarc.org/tech/antenna_my...enna_myths.pdf Several parts of it are pure unadulterated horse doo-doo.. :( I find it to be in very poor taste to post a page claiming to expose and list myths and mysteries, only to replace them with other myths and mysteries. :+ That is akin to chasing one's tail while barking at the moon. I confess to only have 'scan read' the reference and I thought it was still pretty good. What in particular do you have an issue with, please? -- 73 Brian G8OSN/W8OSN Well, I suppose most is OK, but there are a few problems. Take this one.. More recent HF tests by Al Christman - KB8I; "Eleva ted Vertical Antenna Systems," QST, August 1988, p 35; have shown that fewer "elevated" radials will perform about as well as 120 ground mounted ones. A base mounting height above ground of about 1/10 to 1/16 of a wavelength seems optimu m for 4 radials.........but will vary with soil conductivity. I don't know how he ran his tests, but this is not right. 1/10 WL is way too low for four elevated radials to equal 120 on the ground. I was harping about this "myth" in another post not long ago.. There is no free lunch. :| For four radials to equal 120 on the ground, they will need to be nearly 1/2 wave up. Four radials at 1/4 wave up are equal to about 50-60 on the ground. Seems optimum? If the radial system were optimum, it really wouldn't matter what the conductivity of the ground was. This is actually proving my point that four radials at 1/16 to 1/10 WL are not nearly enough to actually equal 120 on the ground. If they were equal, you wouldn't have to raise the antenna and radials. That's the whole point of using so many radials on the ground. So the quality of the ground, good or bad, really doesn't matter. Efficiency will be high either way. Some of his 1/2 vertical design statements could be argued with, but I'll be here all night if I start into that.. :/ Then you have this.. Myth: A 5/8 wave antenna has 3dB more gain than a ground plane. False This can be true in many cases. But it can also be false in many cases. * The losses in the required matching coil at the base of the 5/8 wave antenna reduce the gain difference to a max of about 2dB (with a perfect ground plane) to zero difference in some installations. ** -------------------------------------------------- This is fairly absurd.. The loss of the loading coil is quite negligible. I bet not even enough for most people to accurately measure. The coil has nothing to do with why some types of 5/8 verticals show little or no gain vs a 1/4 GP, or isotropic. Those are a couple of my issues anyway.. The "Free Lunch" elevated radial system being one of my pet peeve myths.. :| |
Antenna article
"Jeff Liebermann" wrote in message ... The coil has nothing to do with why some types of 5/8 verticals show little or no gain vs a 1/4 GP, or isotropic. Just a minor note on this issue. If the coil actually did contribute a 1dB power loss, the lost power would need to be either reflected or dissipated by the coil if it's not being radiated as RF. I haven't seen any high VSWR 5/8 wave antennas, so it's not VSWR. I haven't seen the matching coil on my Larsen 5/8 wave antenna, with a 45 watt VHF mobile, become warm. If it was dissipating 4.5 watts, I should be able to see it get quite warm[1]. I pointed an IR thermometer at the coil and transmitted for about 5 minutes. No change. However, I did see a very slight temperature rise from a mercury thermometer taped to the stainless steel antenna rod (which could be attributable to my sloppiness). The test is easily reproduced. Have fun. Just the same type of easy test that can be done with the old saying of 1 db loss per connector. Connectors have a very small loss in them. If they did have the 1 db or even 1/2 of a db loss and you put 100 to 1000 watts through them you could really feel the heat after a short transmission. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
Antenna article
On Saturday, February 15, 2014 11:39:51 AM UTC-6, Jeff Liebermann wrote:
I haven't seen any high VSWR 5/8 wave antennas, so it's not VSWR. Actually, a 5/8WL antenna is a standing wave antenna. Therefore, there is a standing wave *on the antenna element* no matter what the feedpoint impedance. The SWR on the loading coil will be higher than the SWR on the antenna element. All that a resistive feedpoint on a standing wave antenna means is that the forward wave *on the antenna element* is in phase with the reflected wave *on the antenna element* at the antenna feedpoint. -- 73, Cecil, w5dxp.com |
Antenna article
wrote in message ... On Friday, February 14, 2014 11:21:01 PM UTC-6, Brian Reay wrote: BIG SNIP What in particular do you have an issue with, please? little snip Brian G8OSN/W8OSN Well, I suppose most is OK, but there are a few problems. BIG SNIP 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? "Sal" |
Antenna article
On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote:
As built, both the pattern and the impedance already seem to be what I want. And it already has a counterpoise. The counterpoise is where the current flowing on the inside of the coax shield goes when it leaves the coax. The J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL doesn't radiate (much) and is part of the matching section and counterpoise. Quite often, the outside of the coax braid is also part of the counterpoise. -- 73, Cecil, w5dxp.com |
Antenna article
"W5DXP" wrote in message
... On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote: As built, both the pattern and the impedance already seem to be what I want. And it already has a counterpoise. The counterpoise is where the current flowing on the inside of the coax shield goes when it leaves the coax. The J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL doesn't radiate (much) and is part of the matching section and counterpoise. Quite often, the outside of the coax braid is also part of the counterpoise. -----ooooo----- Cecil, I read with interest your critiques of the article. On the basis that you mentioned eznec, I wonder of perchance you are a professional? Gareth |
Antenna article
"W5DXP" wrote in message ... On Saturday, February 15, 2014 3:27:15 PM UTC-6, Sal wrote: As built, both the pattern and the impedance already seem to be what I want. And it already has a counterpoise. The counterpoise is where the current flowing on the inside of the coax shield goes when it leaves the coax. The J-pole has a radiation pattern of a 1/2WL vertical, i.e. the lower 1/4WL doesn't radiate (much) and is part of the matching section and counterpoise. Quite often, the outside of the coax braid is also part of the counterpoise. -- 73, Cecil, w5dxp.com Thanks, that's what I thought, too. The nature of current flow on the outside of the coax is the reason so many construction articles mention winding a few turns of the coax into a choke, not far* from the antenna. I don't always do it but I probably should. * But what distance is "not far"? A quarter-wave comes to mind, so a measurement is called for. Hm-m-m ... is the propagation velocity on the outside of the coax the same as the inside? My head spins just a little. "Sal" |
Antenna article
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 |
Antenna article
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 |
Antenna article
"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) |
Antenna article
On Sat, 15 Feb 2014 22:29:31 -0800, "Sal" salmonella@food
poisoning.org wrote: 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. You're correct. See my follow up posting. I've been building J-poles driving the short 1/4 wave element instead of the long 1/2 wave element. 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). Correct. The original Zepp antenna was designed to be lowered from a hydrogen filled Zeppelin dirigible. Any sparks or arcing caused by high voltage from the antenna to the dirigible would be considered a really bad idea. So, the antenna was designed to have zero voltage and probably zero current at the closest point of the antenna to the dirigible. 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. Well, here we disagree slightly. I once made a similar comment in this group about the radiation from an end fed collinear antenna came mostly from the bottom of the antenna. I was corrected by Roy Lewallen (W7EL) who pointed out that the current through the length of such an antenna is the same and therefore the radiation from all parts of the antenna are similarly identical. Unless I'm missing something (a real possibility), the situation is the same with a J-pole. https://groups.google.com/forum/?hl=en#!original/rec.radio.amateur.antenna/DREJnRznluQ/bZyCgwa0JvwJ This is interesting: http://www.qikzepp.com/QikZepp_technical_information.html It shows a 1909 German patent for a Zepp antenna. However, the accompanying description once again makes the mistake of feeding the 1/4 WL section and not the longer 1/2 WL section: Early fixed installation Zepp Antennas were a half-wavelength long(or multiple) and fed with a 1/4 WL (or multiple) open wire feed line which uses only one of the wires. The feed line provided a matching section for the transmitter. Please check my reasoning and math Your comments are welcome. Thanks. I hate being wrong, but do I seem to be getting used to it. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna article
On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote:
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. Sorry, that is not correct. Both a Zepp and a J-pole are 1/2WL antennas fed with a 1/4WL matching section of transmission line containing mostly balanced transmission line currents. Here is a plot of the two currents in the matching section. The difference in those two currents is the common-mode current which indeed does radiate. The point is that the average differential transmission line current is more than 10 times the average common-mode radiating current so the matching section is acting primarily as a transmission line, transferring RF energy to the primary radiating element which is indeed 1/2WL element. http://w5dxp.com/ZeppCrnt.jpg Does the matching section radiate? Of course it does. But because the common-mode current is a small percentage of the total current, it does not radiate much and transfers most of the RF energy to the 1/2WL radiating element.. And yes, that small percentage of common-mode current on the matching section indicates that it is also acting as the antenna counterpoise in the above graph. But most of the radiation from the antenna is from the center of the 1/2WL element, just as it is for a 1/2WL dipole. In fact, a Zepp meets the IEEE definition of a dipole because it contains two electrical poles. -- 73, Cecil, w5dxp.com |
Antenna article
On Saturday, February 15, 2014 5:46:21 PM UTC-6, gareth wrote:
I wonder of perchance you are a professional? I've been a ham for 60 years and worked as a EE for 38 years so you might say I am 2/3 professional.:) -- 73, Cecil, w5dxp.com |
Antenna article
On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote:
* But what distance is "not far"? A quarter-wave comes to mind, so a measurement is called for. Hm-m-m ... is the propagation velocity on the outside of the coax the same as the inside? Common-mode chokes work best at the high current points so "not far from the feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL down the feedline, work well. The velocity factor of a common-mode signal on the outside braid of the coax is fairly close to 1.0 only slowed down by the outside insulation layer. -- 73, Cecil, w5dxp.com |
Antenna article
"W5DXP" wrote in message
... On Saturday, February 15, 2014 5:46:21 PM UTC-6, gareth wrote: I wonder of perchance you are a professional? I've been a ham for 60 years and worked as a EE for 38 years so you might say I am 2/3 professional.:) 44 and 42 for me, but mainly diverted into software engineering for 96% of that time. |
Antenna article
On Sun, 16 Feb 2014 06:32:38 -0800 (PST), W5DXP
wrote: On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote: 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. Sorry, that is not correct. Agreed. See my followup to my posting where I noticed that I've been building J-pole antennas with the coax feed connected to the wrong element. Judging by some of the photos I've found, I'm not alone. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna article
On Sunday, February 16, 2014 11:12:51 AM UTC-6, Jeff Liebermann wrote:
On Sun, 16 Feb 2014 06:32:38 -0800 (PST), W5DXP wrote: On Saturday, February 15, 2014 9:58:10 PM UTC-6, Jeff Liebermann wrote: 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. Sorry, that is not correct. Agreed. See my followup to my posting where I noticed that I've been building J-pole antennas with the coax feed connected to the wrong element. Judging by some of the photos I've found, I'm not alone. I never built J-poles myself. Never was that fond of the design for some reason. I always preferred the "ringo" method of feeding a base fed half wave. IE: a flat single turn tapped coil, and coax capacitor. I used them on 10m mostly. They work decently well in most cases with no decoupling section. But they work even better with decoupling. I used a 1/4 wave length of coax to a union which was attached to the mast, and had a set of three radials. But... The 5/8 ground planes were always better than the 1/2 waves on distant 10m local stations. Even the decoupled version. And being low angle space wave stuff, it's a pretty good test. Both antennas were at about 36 ft up at the base. According to a modeling exercise I did once, the best way to run a 5/8 GP is with 5/8 radials. I've also used 3/4 wave radials, which seemed to work well. But according to the modeling, the 5/8 radials will give more gain. Starts to resemble a dual 5/8 collinear. And naturally, you would want to use a decoupling section for the best performance. This is pretty old, but compares the different lengths for 10m use. http://home.comcast.net/~nm5k/acompari.htm |
Antenna article
"Jeff Liebermann" wrote in message ... On Sat, 15 Feb 2014 22:29:31 -0800, "Sal" salmonella@food poisoning.org wrote: 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. You're correct. See my follow up posting. I've been building J-poles driving the short 1/4 wave element instead of the long 1/2 wave element. 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). Correct. The original Zepp antenna was designed to be lowered from a hydrogen filled Zeppelin dirigible. Any sparks or arcing caused by high voltage from the antenna to the dirigible would be considered a really bad idea. So, the antenna was designed to have zero voltage and probably zero current at the closest point of the antenna to the dirigible. 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. Well, here we disagree slightly. I once made a similar comment in this group about the radiation from an end fed collinear antenna came mostly from the bottom of the antenna. I was corrected by Roy Lewallen (W7EL) who pointed out that the current through the length of such an antenna is the same and therefore the radiation from all parts of the antenna are similarly identical. Unless I'm missing something (a real possibility), the situation is the same with a J-pole. https://groups.google.com/forum/?hl=en#!original/rec.radio.amateur.antenna/DREJnRznluQ/bZyCgwa0JvwJ This is interesting: http://www.qikzepp.com/QikZepp_technical_information.html It shows a 1909 German patent for a Zepp antenna. However, the accompanying description once again makes the mistake of feeding the 1/4 WL section and not the longer 1/2 WL section: Early fixed installation Zepp Antennas were a half-wavelength long(or multiple) and fed with a 1/4 WL (or multiple) open wire feed line which uses only one of the wires. The feed line provided a matching section for the transmitter. Please check my reasoning and math Your comments are welcome. Thanks. I hate being wrong, but do I seem to be getting used to it. I'm used to it, too, so there's no chance you will monopolize it. I saw your follow up posting. Several comments. Hams who write construction articles sometimes support the notion of a consensus about feeding a J-pole from the stub side. Consensus doesn't carry any scientific weight. Some construction articles acknowledge such a consensus but claim it doesn't really matter. I've been happy soldering a small mounting tab to the short (stub) side, then bolting on a panel-mount SO-239 and extending the center conductor across to the long side. I'm not sure I care enough to experiment, A versus B. It would involve many hours' work to get it right. Maybe you weren't actually clashing with Roy (whom I admire). If he were regarding the upper two-thirds of the structure as the antenna (and regarding the lower two thirds only as a feed/matching-section/transformer), it might only have been a failure to communicate about what is the "antenna." I risk being wrong again by commenting on brief snatches of somebody else's conversation. With an end-fed half-wave antenna, I believe the current maximum is at the midpoint of the radiating section, 19 inches from the tip of the J-pole's long section, for example. The current minima would then be at the ends, most notably the high voltage end feed point. Cecil's 40m Zepp diagram indicates that the common mode currents are comparatively small, against the differential currents that do the work. Some comfort, there. I think we mostly have it nailed. Please, let any additional nails NOT involve my getting crucified. Again. "Sal" |
Antenna article
"W5DXP" wrote in message ... On Saturday, February 15, 2014 6:59:25 PM UTC-6, Sal wrote: * But what distance is "not far"? A quarter-wave comes to mind, so a measurement is called for. Hm-m-m ... is the propagation velocity on the outside of the coax the same as the inside? Common-mode chokes work best at the high current points so "not far from the feedpoint" of a 50 ohm antenna. Two chokes, one at the antenna and one 1/4WL down the feedline, work well. The velocity factor of a common-mode signal on the outside braid of the coax is fairly close to 1.0 only slowed down by the outside insulation layer. -- 73, Cecil, w5dxp.com Thankew! "Sal" |
Antenna article
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 |
Antenna article
"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) |
Antenna article
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. |
Antenna article
"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 |
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Antenna article
In article ,
W5DXP wrote: Don't forget the Arrow Open Stub J-pole. http://www.arrowantennas.com/osj/j-pole.html There is (or was) a nice writeup of this variety on Cebik's web site. He refers to it as a "variant J-pole". The feed arrangement is different (it's open at the feedpoint, rather than being fed a few inches above a short), and the arm lengths are different than with a "classic" J-pole. The current distributions are different, of course, but the radiation pattern is only very slightly different than the "classic" variety. I have one of these dual-band types in my "go-kit" (it's actually a knock-off, built locally based on the plans that Arrow used to have on their web site) and it's served me well. I usually mount it at the top of a two-or-three-segment aluminum mast, sitting in an old hefty movie-camera tripod base. Much too large and heavy for a vehicle, of course, but I imagine you could build a somewhat-similar open-stub J-pole using much lighter materials (e.g. fiberglass whips with wire fastened inside or outside). |
Antenna article
On Tuesday, February 18, 2014 6:26:40 PM UTC-6, David Platt wrote:
The feed arrangement is different (it's open at the feedpoint, rather than being fed a few inches above a short), and the arm lengths are different than with a "classic" J-pole. If one will ignore the long 2m element and draw a schematic of it just for the 70cm band, one will realize that it is actually just a standard Zepp antenna.:) |
Antenna article
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 |
Antenna article
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 |
Antenna article
In article ,
(David Platt) wrote: In article , W5DXP wrote: Don't forget the Arrow Open Stub J-pole. http://www.arrowantennas.com/osj/j-pole.html I have one of these dual-band types in my "go-kit" (it's actually a knock-off, built locally based on the plans that Arrow used to have on their web site) and it's served me well. David- Using the Wayback Machine https://archive.org/index.php, I was able to find the old Arrow plans listed under instruction sheets. Then I found that the latest version was a single PDF file that has the same information! At the recent Orlando Hamfest, someone was selling dual band J-Pole copies of the Arrow. Now I wish I had bought one. Fred |
Antenna article
"Jeff Liebermann" wrote in message ... ... 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. Yes. I've been making copper pipe and 2-wire transmission line J-poles for almost 20 years. With pipe, I usually fasten the two feed points with clamps and slide the connections up and down. I'll get a VSWR low-point in-band but early-on, I discovered that the best VSWR was often about 1.7:1. I had read about (but never built) a gamma match, so I'd heard about the series cap to tune out the inductance. I tried a series cap at the feed and it helped. 70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading somewhere in the band. Does such a 0.3 dB improvement matter? That's not my call. When I'm essentially playing with the technology, I can take more time than if I'm working, like to a deadline or a budget. "Sal" |
Antenna article
On Wed, 19 Feb 2014 21:16:19 -0800, "Sal" salmonella@food
poisoning.org wrote: "Jeff Liebermann" wrote in message .. . ... 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. Yes. I've been making copper pipe and 2-wire transmission line J-poles for almost 20 years. With pipe, I usually fasten the two feed points with clamps and slide the connections up and down. I'll get a VSWR low-point in-band but early-on, I discovered that the best VSWR was often about 1.7:1. I had read about (but never built) a gamma match, so I'd heard about the series cap to tune out the inductance. I tried a series cap at the feed and it helped. 70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading somewhere in the band. I'm not sure of the frequency of your J-pole. The inductance of about 6 cm of #12AWG solid wire is about 0.05 uH. http://www.consultrsr.com/resources/eis/induct5.htm To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be about 200 pf. Does such a 0.3 dB improvement matter? That's not my call. When I'm essentially playing with the technology, I can take more time than if I'm working, like to a deadline or a budget. 0.3dB is about 6.7% loss. Probably not important or toss a coin? -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna article
"Jeff Liebermann" wrote in message ... I'm not sure of the frequency of your J-pole. The inductance of about 6 cm of #12AWG solid wire is about 0.05 uH. http://www.consultrsr.com/resources/eis/induct5.htm To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be about 200 pf. That's enlightening. I need to admit that no such science was applied. I just tried small caps from my stash to see what would happen. When something good happened, I remembered it for next time. The j-pole that seems to benefit most is the 2m version, which I've built the most of. Perhaps we're not dealing with just the inductance of that 6 cm of wire. There could be some residual inductance in the rest of the antenna which is being brought to resonance. Maybe when my element lengths are not optimal, some inductance would be found there. Not sure why the residual reactance would always be inductive. The first time I add a cap and it gets worse, then I'll know it isn't so. Maybe I should experiment with varying the spacing across the stub and see what that does, with and without a cap. "Sal" |
Antenna article
On Thu, 20 Feb 2014 20:38:22 -0800, "Sal" salmonella@food
poisoning.org wrote: "Jeff Liebermann" wrote in message .. . I'm not sure of the frequency of your J-pole. The inductance of about 6 cm of #12AWG solid wire is about 0.05 uH. http://www.consultrsr.com/resources/eis/induct5.htm To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be about 200 pf. That's enlightening. One must suffer before enlightenment. I need to admit that no such science was applied. I just tried small caps from my stash to see what would happen. When something good happened, I remembered it for next time. Well, I was a bit more scientific when I tried it about 10 years ago in an effort to improve the "American Legion" J-pole. This is from my memory, which is notoriously unreliable. I had the help of a return loss bridge, sweep generator, and oscilloscope. https://www.google.com/search?q=return+loss+bridge&tbm=isch I could tweak the antenna, and see the results to the VSWR immediately. I was finding that the J-pole has a rather narrow operating frequency range and I wanted to know if something could be done about it. Mounting the connector on the base resulted in the narrowest usable bandwidth (at VSWR = 2:1) but also the lowest VSWR. Using the more conventional feed points furthur up from the base, the minimum VSWR was horrid, but the bandwidth was wider. Inserting a series trimmer or wrapping the insulated wire around the 1/4 wave rod reduced the minimum VSWR, but narrowed the bandwidth slightly. Replacing the 1/4" dia aluminum rods, with 3/4" copper water pipe increased the usable bandwidth somewhat, but made the insulated wire wrap capacitor very small and critical. This may sound very scientific, but in reality, it was all done in about an hour, with no photos or recorded data. I had to return the equipment that afternoon. About a year ago, the local radio club had a beginners session on building "emergency antennas", which means a VHF Slim Jim and J-pole type antennas made from 300 ohm twinlead. The only test equipment available was an HT and VSWR bridge. I predicted problems and was invited not to show up. I did anyway, near the end. The problem was that the antennas were VERY sensitive to their surroundings. It was very difficult to cut one to length for minimum VSWR, while people and metal objects moved around the room. The j-pole that seems to benefit most is the 2m version, which I've built the most of. Perhaps we're not dealing with just the inductance of that 6 cm of wire. There could be some residual inductance in the rest of the antenna which is being brought to resonance. Maybe when my element lengths are not optimal, some inductance would be found there. Not sure why the residual reactance would always be inductive. The first time I add a cap and it gets worse, then I'll know it isn't so. Dunno. I can throw together a 4NEC2 model of a j-pole and see what might be the feed point impedance for various styles and locations. No time for that for a few days. Maybe I should experiment with varying the spacing across the stub and see what that does, with and without a cap. It's more fun to fire up the 4NEC2 optimizer, and see what it comes up with for the best possible antenna given a collection of restraints. Yet another project... -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna article
On Friday, February 21, 2014 12:07:31 PM UTC-6, Jeff Liebermann wrote:
It's more fun to fire up the 4NEC2 optimizer, and see what it comes up with for the best possible antenna given a collection of restraints. Yet another project... For EZNEC users, AutoEZ (available from ac6la.com) is an EXCEL-based optimizer - a 30 segment free demo version is also available. (I'm a satisfied EZNEC/AutoEZ user with no monitary connections.) -- 73, Cecil, w5dxp.com |
Antenna article
"Jeff Liebermann" wrote in message .. snip Dunno. I can throw together a 4NEC2 model of a j-pole and see what might be the feed point impedance for various styles and locations. No time for that for a few days. Maybe I should experiment with varying the spacing across the stub and see what that does, with and without a cap. It's more fun to fire up the 4NEC2 optimizer, and see what it comes up with for the best possible antenna given a collection of restraints. Yet another project... Too many projects, not enough me. And I face a big non-hobby project of cleaning and recoating the patio stones. Yuck! "Sal" (KD6VKW) |
Antenna article
"W5DXP" wrote in message ... On Friday, February 21, 2014 12:07:31 PM UTC-6, Jeff Liebermann wrote: It's more fun to fire up the 4NEC2 optimizer, and see what it comes up with for the best possible antenna given a collection of restraints. Yet another project... For EZNEC users, AutoEZ (available from ac6la.com) is an EXCEL-based optimizer - a 30 segment free demo version is also available. (I'm a satisfied EZNEC/AutoEZ user with no monitary connections.) -- 73, Cecil, w5dxp.com I downloaded and used the demo version of EZNEC and I can say it was pretty easy to get up and running with simple structures. I used it for a 20 m dipole and I was quite happy when I raised the antenna at FD for its first outing at full 30-ft height. I keep meaning to buy the full version of EZNEC. Maybe this will push me into it. "Sal" |
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