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J_Pole Trials and Tribulations
I have been trying to get an acceptable 2 meter antenna installed in
my attic. A J-Pole seemed to me the best solution for my attic space. You may recall an earlier post where I discussed SWR on one of the ladder line J-Poles I was working with. Well, I gave up on ladder line and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my basement work shop to give the best SWR and then took it up to the attic. The following table shows the results. Basement Attic 144 1.90 1.80 145 1.65 2.00 146 1.50 2.10 147 1.60 2.25 147.995 1.90 2.25 Actually, probably not that bad for a first attempt at copper tubing. But, I didn't like the fact that SWR was above 2.0 in my attic and that things changed between my basement shop and the attic. I guess this design is influenced by objects close by. So, somewhere in my surfing I came across the Arrow J-Pole. This is an end fed, open stub commercial design that promises less that 1.5 SWR across the band. Here are the results of my experiment with this antenna. Basement Attic 144 1.10 1.22 145 1.18 1.10 146 1.25 1.20 147 1.30 1.32 147.995 1.40 1.42 Not 1.0 SWR but certainly met the promise. And the best thing about this design is that near by objects don't seem to have an impact on performance. If I built one of these designs and tuned it in my basement shop, it should give the same results in the attic. Anyway, just my subjective observations. jimbo - AJ7IM |
J_Pole Trials and Tribulations
jimbo wrote: I have been trying to get an acceptable 2 meter antenna installed in my attic. A J-Pole seemed to me the best solution for my attic space. You may recall an earlier post where I discussed SWR on one of the ladder line J-Poles I was working with. Well, I gave up on ladder line and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my basement work shop to give the best SWR and then took it up to the attic. The following table shows the results. Basement Attic 144 1.90 1.80 145 1.65 2.00 146 1.50 2.10 147 1.60 2.25 147.995 1.90 2.25 Actually, probably not that bad for a first attempt at copper tubing. But, I didn't like the fact that SWR was above 2.0 in my attic and that things changed between my basement shop and the attic. I guess this design is influenced by objects close by. So, somewhere in my surfing I came across the Arrow J-Pole. This is an end fed, open stub commercial design that promises less that 1.5 SWR across the band. Here are the results of my experiment with this antenna. Basement Attic 144 1.10 1.22 145 1.18 1.10 146 1.25 1.20 147 1.30 1.32 147.995 1.40 1.42 Not 1.0 SWR but certainly met the promise. And the best thing about this design is that near by objects don't seem to have an impact on performance. If I built one of these designs and tuned it in my basement shop, it should give the same results in the attic. Anyway, just my subjective observations. jimbo - AJ7IM Hi Jimbo, it has been my experience, and everybody else's that an indoor or attic J-pole is unpredictable. A high impedance end fed antenna in an attic environment will be hard to predict. Moving it one foot can effect things. You might consider the SO-239 ground plane. It can be built and tuned in 30 minutes, is 50 ohms, so it will not be so concerned about its environment. Likely you could not tell a difference in performance from a J-pole especially if they are both in the attic. Oh yes, the standard-if the SWR is below 3.0, don't worry about it. Gary N4AST |
J_Pole Trials and Tribulations
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J_Pole Trials and Tribulations
"jimbo" wrote in message . .. I have been trying to get an acceptable 2 meter antenna installed in my attic. A J-Pole seemed to me the best solution for my attic space. You may recall an earlier post where I discussed SWR on one of the ladder line J-Poles I was working with. Well, I gave up on ladder line and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my basement work shop to give the best SWR and then took it up to the attic. The following table shows the results. Basement Attic 144 1.90 1.80 145 1.65 2.00 146 1.50 2.10 147 1.60 2.25 147.995 1.90 2.25 Actually, probably not that bad for a first attempt at copper tubing. But, I didn't like the fact that SWR was above 2.0 in my attic and that things changed between my basement shop and the attic. I guess this design is influenced by objects close by. So, somewhere in my surfing I came across the Arrow J-Pole. This is an end fed, open stub commercial design that promises less that 1.5 SWR across the band. Here are the results of my experiment with this antenna. Basement Attic 144 1.10 1.22 145 1.18 1.10 146 1.25 1.20 147 1.30 1.32 147.995 1.40 1.42 Not 1.0 SWR but certainly met the promise. And the best thing about this design is that near by objects don't seem to have an impact on performance. If I built one of these designs and tuned it in my basement shop, it should give the same results in the attic. Anyway, just my subjective observations. Not to detract from the other posters ... IMO they all made valid comments, but I offer this: As the builder of about 20 copper pipe j-poles, I discovered that I can get a better final VSWR if I include a 100 pF (or so) cap in the side fed by the coax center. I had been attaching an SO-239-type bulkhead connector to the short side of the J and running a piece of solid wire from the center pin over to the long side of the J. I could get the VSWR to a dip near the middle of the band by monkeying with the feed point and/or trimming the length but the lowest I usually got was around 1.5 : 1. As soon as I used the cap in place of the straight wire, I could get 1:1. It works for me ... your mileage may vary. I built more j-poles for 2M than all the others combined and 20 is not a lot of antennas. For some reason, the cap seems to help the 2M size more than it helps the others, but these antennas are too few in number for scientific judgements by this amateur plumber. 73, John |
J_Pole Trials and Tribulations
jimbo wrote:
I have been trying to get an acceptable 2 meter antenna installed in my attic. A J-Pole seemed to me the best solution for my attic space. You may recall an earlier post where I discussed SWR on one of the ladder line J-Poles I was working with. Well, I gave up on ladder line and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my basement work shop to give the best SWR and then took it up to the attic. The following table shows the results. Basement Attic 144 1.90 1.80 145 1.65 2.00 146 1.50 2.10 147 1.60 2.25 147.995 1.90 2.25 Actually, probably not that bad for a first attempt at copper tubing. But, I didn't like the fact that SWR was above 2.0 in my attic and that things changed between my basement shop and the attic. I guess this design is influenced by objects close by. So, somewhere in my surfing I came across the Arrow J-Pole. This is an end fed, open stub commercial design that promises less that 1.5 SWR across the band. Here are the results of my experiment with this antenna. Basement Attic 144 1.10 1.22 145 1.18 1.10 146 1.25 1.20 147 1.30 1.32 147.995 1.40 1.42 Not 1.0 SWR but certainly met the promise. And the best thing about this design is that near by objects don't seem to have an impact on performance. If I built one of these designs and tuned it in my basement shop, it should give the same results in the attic. Anyway, just my subjective observations. jimbo - AJ7IM I guess my conclusion is that the conventional J-Pole design seems to be more susceptible to near by objects than the much less common end fed, open stub, J-Pole. My conventional copper J-pole changed much more than the end fed Arrow J-Pole when moved from the basement to the attic. And I have noticed on other occasions that even very small changes in location and/or orientation had a large effect on SWR. The ARRL Antenna Book has a small section on J-Pole antennas. They show both designs, conventional shorted stub and the uncommon end fed, open stub designs. They say that the shorted stub design should have a 4/1 balun at the feed point because a direct coax connection results in extreme sensitivity to near by objects. And they also say the end fed, open stub design doesn't require a balun and is much less sensitive to near by objects, but is harder to tune. (I don't understand that point, there is one less parameter to fiddle with.) All of this leads me to wonder why almost all J-Pole designs one sees are of the shorted stub version and almost none use a balun. I suspect ease of construction and low cost. My "store bought" Arrow end fed, open stub, J-Pole seems to verify the ARRL Antenna Book's conclusions about sensitivity to near by objects. However, do-it-yourself construction does appear to be much more difficult. I may give it a try, just out of curiosity. A final note. My attic is a very difficult place to work. There is no way I can install an antenna and then make numerous adjustments to get it tuned to the space. I must be able to do any required tuning in my basement shop or maybe outside on my deck and then take it to the attic for installation. So for my application, I want something that is not sensitive to near by objects. The end fed, open stub J-Pole seems to fit that requirement. 73s jimbo |
J_Pole Trials and Tribulations
On Wed, 26 Apr 2006 09:14:24 -0600, jimbo wrote:
[snip] The ARRL Antenna Book has a small section on J-Pole antennas. They show both designs, conventional shorted stub and the uncommon end fed, open stub designs. They say that the shorted stub design should have a 4/1 balun at the feed point because a direct coax connection results in extreme sensitivity to near by objects. And they also say the end fed, open stub design doesn't require a balun and is much less sensitive to near by objects, but is harder to tune. (I don't understand that point, there is one less parameter to fiddle with.) All of this leads me to wonder why almost all J-Pole designs one sees are of the shorted stub version and almost none use a balun. I suspect ease of construction and low cost. My "store bought" Arrow end fed, open stub, J-Pole seems to verify the ARRL Antenna Book's conclusions about sensitivity to near by objects. However, do-it-yourself construction does appear to be much more difficult. I may give it a try, just out of curiosity. [snip] A j-pole (open or closed stub) will perform better with a common mode choke at the feed point. Even better, use two chokes. One at the feed point and the other 1/4-wave down the line. This will reduce the higher angle radiation that results from the feed line being part of the antenna. Using a choke on ANY antenna fed with coax is just good engineering practice. It can't hurt - only help. A few turns of coax costs very little. 73, Danny, K6MHE In my many years I have come to a conclusion that one useless man is a shame, two is a law firm, and three or more is a congress. - John Adams email: k6mheatarrldotnet http://www.k6mhe.com/ |
J_Pole Trials and Tribulations
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J_Pole Trials and Tribulations
"jimbo" wrote in message . .. I have been trying to get an acceptable 2 meter antenna installed in my attic. A J-Pole seemed to me the best solution for my attic space. You may recall an earlier post where I discussed SWR on one of the ladder line J-Poles I was working with. Well, I gave up on ladder line and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my basement work shop to give the best SWR and then took it up to the attic. The following table shows the results. Basement Attic 144 1.90 1.80 145 1.65 2.00 146 1.50 2.10 147 1.60 2.25 147.995 1.90 2.25 Actually, probably not that bad for a first attempt at copper tubing. But, I didn't like the fact that SWR was above 2.0 in my attic and that things changed between my basement shop and the attic. I guess this design is influenced by objects close by. So, somewhere in my surfing I came across the Arrow J-Pole. This is an end fed, open stub commercial design that promises less that 1.5 SWR across the band. Here are the results of my experiment with this antenna. Basement Attic 144 1.10 1.22 145 1.18 1.10 146 1.25 1.20 147 1.30 1.32 147.995 1.40 1.42 Not 1.0 SWR but certainly met the promise. And the best thing about this design is that near by objects don't seem to have an impact on performance. If I built one of these designs and tuned it in my basement shop, it should give the same results in the attic. Anyway, just my subjective observations. jimbo - AJ7IM |
J_Pole Trials and Tribulations
"jimbo" wrote in message . .. I have been trying to get an acceptable 2 meter antenna installed in my attic. A J-Pole seemed to me the best solution for my attic space. You may recall an earlier post where I discussed SWR on one of the ladder line J-Poles I was working with. Well, I gave up on ladder line and constructed a J-Pole from 1/2 inch copper tubing. I tuned it in my basement work shop to give the best SWR and then took it up to the attic. The following table shows the results. Basement Attic 144 1.90 1.80 145 1.65 2.00 146 1.50 2.10 147 1.60 2.25 147.995 1.90 2.25 Actually, probably not that bad for a first attempt at copper tubing. But, I didn't like the fact that SWR was above 2.0 in my attic and that things changed between my basement shop and the attic. I guess this design is influenced by objects close by. So, somewhere in my surfing I came across the Arrow J-Pole. This is an end fed, open stub commercial design that promises less that 1.5 SWR across the band. Here are the results of my experiment with this antenna. Basement Attic 144 1.10 1.22 145 1.18 1.10 146 1.25 1.20 147 1.30 1.32 147.995 1.40 1.42 Not 1.0 SWR but certainly met the promise. And the best thing about this design is that near by objects don't seem to have an impact on performance. If I built one of these designs and tuned it in my basement shop, it should give the same results in the attic. Anyway, just my subjective observations. jimbo - AJ7IM You should be able to tweak the antenna on the bench until the SWR is more like 1.15:1 or so. Once you get it right there will probably be a lot less detuning once it is put in the attic |
J_Pole Trials and Tribulations
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J_Pole Trials and Tribulations
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J_Pole Trials and Tribulations
Roy Lewallen wrote:
What's the mechanism which gives this half wave antenna 2.4 dB gain over a half wave dipole? A J-Pole over ground obviously has gain over a half wave dipole in free space? :-) -- 73, Cecil http://www.qsl.net/w5dxp |
J_Pole Trials and Tribulations
wrote:
Cecil Moore wrote: A 1/4WL ground plane takes one SO-239 and five pieces of 19 inch copper wire (or stingers). :-) Now drive over one with a Chevy Pickup, the Jpole survived that at field day one year. Well, I just did that with a GMC pickup (hope that counts). Nothing broke. I just straightened out the wires and it still works just fine. If, Heaven forbid, a wire broke, it would only take a butt connector and 20 seconds to repair it. -- 73, Cecil http://www.qsl.net/w5dxp |
J_Pole Trials and Tribulations
Same here..in the attic...works great...even at 50w.
73 de KB9BVN -- ========================================= Radio Amateurs - Fill your junk box, from my junkbox! http://www.hamradparts.com 73 de KB9BVN ========================================= "Buck" wrote in message ... On 25 Apr 2006 16:43:11 -0700, wrote: jimbo wrote: SNIP jimbo - AJ7IM Hi Jimbo, it has been my experience, and everybody else's that an indoor or attic J-pole is unpredictable. A high impedance end fed antenna in an attic environment will be hard to predict. Moving it one foot can effect things. You might consider the SO-239 ground plane. It can be built and tuned in 30 minutes, is 50 ohms, so it will not be so concerned about its environment. Likely you could not tell a difference in performance from a J-pole especially if they are both in the attic. Oh yes, the standard-if the SWR is below 3.0, don't worry about it. Gary N4AST I started building dipole antennas. they can be mounted horizontal or vertical and easily moved around. I use cpvc with a dipole wire inside and a coax-choke balun. They work quite well. Just another option. -- 73 for now Buck N4PGW |
J_Pole Trials and Tribulations
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J_Pole Trials and Tribulations
Bob wrote:
snip My point is, why not make a simple ground plane for the attic that may not get bothered by surroundings as much as the J-pole. Bob I made one of those antennas right after I got my Tech license. Hit the local repeater with no problems. I guess I forgot or thought it was too simple for the current application. jimbo |
J_Pole Trials and Tribulations
"Bob" wrote in message . com... The J-pole crowd seems to tout fantastic claims about this antenna and I have never been impressed. Many people have tested the J-pole against other easy to make antennas and the end fed half wave J-pole usually does not stand up to even it’s brother, the center fed half wave dipole. I assume the decrease in performance is due to mistakes in assembling and tuning the matching section. Here is a link to a group that tested some J-poles against other antennas in the CA desert and the J-pole lost to even simple 1/4 wave ground plane types. http://www.geocities.com/SiliconVall...5/anttest.html snip A quick look at the numbers doesn't make the j-pole look so all-fired bad. "Losing" by 1 dB or so isn't a serious loss. Ham radio isn't exactly the Kentucky Derby. I don't know what "fantastic claims" you've heard, but my affinity for the j-pole comes from its simplicity and durability. It's a halfwave vertical that requires no ground plane and can often be matched to 1:1 at the sweet spot. Mounting is a dream -- it even works if you drop the low end into a plumbing vent pipe. (How would I know that? ;-) I have never built a center-fed halfwave, but we use a bunch of them in the Navy -- call them "stovepipes" because of the way the fiberglass housings look -- and they work very, very well. The elements are large metal cylinders, tending to make them quite broadband. I do not know how they're fed ... balun or what. John , KD6VKW |
J_Pole Trials and Tribulations
I happen to like them as they can be made from common materials with
excellent ruggedness. A 1/4WL ground plane takes one SO-239 and five pieces of 19 inch copper wire (or stingers). :-) AND if you make it out of stiff wire (say #12 or so), it can be folded to fit in a grab-n-go box and then unfolded when needed! The slight bends caused by folding/unfolding may not look neat, but it's mighty handy! -- --Myron A. Calhoun. Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |
J_Pole Trials and Tribulations
The J-pole crowd seems to tout fantastic claims about this antenna and I
....[snip].... ... my affinity for the j-pole comes from its simplicity and durability. ... Mounting is a dream -- it even works if you drop the low end into a plumbing vent pipe. (How would I know that? ;-) ....[snip].... I once made a J-pole from 300-ohm twinlead and tweaked it until it worked OK while hanging from the ceiling, but when I moved it over next to a wood-framed picture window (hung it from the window casing), the SWR went to pot. Did I do something wrong? (I concluded the J-pole must be VERY sensitive to its surroundings.) -- --Myron A. Calhoun. Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |
J_Pole Trials and Tribulations
wrote:
I once made a J-pole from 300-ohm twinlead and tweaked it until it worked OK while hanging from the ceiling, but when I moved it over next to a wood-framed picture window (hung it from the window casing), the SWR went to pot. Did I do something wrong? What kind of metal was behind the window frame? -- 73, Cecil http://www.qsl.net/w5dxp |
J_Pole Trials and Tribulations
On Sun, 07 May 2006 00:39:24 GMT,
wrote: Measurements are required. A suitable SWR meter of some form is needed or your just guessing. Judging an antenna's performance based upon SWR is a very poor indicator of how it will radiate. Danny, K6MHE In my many years I have come to a conclusion that one useless man is a shame, two is a law firm, and three or more is a congress. - John Adams email: k6mheatarrldotnet http://www.k6mhe.com/ |
J_Pole Trials and Tribulations
jimbo wrote:
I have been trying to get an acceptable 2 meter antenna installed in my attic. A J-Pole seemed to me the best solution for my attic space. You snip jimbo - AJ7IM Here is a link to one I have had good luck with. Works quite well out of the box. I use two chaokes, one at the base, and one 19 inches below that. You may need to trim the length of the half wave section, so cut it a smidge long. http://www.hamuniverse.com/2mladjpole.html tom K0TAR |
J_Pole Trials and Tribulations
I once made a J-pole from 300-ohm twinlead and tweaked it until it worked
OK while hanging from the ceiling, but when I moved it over next to a wood-framed picture window (hung it from the window casing), the SWR went to pot. Did I do something wrong? What kind of metal was behind the window frame? The house was old enuf that I'm sure it was made of wood, and since it was a PICTURE window, there were no sash weights, either. So the question remains: are J-poles sensitive to nearby materials? -- --Myron A. Calhoun. Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |
J_Pole Trials and Tribulations
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J_Pole Trials and Tribulations
In article , wrote:
What kind of metal was behind the window frame? The house was old enuf that I'm sure it was made of wood, and since it was a PICTURE window, there were no sash weights, either. So the question remains: are J-poles sensitive to nearby materials? That depends on the materials. Conductive, or lossy-dielectric materials seem to have a significant effect especially when located near the high-impedance points. When I built a ladder-line J-pole and hung it next to my house's outside wall, it de-tuned quite badly. Its performance stank so badly that I could barely reach a nearby repeater. When I hung it by an inside wall, it was not de-tuned enough to affect the radio's performance appreciably (I didn't have an MFJ meter at the time and so don't know the actual degree of de-tuning). I was able to hit the repeater quite well even though the antenna was further inside the house. The first (outside) wall was stucco... with embedded chicken wire. The second (inner) wall was drywall and lumber. I suspect that exterior brick, or stone, or wet wood, would fall somewhere in between the two in terms of de-tuning potential. My guess is that the classic J-pole may be somewhat more vulnerable to being de-tuned by nearby materials than a center-fed half-wave radiator. The quarter-wave matching section is often fairly critical of adjustment - I've read reports that even small changes in the configuration of the upper potion of the matching section (e.g. material near the open end of the stub, or changes in the spacing) can cause some fairly large changes in the J-pole's feedpoint impedance. Standard J-pole antennas seem to work most stably when built of rigid materials and mounted "in the clear". I haven't read any reports of actual experiments which attempted to quantify the tuning stability of the classic J-pole, compared with variants like the Arrow (a.k.a. Cebik's nontypical design), the open-sleeve, and the sperrtopf coaxial design, or compared with a center-fed dipole or other varieties of halfwave radiator. It'd be an interesting study to try to put together. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
J_Pole Trials and Tribulations
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J_Pole Trials and Tribulations
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J_Pole Trials and Tribulations
"Dave Platt" wrote in message ... snip I haven't read any reports of actual experiments which attempted to quantify the tuning stability of the classic J-pole ... snip That, sir is about to change: I wondered about the effect of mast material, so I took a freshly built j-pole and mounted it in the clear. I noted the XMIT reflected power (low) and moved a ten-foot piece of galvanized steel mast around the antenna, I found I had to get within about an inch to see the reflected power increase. Next, I found a repeater with nearly continuous chatter and connected the j-pole through an attenuator to my radio. I set the attenuator so the received audio was very noisy -- another dB or two and the conversation was unreadable. That same piece of mast material had essentially the same effect: had to get really close to louse up the antenna's performance. That said, if I take a j-pole and walk around with it, periodically measuring the reflected power, I will see physical locations where it bumps up. That's probably not detuning; it's probably reflected power off a wall or fence, which could be what's happening in the OP's attic. (Navy ship radars' reflected power monitors will "bump up" every time the antenna rotates past the mast or some other bit of ship's structure.) Is this great science? No. Is it worth noting? I hope so. |
J_Pole Trials and Tribulations
Sal M. Onella wrote:
"Dave Platt" wrote in message ... snip I haven't read any reports of actual experiments which attempted to quantify the tuning stability of the classic J-pole ... snip That, sir is about to change: I wondered about the effect of mast material, so I took a freshly built j-pole and mounted it in the clear. I noted the XMIT reflected power (low) and moved a ten-foot piece of galvanized steel mast around the antenna, I found I had to get within about an inch to see the reflected power increase. Next, I found a repeater with nearly continuous chatter and connected the j-pole through an attenuator to my radio. I set the attenuator so the received audio was very noisy -- another dB or two and the conversation was unreadable. That same piece of mast material had essentially the same effect: had to get really close to louse up the antenna's performance. That said, if I take a j-pole and walk around with it, periodically measuring the reflected power, I will see physical locations where it bumps up. That's probably not detuning; it's probably reflected power off a wall or fence, which could be what's happening in the OP's attic. (Navy ship radars' reflected power monitors will "bump up" every time the antenna rotates past the mast or some other bit of ship's structure.) Is this great science? No. Is it worth noting? I hope so. Maybe reflected power off of nearby objects is the cause of poor performance of my J-Pole in the basement shop. And the attic has the roof overhead and many, many cross braces, typical of pre-fab trusses. Anyway, I am satisfied with the Arrow end fed antenna in the attic. But, I am disappointed with my home made J-Poles. Maybe I will try a little SO239 1/4 wave, just to see if there is any difference. Thanks for the help, jimbo |
J_Pole Trials and Tribulations
On Thu, 11 May 2006 08:53:05 -0600, jimbo wrote:
Maybe reflected power off of nearby objects is the cause of poor performance of my J-Pole in the basement shop. And the attic has the roof overhead and many, many cross braces, typical of pre-fab trusses. Anyway, I am satisfied with the Arrow end fed antenna in the attic. But, I am disappointed with my home made J-Poles. Maybe I will try a little SO239 1/4 wave, just to see if there is any difference. I'm wondering if you are using a common mode choke? Also what type of mast are you using to support the antenna? Danny, K6MHE In my many years I have come to a conclusion that one useless man is a shame, two is a law firm, and three or more is a congress. - John Adams email: k6mheatarrldotnet http://www.k6mhe.com/ |
J_Pole Trials and Tribulations
Dan Richardson wrote:
On Thu, 11 May 2006 08:53:05 -0600, jimbo wrote: Maybe reflected power off of nearby objects is the cause of poor performance of my J-Pole in the basement shop. And the attic has the roof overhead and many, many cross braces, typical of pre-fab trusses. Anyway, I am satisfied with the Arrow end fed antenna in the attic. But, I am disappointed with my home made J-Poles. Maybe I will try a little SO239 1/4 wave, just to see if there is any difference. I'm wondering if you are using a common mode choke? Also what type of mast are you using to support the antenna? Danny, K6MHE In my many years I have come to a conclusion that one useless man is a shame, two is a law firm, and three or more is a congress. - John Adams email: k6mheatarrldotnet http://www.k6mhe.com/ I have tried with and without a coax loop, 5 turns about 6 inches in diameter. I couldn't see any difference. I use a 1/8 inch diameter rope to hang the J-Pole from a joist in the basement or from a roof rafter in the attic. jimbo |
J_Pole Trials and Tribulations
jimbo wrote:
Dan Richardson wrote: I'm wondering if you are using a common mode choke? Also what type of mast are you using to support the antenna? I have tried with and without a coax loop, 5 turns about 6 inches in diameter. I couldn't see any difference. I use a 1/8 inch diameter rope to hang the J-Pole from a joist in the basement or from a roof rafter in the attic. A single choke blocks conducted common mode current. But a J pole (and ground plane, for that matter) can also have considerable common mode current due to coupling between the antenna and feedline, which can distort the pattern and reduce the gain at the horizon. This can actually be made worse by a choke in some cases. Two chokes, about a quarter wave apart, are necessary to reliably reduce the coupled current. The amount of conducted and coupled current you have without chokes depend on the feedline length and orientation, so it can be small in some installations and large in others. I suspect that's one reason why people report such widely varying degrees of success with this antenna. Roy Lewallen, W7EL |
J_Pole Trials and Tribulations
Roy Lewallen wrote:
jimbo wrote: I have tried with and without a coax loop, 5 turns about 6 inches in diameter. I couldn't see any difference. I use a 1/8 inch diameter rope to hang the J-Pole from a joist in the basement or from a roof rafter in the attic. A single choke blocks conducted common mode current. But a J pole (and ground plane, for that matter) can also have considerable common mode current due to coupling between the antenna and feedline, which can distort the pattern and reduce the gain at the horizon. This can actually be made worse by a choke in some cases. Two chokes, about a quarter wave apart, are necessary to reliably reduce the coupled current. The amount of conducted and coupled current you have without chokes depend on the feedline length and orientation, so it can be small in some installations and large in others. I suspect that's one reason why people report such widely varying degrees of success with this antenna. Roy Lewallen, W7EL Isn't 6 inch diameter a little large for 2 meters? tom K0TAR |
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