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j-pole 5/8 wave
Hello all,
As a new Novice amateur I am considering to build my first antenna. I know that starting with a j-pole is simple and easy. However. I see many plans on the internet. One of interest is the stacked 5/8 wave j-pole. I only see this in a stacked version. In a not stacked way I only see the half wave version with a quarter stub. Can a 5/8 wave length and 1/2 wave stub work as a real 5/8 wave antenna? Looking forward for any suggestion! |
j-pole 5/8 wave
michel wrote:
Hello all, As a new Novice amateur I am considering to build my first antenna. I know that starting with a j-pole is simple and easy. However. I see many plans on the internet. One of interest is the stacked 5/8 wave j-pole. I only see this in a stacked version. In a not stacked way I only see the half wave version with a quarter stub. Can a 5/8 wave length and 1/2 wave stub work as a real 5/8 wave antenna? Looking forward for any suggestion! ------------ The homemade (very crude) 1/2 wave with a 1/4 wave matching section that I described in another article easily made great contacts from Pennsville, NJ (Wilmington, DE) to Philadelphia on simplex with ease. It was even indoors when I did it and I was running 10 watts or less. That is about a 30 mile distance. Mine was constructed of two pieces of #8 aluminum grounding wire stuck into two holes drilled into a piece of 2x4 as a base, sitting on my table a few feet away from my rig inside of the house. That was good enough for me to hold QSO's on FM and sideband with all of my Philadelphia friends (WA3RXE and several others). Ed Cregger |
j-pole 5/8 wave
michel wrote:
Can a 5/8 wave length and 1/2 wave stub work as a real 5/8 wave antenna? A 5/8WL needs inductive reactance to resonate it. An inductive stub will do that but a 1/2WL stub will not. FYI, for a 2x5/8WL dipole, a 0.2 WL series section of 450 ohm ladder-line is a pretty good match to 50 ohms. -- 73, Cecil http://www.w5dxp.com |
j-pole 5/8 wave
"Cecil Moore" schreef in bericht . net... michel wrote: Can a 5/8 wave length and 1/2 wave stub work as a real 5/8 wave antenna? A 5/8WL needs inductive reactance to resonate it. An inductive stub will do that but a 1/2WL stub will not. FYI, for a 2x5/8WL dipole, a 0.2 WL series section of 450 ohm ladder-line is a pretty good match to 50 ohms. In a simple way... I have a vertical radiator of 2 x 5/8 wave length. That is serie with 450 ladder line one side connected to the radiated element and the other to a groundplane will work? |
j-pole 5/8 wave
michel wrote:
In a simple way... I have a vertical radiator of 2 x 5/8 wave length. That is serie with 450 ladder line one side connected to the radiated element and the other to a groundplane will work? I cannot tell from your description what is your exact configuration. Is this it? (fixed font) GndPlane--+ +------------------------------------- | | 1.25WL vertical monopole | | | | | | 450 ohm ladder-line | | to transmitter Simple vertical monopoles should not be longer than 5/8WL. -- 73, Cecil http://www.w5dxp.com |
j-pole 5/8 wave
"Cecil Moore" schreef in bericht . .. michel wrote: In a simple way... I have a vertical radiator of 2 x 5/8 wave length. That is serie with 450 ladder line one side connected to the radiated element and the other to a groundplane will work? I cannot tell from your description what is your exact configuration. Is this it? (fixed font) GndPlane--+ +------------------------------------- | | 1.25WL vertical monopole | | | | | | 450 ohm ladder-line | | to transmitter Hi Cecil, In your first reply you talked about 2x 5/8 wave with 0,2 wave in serie to 50 ohm. That is the same as in your example here? Or must we use it al the way up to my transciever? I am still talking about 5/8 wave for 2 meters. Michel |
j-pole 5/8 wave
michel wrote:
In your first reply you talked about 2x 5/8 wave with 0,2 wave in series to 50 ohm. That is the same as in your example here? No, my first reply was about a 2x5/8WL dipole fed in the center with 0.2WL of ladder-line and no ground plane. Or must we use it al the way up to my transciever? I am still talking about 5/8 wave for 2 meters. You can turn the 2x5/8WL center-fed dipole vertical in which case, like a vertical center-fed 1/2WL dipole, it doesn't need a ground plane. -- 73, Cecil http://www.w5dxp.com |
j-pole 5/8 wave
michel wrote: Hello all, As a new Novice amateur I am considering to build my first antenna. I know that starting with a j-pole is simple and easy. However. I see many plans on the internet. One of interest is the stacked 5/8 wave j-pole. I only see this in a stacked version. In a not stacked way I only see the half wave version with a quarter stub. Can a 5/8 wave length and 1/2 wave stub work as a real 5/8 wave antenna? Looking forward for any suggestion! I don't know what construction technique you have in mind, but I prefer to build them from 1/2" copper pipe. If that's what you intend to do, then just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the rest the same (1/4 wave stub). Attach your coax to the stub section using hose clamps, and tune by sliding the clamps up or down until you achieve best SWR. Works great. ac6xg |
j-pole 5/8 wave
I don't know what construction technique you have in mind, but I prefer to build them from 1/2" copper pipe. If that's what you intend to do, then just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the rest the same (1/4 wave stub). Attach your coax to the stub section using hose clamps, and tune by sliding the clamps up or down until you achieve best SWR. Works great. ac6xg Should it be that simple? I understood from other posters that it will not work with the 1/4 wave stub.. Maybe I must try it myself? What I found is 2x5/8 wave with a 1/4 wave stub.. ? |
j-pole 5/8 wave
michel wrote: I don't know what construction technique you have in mind, but I prefer to build them from 1/2" copper pipe. If that's what you intend to do, then just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the rest the same (1/4 wave stub). Attach your coax to the stub section using hose clamps, and tune by sliding the clamps up or down until you achieve best SWR. Works great. ac6xg Should it be that simple? It can be. I understood from other posters that it will not work with the 1/4 wave stub.. For some posters, a view of the forest is obscured by trees. A j-pole is a quarter wave stub with a 1/2 wave (or longer) radiator attached to one side of the open end. The feed is connected nearer to the shorted end of the stub. Build it and they (the QSOs) will come. Get it up as high above the roof as you can. ac6xg |
j-pole 5/8 wave
michel wrote:
Should it be that simple? I understood from other posters that it will not work with the 1/4 wave stub.. A 1/2WL + (1/4WL series stub) is a resonant Zepp. When the stub is shorted at the bottom and tapped for 50 ohms, it becomes a typical J-Pole. A 5/8WL + (1/5WL series stub) is a resonant Extended Zepp. There is only one resonant point and it may not be at 50 ohms but it may be close enough. You won't know till you try it. The feedpoint impedance can be varied by varying the spacing between the stub elements. A 5/8WL + (1/4WL series stub) is non-resonant. Anywhere you tap on the stub will result in some reactance. There's really no reason to build this inferior design. -- 73, Cecil http://www.w5dxp.com |
j-pole 5/8 wave
"Cecil Moore" schreef in bericht t... michel wrote: Should it be that simple? I understood from other posters that it will not work with the 1/4 wave stub.. A 1/2WL + (1/4WL series stub) is a resonant Zepp. When the stub is shorted at the bottom and tapped for 50 ohms, it becomes a typical J-Pole. A 5/8WL + (1/5WL series stub) is a resonant Extended Zepp. There is only one resonant point and it may not be at 50 ohms but it may be close enough. You won't know till you try it. The feedpoint impedance can be varied by varying the spacing between the stub elements. A 5/8WL + (1/4WL series stub) is non-resonant. Anywhere you tap on the stub will result in some reactance. There's really no reason to build this inferior design. -- Cecil, That is some clear information!! The 5/8 wave version needs a shorted stub as well? |
j-pole 5/8 wave
michel wrote:
That is some clear information!! The 5/8 wave version needs a shorted stub as well? What we are looking for is the length of a series matching stub section that will bring the system to resonance, i.e. we are looking for the current maximum point. The graph at http://www.w5dxp.com/majic.gif gives the length of the series matching stub section for dipoles of various lengths from 1/2WL to 1.5WL. The graph can be used for monopoles by doubling the monopole length. For instance, doubling the 5/8WL monopole length gives a 10/8WL dipole length which is 1.25WL. A 1.25WL dipole needs a series matching stub section of 0.19WL, i.e. the bottom stub on a 5/8WL J-Pole needs to be 0.19WL. Make it a little too long and trim for resonance. If you double the 1/2WL monopole length of the J-Pole you get a 1.0WL dipole. From the graph, a 1.0WL dipole needs a 0.25WL series matching stub section, i.e. the bottom stub on a 1/2WL J-Pole needs to be 0.25WL. Of course, that is the standard J-Pole design. If you happened to want to design a 0.35WL J-Pole, the series matching stub would need to be 0.33WL long. 2(0.35WL) = 0.7WL Let me know if you understand the graph. -- 73, Cecil http://www.w5dxp.com |
j-pole 5/8 wave
michel wrote:
That is some clear information!! The 5/8 wave version needs a shorted stub as well? After my first cup of coffee, I canceled my previous posting. The matching method I described works well for balanced dipoles but is probably not applicable to monopoles with no ground plane because the current in the other leg of the series matching section has no place to flow in a J-Pole configuration. So a 0.2WL matching stub is not a good feed design for a 5/8WL monopole and will generate common-mode problems unless there is a ground plane into which the current can flow. Best to stick with the standard 1/2WL J-Pole design. I apologize for my fuzzy thinking - it made sense until I woke up. And it would work for a 5/8WL monopole if it already had ground plane radials. -- 73, Cecil http://www.w5dxp.com |
j-pole 5/8 wave
On Oct 2, 12:23 pm, Jim Kelley wrote:
michel wrote: I don't know what construction technique you have in mind, but I prefer to build them from 1/2" copper pipe. If that's what you intend to do, then just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the rest the same (1/4 wave stub). Attach your coax to the stub section using hose clamps, and tune by sliding the clamps up or down until you achieve best SWR. Works great. ac6xg Should it be that simple? It can be. I understood from other posters that it will not work with the 1/4 wave stub.. For some posters, a view of the forest is obscured by trees. A j-pole is a quarter wave stub with a 1/2 wave (or longer) radiator attached to one side of the open end. The feed is connected nearer to the shorted end of the stub. Build it and they (the QSOs) will come. Get it up as high above the roof as you can. ac6xg I suppose that since there will be an antenna current on the stub, and the 5/8 section would show a reactive feedpoint if fed against a ground plane, things are a bit more complicated than just a resistive matching section (the 1/4 wave stub). I would expect that (1) the antenna won't behave quite like a 5/8 wave fed against ground, nor like a 5/4 wave center fed doublet, and (2) the stub will have to be adjusted in length as well as in transformation ratio to get a "perfect" match. In addition, if the feedline is not decoupled from the antenna, the antenna current on the feedline will change both the pattern and the feedpoint impedance (match) from what it would be if the stub+radiator were in freespace. I'd do some NEC simulating to get an idea of a starting point AND an idea if the pattern was really an improvement over the normal half- wave over a quarter-wave stub, before trying to build one; and I'd put some effort into decoupling the antenna from other nearby metal (including the feedline)--or at least include other elements in the simulation. Even with simulating, I'd expect to have to do some fine tuning (of stub spacing or stub tap point, and possibly of stub length) if I really cared about a good match. Cheers, Tom |
j-pole 5/8 wave
On Oct 2, 12:23 pm, Jim Kelley wrote:
michel wrote: I don't know what construction technique you have in mind, but I prefer to build them from 1/2" copper pipe. If that's what you intend to do, then just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the rest the same (1/4 wave stub). Attach your coax to the stub section using hose clamps, and tune by sliding the clamps up or down until you achieve best SWR. Works great. ac6xg Should it be that simple? It can be. I understood from other posters that it will not work with the 1/4 wave stub.. For some posters, a view of the forest is obscured by trees. A j-pole is a quarter wave stub with a 1/2 wave (or longer) radiator attached to one side of the open end. The feed is connected nearer to the shorted end of the stub. Build it and they (the QSOs) will come. Get it up as high above the roof as you can. ac6xg I suppose that since there will be an antenna current on the stub, and the 5/8 section would show a reactive feedpoint if fed against a ground plane, things are a bit more complicated than just a resistive matching section (the 1/4 wave stub). I would expect that (1) the antenna won't behave quite like a 5/8 wave fed against ground, nor like a 5/4 wave center fed doublet, and (2) the stub will have to be adjusted in length as well as in transformation ratio to get a "perfect" match. In addition, if the feedline is not decoupled from the antenna, the antenna current on the feedline will change both the pattern and the feedpoint impedance (match) from what it would be if the stub+radiator were in freespace. I'd do some NEC simulating to get an idea of a starting point AND an idea if the pattern was really an improvement over the normal half- wave over a quarter-wave stub, before trying to build one; and I'd put some effort into decoupling the antenna from other nearby metal (including the feedline)--or at least include other elements in the simulation. Even with simulating, I'd expect to have to do some fine tuning (of stub spacing or stub tap point, and possibly of stub length) _if_ I really cared about a good match. (Even a half-wave over a nominally quarter-wave stub can benefit from tuning the stub length to get rid of reactance, if you really care about a good match [though that in itself is somewhat over-rated]. With the stub attached, it's not a simple half-wave radiator. There will be antenna current on the stub.) Cheers, Tom |
j-pole 5/8 wave
On Oct 3, 9:59 am, Cecil Moore wrote:
So a 0.2WL matching stub is not a good feed design for a 5/8WL monopole and will generate common-mode problems unless there is a ground plane into which the current can flow. Best to stick with the standard 1/2WL J-Pole design. I apologize for my fuzzy thinking - it made sense until I woke up. And it would work for a 5/8WL monopole if it already had ground plane radials. -- 73, Cecil http://www.w5dxp.com It's not the matching scheme that is the real problem.. A single 5/8 radiator over no radials is going to be a real dog no matter how you match it. And if you do use radials and a 5/8 radiator, you might as well feed it with a simple series loading coil. I've never been a great fan of J-poles, but if I were to build one, it would be the standard 1/2 wave version. If one is going to build a copper J pole using 5/8 elements, they should use two and build it as a collinear. And even in that case, there should be a decoupling section added for the best performance. My favorite "cheap and easy" antenna for VHF is not the j pole.. It's the 1/4 wave ground plane with sloping radials. It's easier to build, needs no matching, and the gain should be very close to most 1/2 wave j poles. MK |
j-pole 5/8 wave
On Oct 2, 2:06 pm, "Jimmie D" wrote:
The small theroretical gain advantage that a 5/8wl radiator has over a 1/2wl radiator is dependent on it having a very good counterpoise. However one may be able to fabricate a 5/8wl antenna with a couterpoise matched to the feedline with a stub made of metal tubing. Jimmie The best "counterpoise" for a 5/8 radiator is a set of 5/8 radials... But in that case, it's more of a collinear. Myself, I think a 5/8 radiator should always be used with a 5/8 lower section of you want the full performance. IE: dual 5/8 collinear. Anything else is a perversion... :/ My 2nd choice would be to use 3/4 wave radials. 1/2 wave radials would be useless. 1/4 wave radials are semi useless, and give a lousy pattern.. MK |
j-pole 5/8 wave
schreef in bericht oups.com... On Oct 2, 2:06 pm, "Jimmie D" wrote: The small theroretical gain advantage that a 5/8wl radiator has over a 1/2wl radiator is dependent on it having a very good counterpoise. However one may be able to fabricate a 5/8wl antenna with a couterpoise matched to the feedline with a stub made of metal tubing. Jimmie The best "counterpoise" for a 5/8 radiator is a set of 5/8 radials... But in that case, it's more of a collinear. Myself, I think a 5/8 radiator should always be used with a 5/8 lower section of you want the full performance. IE: dual 5/8 collinear. Anything else is a perversion... :/ My 2nd choice would be to use 3/4 wave radials. 1/2 wave radials would be useless. 1/4 wave radials are semi useless, and give a lousy pattern.. MK In my search on vertical antennas I also found a option to add on a 1/4 wave element a 5/8 element. This needs a phasing coil? I also found a 5/8 element mounted over a 1/4 innerelement.. ? But wat is the best for homebrew, and a few dB gain? |
j-pole 5/8 wave
WELL, I WOULD DO A 3/4 WAVE BY A 1/4 WAVE. THIS WORKS REAL WELL ON 10 METERS WITH A LOT OF GAIN. CHECK OUT MINE ON MY WEBSITE. http://www.dxradioworld.com |
j-pole 5/8 wave
K7ITM wrote: On Oct 2, 12:23 pm, Jim Kelley wrote: michel wrote: I don't know what construction technique you have in mind, but I prefer to build them from 1/2" copper pipe. If that's what you intend to do, then just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the rest the same (1/4 wave stub). Attach your coax to the stub section using hose clamps, and tune by sliding the clamps up or down until you achieve best SWR. Works great. ac6xg Should it be that simple? It can be. I understood from other posters that it will not work with the 1/4 wave stub.. For some posters, a view of the forest is obscured by trees. A j-pole is a quarter wave stub with a 1/2 wave (or longer) radiator attached to one side of the open end. The feed is connected nearer to the shorted end of the stub. Build it and they (the QSOs) will come. Get it up as high above the roof as you can. ac6xg I suppose that since there will be an antenna current on the stub, and the 5/8 section would show a reactive feedpoint if fed against a ground plane, things are a bit more complicated than just a resistive matching section (the 1/4 wave stub). I would expect that (1) the antenna won't behave quite like a 5/8 wave fed against ground, nor like a 5/4 wave center fed doublet, and (2) the stub will have to be adjusted in length as well as in transformation ratio to get a "perfect" match. In addition, if the feedline is not decoupled from the antenna, the antenna current on the feedline will change both the pattern and the feedpoint impedance (match) from what it would be if the stub+radiator were in freespace. I'd do some NEC simulating to get an idea of a starting point AND an idea if the pattern was really an improvement over the normal half- wave over a quarter-wave stub, before trying to build one; and I'd put some effort into decoupling the antenna from other nearby metal (including the feedline)--or at least include other elements in the simulation. Even with simulating, I'd expect to have to do some fine tuning (of stub spacing or stub tap point, and possibly of stub length) if I really cared about a good match. Cheers, Tom Hi Tom - I've built several of them and achieved a good (1.5:1) match in the middle of the band. 73, ac6xg |
j-pole 5/8 wave
K7ITM wrote: On Oct 2, 12:23 pm, Jim Kelley wrote: michel wrote: I don't know what construction technique you have in mind, but I prefer to build them from 1/2" copper pipe. If that's what you intend to do, then just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the rest the same (1/4 wave stub). Attach your coax to the stub section using hose clamps, and tune by sliding the clamps up or down until you achieve best SWR. Works great. ac6xg Should it be that simple? It can be. I understood from other posters that it will not work with the 1/4 wave stub.. For some posters, a view of the forest is obscured by trees. A j-pole is a quarter wave stub with a 1/2 wave (or longer) radiator attached to one side of the open end. The feed is connected nearer to the shorted end of the stub. Build it and they (the QSOs) will come. Get it up as high above the roof as you can. ac6xg I suppose that since there will be an antenna current on the stub, and the 5/8 section would show a reactive feedpoint if fed against a ground plane, things are a bit more complicated than just a resistive matching section (the 1/4 wave stub). I would expect that (1) the antenna won't behave quite like a 5/8 wave fed against ground, nor like a 5/4 wave center fed doublet, and (2) the stub will have to be adjusted in length as well as in transformation ratio to get a "perfect" match. In addition, if the feedline is not decoupled from the antenna, the antenna current on the feedline will change both the pattern and the feedpoint impedance (match) from what it would be if the stub+radiator were in freespace. I'd do some NEC simulating to get an idea of a starting point AND an idea if the pattern was really an improvement over the normal half- wave over a quarter-wave stub, before trying to build one; and I'd put some effort into decoupling the antenna from other nearby metal (including the feedline)--or at least include other elements in the simulation. Even with simulating, I'd expect to have to do some fine tuning (of stub spacing or stub tap point, and possibly of stub length) if I really cared about a good match. Cheers, Tom I found them to work quite well, though I fine tuned them on the mast - not in the garage next to the transmitter. 73, jk |
j-pole 5/8 wave
On Oct 4, 7:40 am, wrote:
On Oct 3, 9:59 am, Cecil Moore wrote: So a 0.2WL matching stub is not a good feed design for a 5/8WL monopole and will generate common-mode problems unless there is a ground plane into which the current can flow. Best to stick with the standard 1/2WL J-Pole design. I apologize for my fuzzy thinking - it made sense until I woke up. And it would work for a 5/8WL monopole if it already had ground plane radials. -- 73, Cecil http://www.w5dxp.com It's not the matching scheme that is the real problem.. A single 5/8 radiator over no radials is going to be a real dog no matter how you match it. And if you do use radials and a 5/8 radiator, you might as well feed it with a simple series loading coil. I've never been a great fan of J-poles, but if I were to build one, it would be the standard 1/2 wave version. If one is going to build a copper J pole using 5/8 elements, they should use two and build it as a collinear. And even in that case, there should be a decoupling section added for the best performance. My favorite "cheap and easy" antenna for VHF is not the j pole.. It's the 1/4 wave ground plane with sloping radials. It's easier to build, needs no matching, and the gain should be very close to most 1/2 wave j poles. MK Amen, bro. Another advantage of the ground plane: the radials decouple the feedline from the antenna. In a J-pole installation, either you put some effort into decoupling the feedline, or you accept that the feedline is going to radiate (and change the pattern). (I suppose yet another option is to deny the fact that feedlines can radiate...) It's easy to build a "quick-and-dirty" 146MHz ground plane with an SO-239, some 4-40 screws, washers and nuts, and three lengths of 12 or 14 AWG copper wire. A couple half-wave lengths attach to the flange of the SO-239 with the 4-40 hardware (or just solder them) so there are 4 1/4 wave wires sticking out from the flange; a 1/4 wave piece solders into the center pin. The radials can be bent down a bit to get a match to 50 ohms. You can put a little loop in the top of the radiator and hoist it into a tree with fishing line or the like. There are several ways you can attach it to the top of a mast; it doesn't care a lot about what you do below it. Cheers, Tom |
j-pole 5/8 wave
On Oct 4, 11:01 am, "michel" wrote:
In my search on vertical antennas I also found a option to add on a 1/4 wave element a 5/8 element. This needs a phasing coil? I also found a 5/8 element mounted over a 1/4 innerelement.. ? But wat is the best for homebrew, and a few dB gain? With VHF verticals, half the battle is decoupling the feedline. It's no use worrying about extended verticals, unless you also consider decoupling the line. Radiation from the line will skew the pattern upwards off the horizon, and any gain from extended radiators will be useless. If you are going to use a 5/8 radiator, I would go whole hog and build a copper collinear with dual 5/8 elements. I wouldn't waste time with other versions. And even the dual 5/8 collinear needs a decoupling section of some type. One reason I like the 1/4 wave ground planes is that they naturally decouple from the feedline fairly well. The more radials you use, the better the decoupling, and if you add a 2nd set that is 1/4 below, you will have very good decoupling. That method can be used to decouple a dual 5/8 vertical. I would worry more about how the line will be decoupled, than I would element length. It's more important. MK |
j-pole 5/8 wave
Gary #203 wrote:
WELL, I WOULD DO A 3/4 WAVE BY A 1/4 WAVE. A 3/4WL vertical monopole has a take-off-angle of 47 degrees. 5/8WL is the length limit for good monopole DX performance. -- 73, Cecil http://www.w5dxp.com |
j-pole 5/8 wave
wrote in message ups.com... snip I've never been a great fan of J-poles, but if I were to build one, it would be the standard 1/2 wave version. If one is going to build a copper J pole using 5/8 elements, they should use two and build it as a collinear. And even in that case, there should be a decoupling section added for the best performance. My favorite "cheap and easy" antenna for VHF is not the j pole.. It's the 1/4 wave ground plane with sloping radials. Having made a couple of whip+radials antennas out of old coathangers, I am not at all hostile to your views. However, as a fan of the j-pole, myself, I offer their ruggedness as a distinct advantage. My first copper pipe j-pole (from the early 1990's) is still on my roof. It has turned a slightly darker color but is otherwise as good as new. I made three for our Fire House RACES station (two 2m & a 6m) and they will outlive me. Just my $0.02. 73, "Sal" (KD6VKW) |
j-pole 5/8 wave
Hi Tom,
K7ITM wrote: Amen, bro. Another advantage of the ground plane: the radials decouple the feedline from the antenna. . . Here's a fun experiment with EZNEC. 1. Open the VHFGP.EZ example file. Click View Ant to open the View Antenna display. In the View Antenna display control section, click Center Ant Image so you can see the antenna better. 2. Add the following wi End 1: 0, 0, 5 (wavelengths) End 2: 0, 0, 4.727 (wavelengths) Diameter: 0.25 (inches) Segments: 6 This represents the outside of a feedline connected to the feedpoint. 3. Click the Currents button. Look at the display and, in the Currents box, compare the current on the outside of the "feedline" (Wire 6, Segment 1) to the main radiator current (Wire 5, Segment 1). 4. Change Plot Type to Elevation. Click FF Plot to see the 2D elevation pattern. 5. Reconsider the statement about decoupling. . . This is admittedly contrived to show a particularly extreme case. But try different lengths of "feedline" either open or connected to ground and you'll find other cases where the feedline current is high and the pattern distorted. You'll also find cases where inserting a "balun" (high impedance load) in the "feedline" will actually increase the feedline current due to changing the current distribution to a value more favorable for the particular feedline length. Roy Lewallen, W7EL |
j-pole 5/8 wave
On Oct 4, 11:33 pm, Roy Lewallen wrote:
Here's a fun experiment with EZNEC. 1. Open the VHFGP.EZ example file. Click View Ant to open the View Antenna display. In the View Antenna display control section, click Center Ant Image so you can see the antenna better. 2. Add the following wi End 1: 0, 0, 5 (wavelengths) End 2: 0, 0, 4.727 (wavelengths) Diameter: 0.25 (inches) Segments: 6 This represents the outside of a feedline connected to the feedpoint. 3. Click the Currents button. Look at the display and, in the Currents box, compare the current on the outside of the "feedline" (Wire 6, Segment 1) to the main radiator current (Wire 5, Segment 1). 4. Change Plot Type to Elevation. Click FF Plot to see the 2D elevation pattern. 5. Reconsider the statement about decoupling. . . This is admittedly contrived to show a particularly extreme case. But try different lengths of "feedline" either open or connected to ground and you'll find other cases where the feedline current is high and the pattern distorted. You'll also find cases where inserting a "balun" (high impedance load) in the "feedline" will actually increase the feedline current due to changing the current distribution to a value more favorable for the particular feedline length. Roy Lewallen, W7EL I'd be curious to see what you get doing the same with a 1/2 wave whip. IE: usual j-pole, 1/2 wave whip... I tried doing a test using the demo version. Being it was limited segments, I tried to keep it even, and used twice the segments for the 1/2 wave radiator vs the 1/4 wave, but used the same amount of segments for the "feedline" . I also used "real ground", and the elevation plot so I could more easily see the appx real world plots. With the GP, I notice heavy current when the feedline is appx 1/4 wave, but not so bad when it's longer. In some cases I saw a gain where the feed currents seem to be in phase with the antenna currents. When trying the 1/2 wave, I didn't see the problem too much using a short 1/4 wave feed, but the longer lengths were much worse than the plots for the GP. 1/4 WL GP 6.58 dbi at 3 degrees -no feedline 5.26 dbi at 42 degrees -feedline .28 wl 7.39 dbi at 3 degrees -feedline 1 wl 6.09 dbi at 3 degrees - feedline 2 wl 6.77 dbi at 3 degrees - feedline 3 wl 7.05 dbi at 3 degrees - feedline 4 wl 7.87 dbi at 3 degrees- feedline 5 wl and grounded at "0" 1/2 WL whip 6.43 dbi at 3 degrees -no feedline 6.84 dbi at 3 degrees -feedline .28 wl 6.23 dbi at 3 degrees -feedline 1 wl 6.67 dbi at 55 degrees - feedline 2 wl 14.82 dbi at 68 degrees - feedline 3 wl 21.42 dbi at 67 degrees - feedline 4 wl 6.68 dbi at 3 degrees- feedline 5 wl and grounded at "0" Anyway, I may have had problems running this test with the limited segments, but it seems to show the 1/2 wave as having the worse problems of the two overall. Really bad at 3-4 waves length of line. So while the decoupling for the GP is not always the greatest, I think it's still probably less a problem than the usual 1/2 wave whip with no decoupling. I've never used chokes or baluns per say to decouple a VHF vertical. I always use radial sets, cones, sleeves, etc.. The usual ground plane really needs two radial sets to decouple the line well. Most good sleeve dipoles will use an extra sleeve for decoupling the line. You might get a bit different results using unlimited segments. But I betting the trend will still be fairly close, with the non-decoupled 1/2 wave being the worst overall at the longer line lengths. MK |
j-pole 5/8 wave
On Oct 5, 8:39 am, wrote:
BTW, you may notice the no feedline 1/2 wave shows a higher gain than the sloping radial GP. I'm not sure if this is right or not... But I didn't tweak the 1/2 radiator.. It's exactly .50 wl long.. MK |
j-pole 5/8 wave
On Oct 5, 6:45 am, wrote:
On Oct 5, 8:39 am, wrote: BTW, you may notice the no feedline 1/2 wave shows a higher gain than the sloping radial GP. I'm not sure if this is right or not... But I didn't tweak the 1/2 radiator.. It's exactly .50 wl long.. MK Higher?? You posted 6.34dBi for the half wave, and 6.58dBi for the quarter wave GP ... Am I misreading something, or was that a typo about which has higher gain, or what? Thanks for taking the time to make the simulations and posting the results, Mark. And thanks to Roy for pointing out that the GP doesn't fully isolate the feedline from antenna currents. I should say "pointing out once again" as I know he's posted it before, now that I'm reminded about it. I don't have time at the moment, but put it on the list to play with in simulations when I have some time. Still, the 1/4 wave GP is extremely easy to hack together in an emergency -- you can even strip a quarter wave of braid off coax and twist copper wire or coat hangers around the top of the braid for the GP -- and should perform comparably with other more complicated antennas. A plus is that you only need to remember (or figure out) how long 1/4 wave is to know how to cut it to get a decent, if not perfect, match. Cheers, Tom |
j-pole 5/8 wave
On Oct 5, 11:49 am, K7ITM wrote:
On Oct 5, 6:45 am, wrote: Higher?? You posted 6.34dBi for the half wave, and 6.58dBi for the quarter wave GP ... Am I misreading something, or was that a typo about which has higher gain, or what? Dang.. I guess I had it backwards in the 2nd post... Normally, I would think the 1/2 whip would show slightly higher, but dunno.. Quiver in the force I guess. I have no real problems with either type. A 1/2 wave whip is fine if it's decoupled. But few J pole users seem to add decoupling sections. Most of the ringos sold for VHF lack decoupling also.. One note.. If I build a 1/2 wave, I usually prefer to feed as a ringo, vs as a J pole.. But I still mostly use the GP's as they are simple. I've got one up in the attic hanging from the rafters as an emergency antenna when T-storms are in the area and I'm chicken to use my outside antennas. MK |
j-pole 5/8 wave
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j-pole 5/8 wave
K7ITM wrote in news:1191602989.414910.321210
@r29g2000hsg.googlegroups.com: .... results, Mark. And thanks to Roy for pointing out that the GP doesn't fully isolate the feedline from antenna currents. I should say "pointing out once again" as I know he's posted it before, now that I'm reminded about it. I don't have time at the moment, but put it on the list to play with in simulations when I have some time. It is an interesting topic for modelling. I wrote some notes up after modelling an unloaded elevated vertical as a multi-band HF antenna. I was particularly interested in the effectiveness of decoupling of the mast / feedline and the loss implications of mast current flowing to ground. The article is at http://www.vk1od.net/multibandunload...ical/13mEV.htm . The spikes in loss in Fig 3 and 4 are mainly due to loss in the simulated resistance of the earth connection at the lower end of the mast. So, the issue is not just about pattern distortion, efficiency may be impacted significantly. One of the popular antennas amongst our new six hour hams is to use their StationMaster on 40m and 80m The StationMaster is a half wave vertical for 27MHz base fed with a parallel tuned circuit and the 50 ohm coax is tapped onto the coil of the tuned circuit. (A 27MHz version of the once popular ham ringo... but the coil is multi turn.) When modelled on a 10m high mast on 40m and 80m, most of the current moment contribution is from the support mast and the efficiency is low due to losses in mast current flowing to lossy ground. Additionally, there a huge transmission line losses. It is a top fed dummy load... but with the magic of an ATU, it has a VSWR of 1:1! But, when their investment in ham radio is 6 hours of learning, they aren't well equipped to appreciate that the StationMaster isn't an efficient antenna on 40m and 80m. Owen |
j-pole 5/8 wave
On Oct 5, 2:42 pm, Ed Cregger wrote:
wrote: On Oct 5, 11:49 am, K7ITM wrote: On Oct 5, 6:45 am, wrote: Higher?? You posted 6.34dBi for the half wave, and 6.58dBi for the quarter wave GP ... Am I misreading something, or was that a typo about which has higher gain, or what? Dang.. I guess I had it backwards in the 2nd post... Normally, I would think the 1/2 whip would show slightly higher, but dunno.. Quiver in the force I guess. I have no real problems with either type. A 1/2 wave whip is fine if it's decoupled. But few J pole users seem to add decoupling sections. Most of the ringos sold for VHF lack decoupling also.. One note.. If I build a 1/2 wave, I usually prefer to feed as a ringo, vs as a J pole.. But I still mostly use the GP's as they are simple. I've got one up in the attic hanging from the rafters as an emergency antenna when T-storms are in the area and I'm chicken to use my outside antennas. MK Yes, BUT, the 5/8th wave radiator will put more of the signal toward the horizon, instead of launching it at a 40 degree plus angle away from the horizon. So while one configuration can have higher dbi ratings, it doesn't count unless the signal goes where it will be most effective. Dunno.. You sure you ain't got it backwards? Unless the 5/8 is on a large ground plane, etc, it's usually the one with the higher avg launch angles vs the 1/2 wave. The 5/8 with no radials should be pretty bad at that.. Even a set of 1/4 wave radials under a 5/8 will give a fairly poor pattern. If I were to build a j pole, it would be a 1/2 wave. If I run a 5/8, I'd have two elements as a collinear, or at least have 5/8 or 3/4 wave radials. I'd never run just a single 5/8 wave radiator on it's own. It's not a "complete" antenna like a 1/2 wave j pole is. Or to my way of qualifying anyway... MK |
j-pole 5/8 wave
Ed Cregger wrote: Yes, BUT, the 5/8th wave radiator will put more of the signal toward the horizon, instead of launching it at a 40 degree plus angle away from the horizon. So while one configuration can have higher dbi ratings, it doesn't count unless the signal goes where it will be most effective. That is born out he http://www.cebik.com/gp/58.html I have a friend (engineer) that designs and builds his boats (some rather large sailboats) with everything quantized mathematically. I showed up one day with a model sailboat sitting on a carry stand that I had made. He asked me how I calculated the angles needed to accommodate the hull accurately. He said that this problem had been bugging him for a while. I was surprised because this guy is really smart. I grabbed two rulers and put each one along the side of the boat and then clamped them at that angle. I then transferred the angle of the two rulers to a sheet of paper by simply drawing lines along the inside of the v that was created. His jaw dropped in surprise. He was amazed at how easy the process was and he realized that the same process would work with his full size boats. No math required. While EZNEC is a fantastic program, it is no better than the programmer that wrote it. No one person can take absolutely every variable into consideration because many of them are very, very complex and nearly impossible to quantize. I suggested a simple 1/2 wave J-pole antenna earlier that was easy to make, super easy to tune and one that worked very effectively. Yet everyone is beating their brains out trying to come up with the best 5/8th wave J-pole, even though this design will require lossy matching devices to get the impedance down to a manageable/acceptable level. What's up wid dat? I do realize that figuring out such a design is fun in and of itself and may be the real purpose of the exercise. Still, I'll betcha no one on the receiving end of the OP's signal could tell if he was using the 5/8th wavelength J-pole or the 1/2 wavelength J-pole. W4RNL may have provided the definitive work on the subject. http://www.cebik.com/vhf/jp4.html Ed, NM2K 73, ac6xg |
j-pole 5/8 wave
wrote: On Oct 2, 2:06 pm, "Jimmie D" wrote: The small theroretical gain advantage that a 5/8wl radiator has over a 1/2wl radiator is dependent on it having a very good counterpoise. However one may be able to fabricate a 5/8wl antenna with a couterpoise matched to the feedline with a stub made of metal tubing. Jimmie The best "counterpoise" for a 5/8 radiator is a set of 5/8 radials... But in that case, it's more of a collinear. Myself, I think a 5/8 radiator should always be used with a 5/8 lower section of you want the full performance. IE: dual 5/8 collinear. Anything else is a perversion... :/ My 2nd choice would be to use 3/4 wave radials. 1/2 wave radials would be useless. 1/4 wave radials are semi useless, and give a lousy pattern.. MK I like the 5/8th on VHF because it has noticably more gain than a 1/4wl groundplane and in my opinon is easier to build and match than an end-fed 1/2wl antenna. Jimmie |
j-pole 5/8 wave
Ed Cregger wrote:
... I suggested a simple 1/2 wave J-pole antenna earlier that was easy to make, super easy to tune and one that worked very effectively. Yet everyone is beating their brains out trying to come up with the best 5/8th wave J-pole, even though this design will require lossy matching devices to get the impedance down to a manageable/acceptable level. What's up wid dat? ... Ed, NM2K Until recently, I owned an all fiberglass houseboat--beautiful rig. However, on fresh water it offered no counterpoise properties whatsoever. A continuously loaded 1/2 wave end fed vertical utilizing a modified gamma match feed ended up the solution, and one which required a minimal counterpoise. Regards, JS |
j-pole 5/8 wave
Ed Cregger wrote:
Yes, BUT, the 5/8th wave radiator will put more of the signal toward the horizon, instead of launching it at a 40 degree plus angle away from the horizon. So while one configuration can have higher dbi ratings, it doesn't count unless the signal goes where it will be most effective. . . . When mounted on a perfect ground plane of infinite extent, any ground mounted vertical monopole higher than 1/2 wavelength will have one or more high angle lobes. As the height increases above 1/2 wavelength, the gain at the horizon increases even though a high lobe appears at around 60 degrees above the horizon. The gain at the horizon peaks out at about 5/8 wavelength, where the high lobe is about 9 dB weaker than the main lobe. As the antenna gets longer than 5/8 wavelength, the power going into the upper lobe starts reducing the gain at the horizon (and the lobe's elevation angle slowly drops) until at one wavelength, all the power goes to the upper lobe and there's no radiation at the horizon at all. The gain increase of 1/2 or 5/8 wavelength antennas over shorter monopoles comes about by a narrowing of the lobe pointing toward the horizon. Unfortunately, though, radiation at the low angles is severely attenuated by reflection from real ground. And this is just where most of the power from longer verticals is going. So a 5/8 wave HF vertical usually won't exhibit the gain over a shorter antenna you see with a perfect ground simulation. Likewise, a finite ground plane like a car roof impacts low angle radiation, so it has more of an effect on a 1/2 or 5/8 wave radiator than a shorter one, and once again you often won't see the gain you might expect. A few minutes with the demo version of EZNEC or a similar program shows the effect of finite ground on various antenna heights very clearly. Use MININEC-type ground to eliminate the separate effect of ground system resistive loss. The full EZNEC program will let you model an antenna on a car top (by using a wire grid to simulate the car top). Roy Lewallen, W7EL |
j-pole 5/8 wave
On Sat, 06 Oct 2007 17:22:07 -0700, Roy Lewallen
wrote: The gain increase of 1/2 or 5/8 wavelength antennas over shorter monopoles comes about by a narrowing of the lobe pointing toward the horizon. Unfortunately, though, radiation at the low angles is severely attenuated by reflection from real ground. And this is just where most of the power from longer verticals is going. So a 5/8 wave HF vertical usually won't exhibit the gain over a shorter antenna you see with a perfect ground simulation. Likewise, a finite ground plane like a car roof impacts low angle radiation, so it has more of an effect on a 1/2 or 5/8 wave radiator than a shorter one, and once again you often won't see the gain you might expect. A few minutes with the demo version of EZNEC or a similar program shows the effect of finite ground on various antenna heights very clearly. Use MININEC-type ground to eliminate the separate effect of ground system resistive loss. The full EZNEC program will let you model an antenna on a car top (by using a wire grid to simulate the car top). It more that just the car's roof that impacts the antennas' radiation patterns. The car's whole body has a significant effect and there will be variations - sometimes large - between different vehicles. http://k6mhe.com/files/mobile_vhf_ant.pdf Danny, K6MHE |
j-pole 5/8 wave
On Oct 6, 3:19 pm, "Jimmie D" wrote:
If I remember correctly the oft quoted theorotical gain of a 5/8ths is with a groundplane that extents to infinity. 5/8ths with sloping 5/8th radials are begining to perform more like a ceterfed collinear. Jimmie Exactly. That is what you want. A 5/8 element is only half an antenna, and the other half wants to be a 5/8 too to work properly. All a 5/8 GP is , is half of an EDZ. You would never want to run only half of an EDZ, or use a 1/4 wave leg on one side, etc.. Like I say, it's a perversion. The 1/2 wave is the only single element that can be a proper complete antenna on it's own. Or according to the laws of NM5K anyway... :/ And of course, even that "complete" antenna needs decoupling from the feedline to live up to it's full potential. The success of a 5/8 antenna varies a lot across the spectrum. On 2m, I hate em... Not a good pattern for VHF, unless you use the collinears, etc.. But... On 10m, it's my favorite vertical.. I've done careful tests over the years, and I've never had any other type single element beat it. But... I was using 3/4 wave radials the last time I did do testing on all those. In one test, I started with a 1/4 wave GP with sloping radials. Worked ok.. Then I built a 1/2 wave whip, and fed as a ringo. I saw an improvement over the 1/4 GP, even with no decoupling. So, I then decided to add a decoupling section, using a 1/4 wave section of the feedline below the feed, and attached to a set of 1/4 radials. This improved the antenna a good bit. It was really humming along at that point. I used mainly stable local signals to test any improvements. When I added the decoupling to the halfwave, I noticed a bit less bandwidth as far as the antenna, but the performance was more stable, and I assume would be about the same no matter what line length I used, etc.. Anyway, I had the half wave working about as well as possible I think. Then I built a 5/8 radiator, and used 4 sloping 3/4 wave radials. Guess what? It beat the decoupled half wave by an easily noticed margin on the low angle space/ground wave paths I was using to test. The other stations were spread across town in various locations, some 30-40 miles away. If the low angle performance of the 5/8 was crippled, I sure didn't see it here on 10m. It still had enough gain at low angles to beat anything else I could try. BTW, all those antennas were mounted at the same 36 ft height. I also ran a 5/8 GP on 17m for a while. That antenna beat every other antenna I had that could be tuned for 17m. IE: wire dipoles, etc.. I've seen a load of people use the usual "perverted" 5/8 GP on the CB band. Even with the short 1/4 radials, I never saw one of those lose to a 1/4 wave GP. On the average CB meter, ' the change from a 1/4 GP to the 5/8 GP was normally good for about 2 S units on the average CB , if you were talking across town a ways. I've seen this too many times for it to be a fluke of nature. This goes back to the early 70's.. Anyway, I like 5/8 verticals on the HF bands. But myself, I don't use the short 1/4 wave radials. As far as 5/8 whips on cars, just depends on the path. Here in Houston, with the flat terrain, a 5/8 will generally beat a 1/4 wave by a noticable amount, and will usually have less picket fencing. But in those cases, the metal under the antenna is fairly large vs wavelength. IE: a 5/8 at 146 mhz is about 48 inches. So it doesn't take a huge vehicle to give a decent ground plane if the antenna is on the roof or trunk. Myself, I think most mobile 5/8 whips work better than elevated 5/8 GP's used on the same band. In some areas, the terrain will favor using the 1/4 wave though. Like I say, the usual 5/8 with 1/4 wave straight radials is generally the pits on 2m, unless some strange quiver in the force kicks in, like say in phase feedline currents, etc. And thats fairly rare in the real world. It can happen though.. A properly decoupled dual 5/8 collinear on 2m is a stout antenna and will be hard to beat unless you take drastic measures. Once I get to that point, I switch to a yagi if I want to do better. A 3 el yagi will pretty much smoke any practical vertical. Anyway, the performance of 5/8 antennas varies quite a bit depending on where you are using them at. The 5/8 with 3/4 radials I used on 10m was pretty stout overall. It was the best out of all the usual lengths. The real world and the modeling programs don't always exactly jive.. IE: On Cevics page, he mentions that in his modeling tests, he seems to think the 5/8 GP was generally not worth the trouble on the upper HF bands.. IE: 10m I know from real world use, it is worth it. No doubt in my mind. But I also know there are much better radial lengths than the straight 1/4 wave. BTW, I agree with one thing he mentioned.. Sloping 1/4 radials with the 5/8 is the pits... You want 1/4 wave radials straight out if they are under a 5/8 whip. Sloping them will really whack out the pattern. But sloping radials are no problem under a 1/4 wave whip. Now, on the other hand, sloping 3/4 or 5/8 radials under a 5/8 whip are ok. MK |
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