heres an interesting article explaining the deal with masts/feedlines
J-Pole Antenna - Should I ground it?
J-Pole Antenna - Should I ground it?
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
MASTS EXPLAINED
- Thread starter bob85
- Start date
Bob and I have fussed and discussed the issues using isolation before. I even experimented with isolating my A99, and basically failed to see the kind of improvement promised, albeit my style of isolating was to insulate the antenna, side by side with the mast instead of raising the antenna well above the mast. I must admit that I would have to consider and real world test Bob's suggestions further in order to really be sure.
I believe Bob when he claimed that he and Multimode 200 did what he suggested on a 1/2 wave fiber glass antenna similar to an A99 that was made in England I think, when Multimode claimed a 2+ Sunit improvement in local signals. The antenna also had short radials that Bob modified to a full 1/4 wavelength, but I don't believe MM200 ever added the choke that was supposed to go with the project. This is basically my real world experience with such an idea.
Now that I can model, I did a project for a simple J-Pole and started at 18' above real Earth, and raised and tuned as I went up to 31', 36', 40', and 42'. This was done to see how the gain, angle, pattern, and most importantly the currents flowing on the mast, noted in red, effected the models. The current flows on the mast is the subject of the link that Bobs posted above.
The issue that catches my attention in this argument is a reference to a worst case scenario discussed in the published report. I also made a similar issue some time back due to the same comment made by W8JI in his piece on the Imax antenna specifically as note below. Scroll down to the topic on the Imax 2000 and the A99.
End-fed Zepp
End fed longwire or random wire antenna
Groundplane Vertical
End-fed half wave
IMAX 2000
Understanding Gain differences
In order to understand gain differences between antennas, we have to understand the signal from a good basic antenna like the dipole. A dipole is the normal reference for comparing antennas.
The dipole is also a basic building block of many antennas. Let's dispel a common gain misconception about dipoles and isotropic radiators.
A dipole does NOT have 2.2dB gain over an isotropic radiator when the dipole is placed over earth. At optimum heights, a common 1/2 wave dipole actually has about 8.5 dB gain over an isotropic radiator! Always remember that when you see antenna models over earth that tell you an antenna's gain in dBi.
If a model over earth shows a "gain" of about 8.5 dBi, the model effectively has the same gain as a dipole at optimum height over typical earth! We cannot add 2.15 dB to the isotropic gain to get the dBi gain unless ALL of the antennas are in free-space! The instant the earth is involved in a model or measurement the 2.15 dB rule flies out the window.
The plots below are for a 145-foot high copper wire dipole modeled with high accuracy ground over medium real earth on EZNEC:
You can see the gain is 8.5 dBi and it is just a simple dipole just over 1/2 wave high. Any antenna we model should be compared to a standard like a dipole over real earth (unless we intend to install the antenna in outer space)!
The J-pole and other end-fed Hertz antennas are prime examples of antenna that can have severe feedline common mode current problems. The coax shield has to be at zero volts potential and have exactly equal and opposite currents to those flowing into and out of the center conductor at the load and source, otherwise the feedline radiates.
When we allow the feedline shield to be part of the radiating system due to poor feed system design or construction, the system can be unstable. Weather changes can affect feedline moisture between the outer jacket and the support for the feedline, and this can change SWR with rain or snow. With improper feedline and mast decoupling, feedline and mast length and grounding can affect SWR. Potentially severe common-mode feedline problems of end-fed 1/2 wave antennas vary with feedline length and feedline routing. This is why some people swear by end-fed antennas, while other people swear at end-fed antennas.
The J-pole is a good example of a poorly implemented feed system.
Here is a zoom of the feedpoint in a correct model of a J-pole. Notice the model includes the coaxial feedline and/or mast attached to the "grounded point" of the J-pole. (red vertical wire extending downward by the "3")
You'll see the feedline or mast grounds directly to what everyone assumes is a "zero voltage" point. This is the electrical equivalent of any J-pole with the coax connected in series with the feedpoint, and the shorter J-pole leg connected to the shield. The shield can be connected to any supporting mast with much change in system performance. The feedline in this case is relatively cold.
Here is the resulting pattern of the shield to short leg (with a split base feed, NOT tapped up on the "J") :
The gain is 2.37 dBi at 4 degrees elevation (compared this to 2.69 dBi for a 1/4wl groundplane). This is actually the best feed system for the J-pole! The shield is connected to the bottom of the short element of the J-pole, with the center conductor connected to the bottom of the longer element of the J-pole.
This antenna model is in freespace, so earth reflection gain is not a factor. It is essentially equal to a vertical dipole in the same environment.
There is some distortion of pattern cause by the imperfect feed, even though it is the best feed.
Here is the pattern with the feedpoint connections reversed. The shield is connected to the longer element and the center conductor to the short element:
Low-angle gain dropped about 5dB with just a simple reversal of feedline connections! If the model did not include the feedline, the model would never show this problem. In both cases, the SWR stayed near 1:1, yet low angle gain was reduced 5dB by reversing the shield and center conductor positions on the antenna!
The above shows the sensitivity of the J-pole to feedline connections, yet virtually all articles on construction of J-pole ignore this sensitivity
I-Max 2000 Solarcon A-99 Antenna
The following model is an I-Max 2000 5/8th wave vertical with a vertical feedline or mast connected to the antenna base, and no radials. In this case I picked one of many worse-case feedline or mast lengths:
Feedline shield current is 100% of antenna current. This illustrates why some users complain about SWR problems and RF in the shack with end-fed verticals like the I-MAX 2000, while other people do not complain and seem to love the antenna. This is because some people pick a lucky mast height or feedline length, while others are not so lucky. Unlucky people happened to choose a mast height, feedline length, or grounding system length that enhanced common mode problems.
*****end of article*****
I argue if there is such a case as the worst case scenario for the installation of the Imax, then that would also suggest that, at the very least, there were better installation heights to be considered...where the currents were a lot less significant to be causing pattern skewing and other problems. IMO, my J-P models attempt to demonstrate such a situation. Remember antenna models are not necessarily duplicable in the real world, but there could be some trends to be noted that suggest another way to solving such a problem.
Below are my models mentioned above and are supplied for some consideration for what might be going on with your J-Pole installation and that there may be an alternative method to solve the problems discussed in the article. Of course these models do not include the feed line which would necessarily provide an easy pathway for currents to flow, and something would also have to be done to mitigate such a problem. But with that said, I contend that there may be a better way to solve such problems aside for isolation, which the article describes as an iffy installation. You will note that the models suggest modest changes in height trend toward a mitigating type solution, and warns of the possibilities for a worst case scenario. Only real world test may prove this one way or another.
Another thing to notice is how bad the patterns look at 1/2 wave points, 18', 36', and likely at 54', and 72' feet which I did not model.
I also have an opinion regarding such a setup maybe needing radials along with the choke, but I have not tested such and unfortunately I cannot model a choke of a feed line with good understanding.
I'm sure there are issues I have left out in my thinking, but this is what is mullin around in my mind on the subject.
View attachment Bob's issues on Masts on J=Poles.pdf
I believe Bob when he claimed that he and Multimode 200 did what he suggested on a 1/2 wave fiber glass antenna similar to an A99 that was made in England I think, when Multimode claimed a 2+ Sunit improvement in local signals. The antenna also had short radials that Bob modified to a full 1/4 wavelength, but I don't believe MM200 ever added the choke that was supposed to go with the project. This is basically my real world experience with such an idea.
Now that I can model, I did a project for a simple J-Pole and started at 18' above real Earth, and raised and tuned as I went up to 31', 36', 40', and 42'. This was done to see how the gain, angle, pattern, and most importantly the currents flowing on the mast, noted in red, effected the models. The current flows on the mast is the subject of the link that Bobs posted above.
The issue that catches my attention in this argument is a reference to a worst case scenario discussed in the published report. I also made a similar issue some time back due to the same comment made by W8JI in his piece on the Imax antenna specifically as note below. Scroll down to the topic on the Imax 2000 and the A99.
End-fed Zepp
End fed longwire or random wire antenna
Groundplane Vertical
End-fed half wave
IMAX 2000
Understanding Gain differences
In order to understand gain differences between antennas, we have to understand the signal from a good basic antenna like the dipole. A dipole is the normal reference for comparing antennas.
The dipole is also a basic building block of many antennas. Let's dispel a common gain misconception about dipoles and isotropic radiators.
A dipole does NOT have 2.2dB gain over an isotropic radiator when the dipole is placed over earth. At optimum heights, a common 1/2 wave dipole actually has about 8.5 dB gain over an isotropic radiator! Always remember that when you see antenna models over earth that tell you an antenna's gain in dBi.
If a model over earth shows a "gain" of about 8.5 dBi, the model effectively has the same gain as a dipole at optimum height over typical earth! We cannot add 2.15 dB to the isotropic gain to get the dBi gain unless ALL of the antennas are in free-space! The instant the earth is involved in a model or measurement the 2.15 dB rule flies out the window.
The plots below are for a 145-foot high copper wire dipole modeled with high accuracy ground over medium real earth on EZNEC:
You can see the gain is 8.5 dBi and it is just a simple dipole just over 1/2 wave high. Any antenna we model should be compared to a standard like a dipole over real earth (unless we intend to install the antenna in outer space)!
The J-pole and other end-fed Hertz antennas are prime examples of antenna that can have severe feedline common mode current problems. The coax shield has to be at zero volts potential and have exactly equal and opposite currents to those flowing into and out of the center conductor at the load and source, otherwise the feedline radiates.
When we allow the feedline shield to be part of the radiating system due to poor feed system design or construction, the system can be unstable. Weather changes can affect feedline moisture between the outer jacket and the support for the feedline, and this can change SWR with rain or snow. With improper feedline and mast decoupling, feedline and mast length and grounding can affect SWR. Potentially severe common-mode feedline problems of end-fed 1/2 wave antennas vary with feedline length and feedline routing. This is why some people swear by end-fed antennas, while other people swear at end-fed antennas.
The J-pole is a good example of a poorly implemented feed system.
Here is a zoom of the feedpoint in a correct model of a J-pole. Notice the model includes the coaxial feedline and/or mast attached to the "grounded point" of the J-pole. (red vertical wire extending downward by the "3")
You'll see the feedline or mast grounds directly to what everyone assumes is a "zero voltage" point. This is the electrical equivalent of any J-pole with the coax connected in series with the feedpoint, and the shorter J-pole leg connected to the shield. The shield can be connected to any supporting mast with much change in system performance. The feedline in this case is relatively cold.
Here is the resulting pattern of the shield to short leg (with a split base feed, NOT tapped up on the "J") :
The gain is 2.37 dBi at 4 degrees elevation (compared this to 2.69 dBi for a 1/4wl groundplane). This is actually the best feed system for the J-pole! The shield is connected to the bottom of the short element of the J-pole, with the center conductor connected to the bottom of the longer element of the J-pole.
This antenna model is in freespace, so earth reflection gain is not a factor. It is essentially equal to a vertical dipole in the same environment.
There is some distortion of pattern cause by the imperfect feed, even though it is the best feed.
Here is the pattern with the feedpoint connections reversed. The shield is connected to the longer element and the center conductor to the short element:
Low-angle gain dropped about 5dB with just a simple reversal of feedline connections! If the model did not include the feedline, the model would never show this problem. In both cases, the SWR stayed near 1:1, yet low angle gain was reduced 5dB by reversing the shield and center conductor positions on the antenna!
The above shows the sensitivity of the J-pole to feedline connections, yet virtually all articles on construction of J-pole ignore this sensitivity
I-Max 2000 Solarcon A-99 Antenna
The following model is an I-Max 2000 5/8th wave vertical with a vertical feedline or mast connected to the antenna base, and no radials. In this case I picked one of many worse-case feedline or mast lengths:
Feedline shield current is 100% of antenna current. This illustrates why some users complain about SWR problems and RF in the shack with end-fed verticals like the I-MAX 2000, while other people do not complain and seem to love the antenna. This is because some people pick a lucky mast height or feedline length, while others are not so lucky. Unlucky people happened to choose a mast height, feedline length, or grounding system length that enhanced common mode problems.
*****end of article*****
I argue if there is such a case as the worst case scenario for the installation of the Imax, then that would also suggest that, at the very least, there were better installation heights to be considered...where the currents were a lot less significant to be causing pattern skewing and other problems. IMO, my J-P models attempt to demonstrate such a situation. Remember antenna models are not necessarily duplicable in the real world, but there could be some trends to be noted that suggest another way to solving such a problem.
Below are my models mentioned above and are supplied for some consideration for what might be going on with your J-Pole installation and that there may be an alternative method to solve the problems discussed in the article. Of course these models do not include the feed line which would necessarily provide an easy pathway for currents to flow, and something would also have to be done to mitigate such a problem. But with that said, I contend that there may be a better way to solve such problems aside for isolation, which the article describes as an iffy installation. You will note that the models suggest modest changes in height trend toward a mitigating type solution, and warns of the possibilities for a worst case scenario. Only real world test may prove this one way or another.
Another thing to notice is how bad the patterns look at 1/2 wave points, 18', 36', and likely at 54', and 72' feet which I did not model.
I also have an opinion regarding such a setup maybe needing radials along with the choke, but I have not tested such and unfortunately I cannot model a choke of a feed line with good understanding.
I'm sure there are issues I have left out in my thinking, but this is what is mullin around in my mind on the subject.
View attachment Bob's issues on Masts on J=Poles.pdf
Last edited:
eddie,
there are best and worst case scenarios for mast/feedline length and it reverses depending on the mast been grounded or not, i know im repeating myself but thats how antennas work,
clearly the guys who are suffering terrible rfi benefit most from isolation and those who pic a lucky length benefit least or not at all,
mm2000 did the work himself, i gave him the sirio radials and some suggestions,
he does have a correctly sized choke at the feedpoint isolation and spiderplane radials as does gary bowler on his imax,
doing it like we have shown has worked as claimed for several locals on 1/2wave and 5/8wave fiberglass or aluminum no radial endfeds,
your models above show exactly whats going on with mast/feedline currents,
i think the j-pole guy with his 3 way junction sums it up pretty well in laymens terms.
there are best and worst case scenarios for mast/feedline length and it reverses depending on the mast been grounded or not, i know im repeating myself but thats how antennas work,
clearly the guys who are suffering terrible rfi benefit most from isolation and those who pic a lucky length benefit least or not at all,
mm2000 did the work himself, i gave him the sirio radials and some suggestions,
he does have a correctly sized choke at the feedpoint isolation and spiderplane radials as does gary bowler on his imax,
doing it like we have shown has worked as claimed for several locals on 1/2wave and 5/8wave fiberglass or aluminum no radial endfeds,
your models above show exactly whats going on with mast/feedline currents,
i think the j-pole guy with his 3 way junction sums it up pretty well in laymens terms.
Hi guys,
I use an Astroplane and I remember you both saying things about mounting it on a metal mast with a wood mast under, then another metal mast to mount. And at another time using a substitute for the main radiator like a 1/4 wave whip. How did those experiments pan out or did they ever get off the ground? Have either of you been able to optimize the A/P?
3's
I use an Astroplane and I remember you both saying things about mounting it on a metal mast with a wood mast under, then another metal mast to mount. And at another time using a substitute for the main radiator like a 1/4 wave whip. How did those experiments pan out or did they ever get off the ground? Have either of you been able to optimize the A/P?
3's
Below is the 36' model done earlier where I added 6" of isolation at the top of the mast. Just for the heck of it, I did another model where I added two 1/4 wave radials, slanted down. Slanted radials happened to result in a better setup than the ones with horizontal radials, but the results with radials attached seem to indicate that the radials primarily add more high angle RF to the pattern and thus reduced the RF for the lower lobe.
Bob you're right, isolating the mast/antenna would seem to mitigate the problem currents, just as the article argues, but other measures might also need to be done to correctly fix the problem if the modification was not at the right height. Neither method alone, isolating or finding a better height (length for the mast and feed line) would seem to solve the problem. I think the problem is due to Kirchoff's rule as noted in the article, this problem appears to be too complex to provide us a one fix solution.
For sure if I selected the right height, I would also have to effectively choke the feed line, or else the undesirable currents would tend to flow on the shield to ground.
IMHO, antenna height might be a more complex factor to consider in any antenna installations than most think about. This might be the unsung factor for why we see different guys provide us totally different results with the same antenna, and where we hear location, location, location. I think that location matters due to Earth conditions, which I consider significant to antenna performance, but if this idea regarding height is true, and in all case, then antenna feed point height must be considered as well. Height may be more than just raising your antenna higher...hoping for more far field range.
Generally, most would think that height is always better, but if my modeling is even close, it tends to support the idea that height is more than just raising the antenna. Different heights also seems to dramatically changes the antenna performance and characteristics, and dramatic changes seem to be albe to occur within small changes.
Over time messing with my antennas I also saw this inexplicable changing going on, but I could not understand why. I surely couldn't explain it.
The models below seem to support the idea for isolating the antenna from the mast, but it has to be understood that the feed lines are not part of the models, so just isolating the mast might not solve the problem. But IMO, if it does then the antenna height might be what is working for you.
View attachment Bob's Isolation idea.pdf
SD, I think Bob once had an idea to test the Original AstroPlane with the mast extended and isolated below the hoop by a 1/4 wavlength, as an improvement to the angle and/or maybe the gain in performance.
I have real world tested my old Top One using a full 1/4 wave top element and by just operating my radio I was unable to detect any difference at all. I also have some SWR bandwidth curve charts that I think show pretty much that the antennas responses were the about the same curve with the full length element antenna showing a little better match per the SWR.
I understand that a full 1/4 wave element should out perform a shortened element with a top hat. And, these models prove that, but even if there is a difference it is no where near enough for me to tell just using my radio. This is just another CB BS story about a difference that is so insignificant as to be totally irrelevant to argue.
I also model both in Eznec and I attached both below. I also added a model with the 8.5' foot mast isolation idea that I think Bob was considering to do.
View attachment AstroPlane .pdf
IMO, guys that complain about the A/P, Top One, old or new, don't know what they are doing. These are great little antennas, that contrary to another CB BS popular idea...that they don't work well unless installed low, don't work at all, or produce a lot of TVI, are just simply in the wrong hobby and need to take up gardening instead.
Last edited:
eddie,
i agree isolating the mast alone is useless, those currents don't care if its mast or coax outer braid, if the cm impedance is low currents will flow and spoil the party,
you cannot isolate an endfed without adding radials or some other conductor for return currents, kirchhoff won't let us,
i also believe w8ji when he tells us a 5/8wave is worse in this regard than a 1/2wave,
im also certain you can have lip & hand biting rfi and terrible tvi when using an A99 and that adding radials and isolating can reduce that to insignificant levels as i reported in the past,
w8ji tells us he had to add a radial below the choke to make the current near zero, i trust ji is telling us the truth as seen in his models,
we have never put a second radial below the choke but we have reduced mast/feedline currents significantly using the method i described while improving signals and sometimes reducing noise in the receiver,
of course the ground is in the mix and imho should be included in the models too.
i agree isolating the mast alone is useless, those currents don't care if its mast or coax outer braid, if the cm impedance is low currents will flow and spoil the party,
you cannot isolate an endfed without adding radials or some other conductor for return currents, kirchhoff won't let us,
i also believe w8ji when he tells us a 5/8wave is worse in this regard than a 1/2wave,
im also certain you can have lip & hand biting rfi and terrible tvi when using an A99 and that adding radials and isolating can reduce that to insignificant levels as i reported in the past,
w8ji tells us he had to add a radial below the choke to make the current near zero, i trust ji is telling us the truth as seen in his models,
we have never put a second radial below the choke but we have reduced mast/feedline currents significantly using the method i described while improving signals and sometimes reducing noise in the receiver,
of course the ground is in the mix and imho should be included in the models too.
I can't disagree with any of the above with one exception, I've had my best A99, an original from some point back near their product introduction. It is still working and I got it with the GPK for $15, although it was supposedly broken, showing very high SWR. I have never had any problem with its use at my station, and I live in a very congested neighborhood. I have also had other A99's, that talked maybe just as well, but some were terrible with TVI. So I remain open minded about many things.
Well Bob, I don't think that news would set well with Jazzsinger.
BTW, I don't see him posting lately, is he OK?