What W8JI says about a 1/4 wave vs. 5/8 wave

[Moleculo]

Bob made a comment that I was going to make, as well. W8JI specifically mentions the Imax 2000, which is a 5/8 with no GP radials. There are quite a few other 5/8 wave CB / 10 meter antennas out there with either no GP radials, or very small, ineffective ones. As W8JI says, you need the proper length radials to make the 5/8 wave antenna effective; 1/4 wavelength is a good rule of thumb to start with, just like the 1/4 ground plane.

Originally Posted by W8JI

The 5/8th wave depends heavily on a large highly conductive reasonably flat groundplane at the BASE height of the antenna. That groundplane has to extend out in all directions for a considerable distance. The reflection out some distance is how the antenna works and gets gain. Without that groundplane at base height for some distance, the feedline will radiate and the pattern also will not form properly. You ALWAYS wind up with an unpredictable mess because key parts of the system are missing.
[snip]
What you will find is the 5/8th wave can vary from being just slightly better than a 1/4 wave with three or more sloping radials to being much worse at low angles. Statistically the 1/4 wave is more reliable.

Note the emphasis I added on those two sentences, which are one complete thought. The corollary to that quoted paragraph is that with an effective groundplane at the base you get some gain and you won't end up with an unpredictable mess.

Saying something is "statistically...more reliable" isn't really saying much other than it generally works good under most circumstances. My Jeep is statistically more reliable than any motorcycle I've ever owned. That statement really doesn't tell you much about why you would want to use either vehicle, does it? The same can be said about antennas.

There are a lot of circumstances around why you might want to choose one antenna over another. The important thing is to try and sort through all of the hype and BS and develop an understanding around how the antenna is going to perform in both a perfect environment and your own environment and whether or not it will accomplish what you need it to.

BTW, W8JI has a whole section on his website comparing 1/4 and 5/8 wave 2 meter antennas modeled in both a perfect ground and mobile environment. It's really great reading...I'm sure you would like it.

 

[eagle1911]

This is very interesting reading that hits home with me right now..

I'm in the middle of laying out a ground-mounted IMAX2000 install for 10-12m DX work. I live on a tiny city plot of land, fenced in. I have neighbors on all sides, and a homeowner's association that is hostile towards antennae.

I painted the antenna camouflage, and my plan is to mount it near ground level on one side of my property on the fence since there is a large tree there that will completely conceal the antenna. Since it's on the fence, if I use ground radials some of them will be in my neighbors yard, so that's out.

What do you guys think about the idea of grounding the antenna to the chain-link fence itself? It's not a ground plane exactly, and it's not resonant, but would it not suffice as some sort of counterpoise?

 

[MrSuburban]

This is very interesting reading that hits home with me right now..

I'm in the middle of laying out a ground-mounted IMAX2000 install for 10-12m DX work. I live on a tiny city plot of land, fenced in. I have neighbors on all sides, and a homeowner's association that is hostile towards antennae.

I painted the antenna camouflage, and my plan is to mount it near ground level on one side of my property on the fence since there is a large tree there that will completely conceal the antenna. Since it's on the fence, if I use ground radials some of them will be in my neighbors yard, so that's out.

What do you guys think about the idea of grounding the antenna to the chain-link fence itself? It's not a ground plane exactly, and it's not resonant, but would it not suffice as some sort of counterpoise?

Ground mount it and put as many radials on the ground beneath it, Ideally you can use 14 guage wire about 32 of them or more in a spoke type fashion around the base of antenna connected to the ground of antenna or shield of coax. If you cant put the radials on neighbors property put as many as you can on your own. Hell they are only 9' of wire I do it at night the grass will grow and he may never see em. But you need them below the antenna to minimize ground losses. Dont think the fence will work so hot for that.

 

[W5LZ]

Generally, a fence can be used as 'part' of an antenna. But there are consequences to that! Where does that fence go, how close to neighbor's electronics, etc.? As 'part' of that antenna is will radiate. That means it will have more likelihood of getting too near stuff and causing problems.
- 'Doc

 

[Marconi]

Bob made a comment that I was going to make, as well. W8JI specifically mentions the Imax 2000, which is a 5/8 with no GP radials. There are quite a few other 5/8 wave CB / 10 meter antennas out there with either no GP radials, or very small, ineffective ones. As W8JI says, you need the proper length radials to make the 5/8 wave antenna effective; 1/4 wavelength is a good rule of thumb to start with, just like the 1/4 ground plane.

I have a different point of view about what W8JI says in this regard. That said I sure would like to see his modeling that supports his claim, because my models below do not support your idea of ineffectiveness nor does a world of installations using an Imax without radials. This antenna, without radials, may not be quite as efficient, but the radiated pattern, angle, and gain does not show to be ill-affected in the absence of radials, and the Imax is an effective antenna in most cases.

I agree with Shockwave, when he noted that the Imax, in particular, was designed to work without radials, and thus has proved to be efficient in 1000's of installations. I believe the design took care of the inefficiency problem, doing without radials, allowing the radiator to be adequately decoupled from the feed line and mast, which improve efficiency, very similar in part to the New Gain Master as to results. I also don't consider the Imax design to be a ground plane antenna.

I wonder if when W8JI wrote about the Imax, if he was talking about efficiency, or effectivness, or could he have been talking about how to decouple the feed point from the mast and feed line instead? Ask yourself, if W8JI is really saying that the Imax 2000 is ineffective in operations, because to me that would suggest that it doesn't work very well, and I don't believe that to be the case. Again, I don't think W8JI tells us quite enough of the details for his idea, and thus we have to take his word on shear faith alone. Maybe he does have the credentials to do that, but how do we explain the Imax working so well for so many? I might not be able to explain things very well, but IMO there is something missing in this understanding of what W8JI may really be saying.

Originally Posted by W8JI

The 5/8th wave depends heavily on a large highly conductive reasonably flat ground plane at the BASE height of the antenna. That ground plane has to extend out in all directions for a considerable distance. The reflection out some distance is how the antenna works and gets gain. Without that ground plane at base height for some distance, the feed line will radiate and the pattern also will not form properly. You ALWAYS wind up with an unpredictable mess because key parts of the system are missing.
[snip]
What you will find is the 5/8th wave can vary from being just slightly better than a 1/4 wave with three or more sloping radials to being much worse at low angles. Statistically the 1/4 wave is more reliable.

Moleculo, for me this makes sense with a traditionally designed 5/8 wave ground plane, but the attached models do not support that idea completely as noted...unless W8JI was talking strictly about efficiency. The only test that will convince folks in this regard would be to correctly mount an Imax without radials and work it for a while, and you will see it work efficiently and effectively.

Then mount an I-10K without radials and see it work in-effectively and in-efficiently.

Then add radials to the I-10K and see it work effectively and efficiently. Even W8JI, in his article on the Imax, referred to mounting the Imax in the worse case scenario, which for me suggest there is a best case scenario.

The gain and angles for both 5/8 wave radiators in my models, with and without radials, are about the same, but the efficiency for the one without radials leaves much to be desired. Again what if W8JI was talking about efficiency and not effectivness, which are two totally different things. Admittedly, these scientific kind of guys do tend to talk antenna efficiency more so, and us CB'rs tend to talk effectivness, at least that is my opinion.

If the end fed 5/8 wave radiator without radials is in-effective how do we explain this modeling result?
View attachment .625 wave with and without radials.pdf

Note the emphasis I added on those two sentences, which are one complete thought. The corollary to that quoted paragraph is that with an effective ground plane at the base you get some gain and you won't end up with an unpredictable mess.

Saying something is "statistically...more reliable" isn't really saying much other than it generally works good under most circumstances. My Jeep is statistically more reliable than any motorcycle I've ever owned. That statement really doesn't tell you much about why you would want to use either vehicle, does it? The same can be said about antennas.

There are a lot of circumstances around why you might want to choose one antenna over another. The important thing is to try and sort through all of the hype and BS and develop an understanding around how the antenna is going to perform in both a perfect environment and your own environment and whether or not it will accomplish what you need it to.

BTW, W8JI has a whole section on his website comparing 1/4 and 5/8 wave 2 meter antennas modeled in both a perfect ground and mobile environment. It's really great reading...I'm sure you would like it.

I agree this is true of a ground plane, but is the Imax really a ground plane?

BTW Moleculo, I added the wires view for the end fed 5/8 wave radiator with radials, so maybe you and Mr.Suburban can correct my model showing the results that agree with your ideas of what W8JI is saying.

 

[MrSuburban]

Generally, a fence can be used as 'part' of an antenna. But there are consequences to that! Where does that fence go, how close to neighbor's electronics, etc.? As 'part' of that antenna is will radiate. That means it will have more likelihood of getting too near stuff and causing problems.
- 'Doc

Another lame brain post by Doc, he is mounting a vertical at just about ground level, he needs to put a ground system beneath the antenna to minimize ground losses. Can you explain how a vertical fence is gonna provide this? If it is cold out the worms will love your idea The antenna he is mounting doesnt have any ground radials. I am all ears.

 

[W5LZ]

MrSuburban,
Can I explain how a "vertical" fence would provide a 'groundplane' or counterpoise? I have a feeling that I couldn't explain it to -you-, so I won't bother.
- 'Doc

 

[HomerBB]

Marconi, I will quote this sentence and propose a thought -

Without that ground plane at base height for some distance, the feed line will radiate and the pattern also will not form properly. You ALWAYS wind up with an unpredictable mess because key parts of the system are missing.

I do not read where W8JI has said the antenna w/o GP is either inefficient, nor in-effective, although one or the other is implied. I do read that he is saying the vertical 5/8 is unpredictable in its response/performance being without a GP. To this I propose that there is also a world of testimony. The A99 1/2ƛ, and the Imax2k 5/8ƛ both have those who've tried them and liked them, and those who've cursed them because they were dismal performers where they tried to use them. I believe they use the mast, or the coax, or surrounding elements, or some such for the missing part of the system. It seems to me that W8JI is making a case for the repudiation of the idea that antennas, particularly 5/8 verticals, which the Imax2k purports to be, do need a decoupling system in order to maintain stability of operation. He mentions the choke as one such device, but advocates for a sufficient radial GP network as the most effective means of achieving this when consistent performance is sought.

At least that was my read of it. I tend to think he's right.

I am not a software modeler. I can't speak to that. What I can say is that in virtually every instance of software modeling I've looked at people tend to forget that the model does not default to unpredictable variables so the modeling results tend to be the best potential - guidelines at best.

 

[MrSuburban]

MrSuburban,
Can I explain how a "vertical" fence would provide a 'groundplane' or counterpoise? I have a feeling that I couldn't explain it to -you-, so I won't bother.
- 'Doc

Doc I am not the only memeber of the forum perhaps you can explain it to the other members of the forum who will understand it better than I. How does a chain link fence provide a ground system under a ground mounted antenna. The floor is yours I am willing to learn.

 

[eagle1911]

Many thanks for the replies to my question, I see that I should have started a new thread, so I apologize to Marconi for my partial derailment of his thread.

MrSuburban: In my installation specifically, the antenna will be mounted near (but not at) ground level on the fence, with the feedpoint at roughly the same height as the top of the fence. It makes sense to me that the fence itself would work as a counterpoise, but how well it will work I won't know until I try it. I was mostly just looking to see if anyone had heard of doing that before, or if anyone had been in a similar situation and had found a better way to provide a good antenna ground for a vertical.

I think that the part of W5LZ's post that talks about the fact that the fence will radiate and be a potential source of harmful interference to others is pretty wise.. I hadn't thought about that.

 

[Marconi]

eddie,
i posted a link to what w8ji had to say on his website about 1/4waves vs 5/8waves such as imax a few years ago and a couple of times since then, he talks about the imax2000 or similar no radial end feds and what clearly is a starduster style antenna, what he says imho answers why you see a funky pattern when you add the mast to your new top_one model,

Bob, I've already tried to explained that I posted the free space pattern in this case, simply because that it what Avanti displayed in their Patent for the AstroPlane. The mistake I made was posting a free space model with a mast included, which we both should know is not appropriate. IMO the fact is, that a free space model with a mast connected to Earth is not good protocol to follow in free space modeling practices...and thus the funky pattern. This issue you raise has nothing whatever to do with the ideas presented by W8JI/Cebik regarding the short comings with modeling in the absence of mast and/or feed line.

i also posted about what cebik had to say about 5/8waves at different heights above different conductivity ground in his upper hf monopole article,

i don't have an issue with eznec, why you would think myself and other people have no faith in nec because of something lb.cebik told me about modeling the sigma style antenna is beyond my comprehension,
plenty of people use nec to good effect, its out there for peer review,
many of my own ideas come from people who use nec,

I think there is more to your opinion regarding Eznec than you let on, but the issue here is what other people might think on hearing your critical words, concerning Eznec and what Cebik told you. What you said is clear to me, but is it also possible that most readers of these posts may not understand the distinction you make in this regard. Since this idea came up repeatedly in the recent past, there seems to be little to no interest or discussion regarding modeling, and for me that is unfortunate.

we have used ideas from w8ji based on his models and seen positive results more than once that the j-pole camp poopoo,

lb.cebik did not say modeling the sigma style antenna was impossible, what he said was that it would be very difficult to model that antenna and get accurate results, w7el also gives us a warning in the manual.

when we test antennas we use an unmodulated carrier, same mast/feed line, make sure there is no skip and signals are stable, make sure there are no locals pumping the receivers agc, there is always a winner and the winner is always ahead, there is no flipflopping between which antenna is best, its not scientific but it is usually consistant/repeatable,

whenever there is skip/multipath all bets are off, you can get the flipflop even when using two identical antennas on different masts.

Well, at least you heard another opinion and got some feed back regarding the J-pole vs. the Sigma4 design in that case. I have posted numerous models, and I very rarely-if-ever get a question in return. What do you think that tells us about modeling?

 

[BOOTY MONSTER]

"with the feedpoint at roughly the same height as the top of the fence"

so the fence will be below the antenna . thats good , but i think its gonna cause more problems that it will solve , but it can work .
you can also plan on that "large tree there that will completely conceal the antenna" blocking your signal to some degree .

damned if you do and damned if you dont ........ but something is better than nothing .

going by comments on forums it seems to be a hit or miss with folks adding ground elements to them . CMC's are the result of a poor ground plane so its always a good idea to use a choke at the feedpoint of antenna without them . a proper choke has no negative effect so i put them on every antenna .

 

[Marconi]

Marconi, I will quote this sentence and propose a thought -
I do not read where W8JI has said the antenna w/o GP is either inefficient, nor in-effective, although one or the other is implied.

I also agree, but the references were Molucelo's comments and I was trying to draw attention to the distinction that W8JI may have been referring to. Again IMO, sometimes Tom produces a hard read, and is sorely short on details and that makes us either think or guess. In the Cebik article that Bob recently presented entitled "A Common-Mode Current Picture Show" I think Cebik's conclusions about our trying almost in vain to determine what the real coax lenght in a system really is in order to fix the problem with CMC...says it all. And in the boot, Cebik suggests that we not worry about all the details in his 18 page report, but instead use a simple CMC attentuator to solve the problem. Did you get that in reading that report? The details may not be BS, but it is close or might as well be...considering his final conclusions.

I do read that he is saying the vertical 5/8 is unpredictable in its response/performance being without a GP. To this I propose that there is also a world of testimony. The A99 1/2ƛ, and the Imax2k 5/8ƛ both have those who've tried them and liked them, and those who've cursed them because they were dismal performers where they tried to use them. I believe they use the mast, or the coax, or surrounding elements, or some such for the missing part of the system. It seems to me that W8JI is making a case for the repudiation of the idea that antennas, particularly 5/8 verticals, which the Imax2k purports to be, do need a decoupling system in order to maintain stability of operation. He mentions the choke as one such device, but advocates for a sufficient radial GP network as the most effective means of achieving this when consistent performance is sought.

I disagree that he is repudiating the antenna design, I think he tells us the antenna design does present problems, and that an optimum installation or no, can mean the difference in successful use or not. He also talks about how futual it is to try and fully metigate the problem using the proper combination of feed line length and suggests using a choke in such cases. I can't prove my point, but I ask folks to consider the Gain Master which has similar problems, if not worse. Does this mean the GM is in the same category as some claim about the Imax?

This reminds me of the old argument: "...is the glass half full or half empty?"

At least that was my read of it. I tend to think he's right.

I am not a software modeler. I can't speak to that. What I can say is that in virtually every instance of software modeling I've looked at people tend to forget that the model does not default to unpredictable variables so the modeling results tend to be the best potential - guidelines at best.

Homer, I don't disagree that Tom and Mr. Cebik are right in their remarks here, but sometimes I argue that they may be misunderstood on some issues and I always leave open the possibility that I can be wrong also.

 

[eagle1911]

@BOOTY MONSTER: Thanks! I was betting that there might be a very slight blockage of signal from the tree, however I do know that this effect is generally so minimal at even high HF frequencies as to be safely ignored. W8JI did a study on that I think by measuring the losses generated by a ground mounted antenna in a forest. Outside the forest there were some very minimal losses, but we're talking about many trees in that case, not just one.

Good thoughts though, particularly about the choke.. I'm using LMR400, which is pretty stiff.. Creating a choke should only entail looping the coax a few times, right? Thanks again.

 

[BOOTY MONSTER]

choke info

http://www.worldwidedx.com/cb-antennas/62990-coax-choke-info.html

............

Tree Signal Attenuation


Tree Signal Attenuation
November 30th Fact-of-the-Day
if a vertical 28 MHz antenna is surrounded by moderately thick trees that are higher than the top of the antenna signals will be attenuated 2 to 3 dB on average. Tree attenuation with horizontal polarization under the same conditions usually will be negligible.