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.
