Another look at the Imax

[Marconi]

The following model is an I-Max 2000 vertical with a vertical feed line. In this case the worse feed line length has been modeled:

http://www.w8ji.com/images/Zepp/end-fe15.gif

Feed line current is 100% of antenna current. This illustrates why so many people complain about SWR problems and RF in the shack with end-fed verticals like the I-MAX 2000!
Here is the pattern of an antenna that copies the I-MAX dimensions and feed system:

http://www.w8ji.com/images/Zepp/end-fe16.gif

This is a NEGATIVE gain antenna at low angles. A 1/4wl ground plane would seriously out-talk the I-MAX 2000 or any other 5/8th wl antenna that does not have a large ground plane.
Even if we use the optimum feed line and mast length, here is the very best the end-fed antenna will do:

http://www.w8ji.com/images/Zepp/end-fe17.gif

In this case we now have 2.67 dBi, which is actually a little less than a 1/4wl ground plane will do! The severe common-mode mast and feed line currents make "no-radial" verticals extremely sensitive to mounting height, mounting structure, feed line length, and grounding. This is NOT normal for an antenna. It is a sign of a design problem.


My thoughts;

In case you wish to read more of this work, the preceding info was taken from;

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=7799

in the menu at top of page goto (Antennas) then to (End-fed verticals/...)

It should be noted, that the W8JI bases his conclusions upon a rule he sites in the very beginning of his article on end-fed verticals. He further concludes; much of the confusion (lies) about gain is derived from improper modeling techniques used by design engineers.

Note in the results from the graphic above, end-fed16.gif, that the high gain indicated for the Imax in this situation is at 54°. This is certainly not the low angle idea we all consider effective in most CB work.

I suppose it can be concluded here that if the antenna in graphic, end-fed17.gif, had a good counterpoise or the feed line was otherwise isolated well from the antenna, then more of the high angle RF shown in the results would be radiated at a lower angle. Thus further improving the effectiveness of the antenna even more. Question is, will the GPK for the I-2000 do what is suggested here?

 

[Master Chief]

It took me a minute, but I found it here:
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

Interesting reading to be sure! I'll give it a better look when I have more time.

I wonder if Tom took into account the matching section at the base of the I-max 2000. Again, I haven't read it all in depth.

I can say that Tom is well respected in the antenna community!

 

[Devildog]

Very good read there Eddie, even if I dont understand all of it. But what your saying is the I2K might perform like a good ground plane with the GP kit for it?

 

[Marconi]

DD, I don't understand it all either. But, in answer to your question, it is not what I believe that is at question it is what W8JI states. Yes, I think a good ground plane, if it helps to isolate the feed line from the antenna well, then this should reduce or help eliminate the feed line currents referred to as common mode currents. I believe he also suggests that the Imax uses CMC's on the feed line or mast in order to work as effectively or not, as it does.

It is W8JI's contention throughout this piece that end-feds require large ground planes or other extraordinary isolation method(s) to prevent feed line or mast common mode currents. I tend to agree that to fix this would force more of the current into the antenna. This should produce a better, radiated pattern with more RF at lower angles to the earth. If his modeling examples are true then his ideas look like common sense thinking to me.

Master Chief posts:

I wonder if Tom took into account the matching section at the base of the I-max 2000.

Yes MC, I think you will find on a closer reading that W8JI suggests just that. Modeling that does not include the matching section and the feed line attached, results in questionable results. He even suggests this is where inflated gain figures come from that hype these antennas.

Some have suggested to me that EZnec is a flawed modeling tool and has little to do with real world testing in a controlled environment. All I know is that my real world testing of the Imax proved something is going on that I do not understand, so I seek additional information.

I assume that if some of the current, that takes a path to ground via the feed line or the supporting mast, gets to the earth then much of the current will be lost and not be returned to the antenna. I think a good ground plane should accomplish this end. This is not to say that just any old ground plane up at the feed point will accomplish this however. So I ask the final question, "...will the Imax GPK do this?".

 

[freecell]

nothing new here.

 

[Marconi]

FC, you don't think we would come up with something new without talking to you first, do ya!

 

[freecell]

no offense marconi but i have posted along these lines several times in the past. every antenna is a dipole, if it isn't it doesn't radiate, not very efficiently anyway. balanced currents in both "poles" and elimination of common mode current on the feedline and the support structure is a must in any properly designed antenna system. while i don't voice it as often as i once did, low feedpoint heights, feedline and support structure radiation are the main culprits in most tvi and interference situations. in the case of my own antenna systems i make extensive use of baluns, radials and antenna tuners, even with such antennas as the "glass sticks" like the A99 and others and i really don't worry about rf earth-ground returns for antennas that are properly balanced and decoupled. come to think of it, swr is usually the last thing i worry about after poles, (two halves of the antenna) feedline and support current measurements and making sure that the transmitter is always looking into what it takes to keep it happy.

it's not too difficult to understand why most operators fail to realize any benefit from elevated radial systems (or add-on radial kits) when you stop for a moment and ask yourself if there is any antenna current present in the radial system to begin with. if the current has disappeared into the feed and support then what's left for the radials? unfortunately in the majority of cases, there's a lot more involved to proper antenna design and installation than just throwing up the latest entry on the market, adjusting for min. swr and flipping the switch and you and others here know that.

i spend most of my time these days discussing subjects such as this on my website with whoever stops by using the livehelp software currently installed throughout my domain and any of you are welcome to stop by and chat.

 

[rfoverlord]

so then, a ant. like the cushcraft ar-10 is probably not a decent ant.? i homebrewed one for 11mtrs. but i did put an ugly balin on it about a foot from the feed point. i haven't had any problems with rf in the shack or tvi with me or my nieghbors. the only reason i went with it was due to no radials to clutter up the field for the rest of my ants. i know it's not even close to optimal but wanted an outside cb ant. without messing up the others that are higher priori.

 

[Marconi]

RF, I don't think FC is saying that your antenna is good or not good. He is saying that there are certain rules that radiating antennas must follow, and this Ringo is not following the rules. It is using common mode currents to work and that is probably not good even if we know it works.

Why did you use a balun and in doing so, could you tell any difference?