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Marconi
Intermediate Member
Username: Marconi
Post Number: 473
Registered: 11-2001
| | Posted on Friday, November 04, 2005 - 9:36 am: | 
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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:
Modeled Imax 2000
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:
Modeled Imax radiated pattern
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:
Modeled Imax radiated pattern with optimized installation
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.
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.
You will note in the results from the graphic above, end-fed16.gif, that the high gain noted 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 and thus further improve the effectiveness of the antenna even more. Question is, will the GPK for the I-2000 do what is suggested here?
Note: the words in bold are not by W8JI.
Marconi
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Tech833
Moderator
Username: Tech833
Post Number: 1078
Registered: 8-2002
| | Posted on Friday, November 04, 2005 - 10:46 am: |
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There is only one flaw. Your examples have been modeled by software alone. A while ago, I spun an actual Imax 2000 on an antenna range and got the ACTUAL radiation pattern plotted from a real world antenna. There is a BIG difference between the shareware antenna modeling software used here and the real world conclusions.
Although this is very good to know, and many of your conclusions are accurate, I highly CAUTION readers to take actual real-world information over information formed by software that is available on the web for free, developed by a ham, and known for inaccuracies.
In my factual conclusions, the Imax 2000 is still one of the best omni antennas available today. Obviously, not THE best for all installations, but certainly in the top 5. |
Marconi
Intermediate Member
Username: Marconi
Post Number: 474
Registered: 11-2001
| | Posted on Friday, November 04, 2005 - 12:50 pm: |
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Thanks for the response 833. I was not aware that the software used in this article had the known inaccuracies you mention. I will have to check my sources a little closer in the future.
BTW, I agree that the Imax is a very effective antenna, with all the stations using them on the air, they couldn't all be wrong. I was just curious about how it really works and I had never seen such details before. The ideas and the conclusions given by the author all seemed reasonable to my thinking, so I thought I would share the ideas with the guys. |
Hotwire
Advanced Member
Username: Hotwire
Post Number: 678
Registered: 1-2005
| | Posted on Sunday, November 06, 2005 - 11:11 am: |
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thanks Marconi! |
Marconi
Intermediate Member
Username: Marconi
Post Number: 475
Registered: 11-2001
| | Posted on Sunday, November 06, 2005 - 12:09 pm: |
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Sorry guys, that my effort to code the link to W8JI's webpage did not work.
The exact article is at this site:
Good luck and good reading.
833, it sure would be a good deal if you could post the radiated pattern results you got on the Imax you mentioned. Not all the details, just the pattern graphics. I would like to compare your results to those of W8JI. I think you are correct, real world results are always better than modeling results.
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Tech808
Moderator
Username: Tech808
Post Number: 8265
Registered: 8-2002
| | Posted on Sunday, November 06, 2005 - 12:37 pm: |
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If you include a LINK to a Web Site in your Post PLEASE check that link by Clicking on it and make sure ther are NO MORE than 5 other LINKS inside of the Original Link you are trying to Post.
If there are More than 5 Other Links inside of that Link the Copper Forum Moderators do not have the time to check them all and Your Post Will be Refused.
Should anyone have any questions Please feel free to contact me at:
Tech808@copperelectronics.com
Thank You,
Lon
Tech808
CEF808
N9OSN |
Tech833
Moderator
Username: Tech833
Post Number: 1082
Registered: 8-2002
| | Posted on Sunday, November 06, 2005 - 8:09 pm: |
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Marconi,
See my review of the Imax 2000 GPK. It includes radiation patterns for the antenna with and without the GPK installed.
Radiation Patterns |
Marconi
Intermediate Member
Username: Marconi
Post Number: 476
Registered: 11-2001
| | Posted on Monday, November 07, 2005 - 11:28 pm: |
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Thanks again 833. It has been a long time since I look at your report. Thanks for the heads-up.
Also 808, I understand your point about links, but it is real hard to find sites with just a few links anymore. I guess if anyone wanted to really look further into the source of the information, then they could do a search on their browser to W8JI and they should be able to find it.
Thanks |
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