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Author Topic: 20 meter "vertical zepp dipole"  (Read 1557 times)
WB0KSL
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Posts: 94




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« on: March 11, 2013, 12:43:20 PM »

While mindlessly wandering the net yesterday, I came upon the following web page:

http://www.hamuniverse.com/N7AGK20metervertical.html

On hamuniverse.com.  :-) 

A bit like a coaxial vertical dipole where the coax feed is through the center of a quarter wave aluminum tube.  In this case, however, the shield floats at the top of the vertical tube, and the center conductor is attached to the top of the tube instead of continuing for the second quarter wave of the dipole.  I didn't check the date of the article to see if it was April 1.  Assuming it is intended to be a real design, I can only see it working via lots of common mode current on the outside of the shield.  If anyone has seen this, or cares to look, I am curious about it.  I'm guessing the reference to a Zepp in the title comes from the floating shield of the coax feedline.  Any comments?  Other than inviting me on a Snipe Hunt?

73 de wb0ksl, John
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K5LXP
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Posts: 4506


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« Reply #1 on: March 11, 2013, 02:13:39 PM »

From the Hamuniverse site:


The HAM UNIVERSE.COM site accepts material from ALL Amateur Radio Operators and others such as Shortwave and Scanner buffs wanting to submit projects, articles, antenna plans etc, to share with all hams on our site or others interested in ham radio or the radio hobby.
...
There is absolutely NO CHARGE TO YOU!


So, they get what they pay for.  About the best you get is a disclaimer like the one on the page you referenced:

"Editors note: The author of this project arrived at these plans by experimentation so this antenna should be considered an experimental project by the reader. "

If that article were published here on eHam where there's an opportunity for feedback, the author would get torn to pieces.  But I'm guessing the Hamuniverse site owner gets paid by the click, so it doesn't matter what the content is, just as long as there's clicks.

There actually might be some good technical content at that site but you'd have to wade through this crap to find it.  The problem is that newbies will see this stuff and not know any better.
 

Mark K5LXP
Albuquerque, NM
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W1JKA
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Posts: 1763




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« Reply #2 on: March 11, 2013, 02:55:35 PM »

   I don't do antenna modeling,just seat of the pants experimentation but am curious if the NAGK 20m vert. could be modeled and what the results might be.
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WB6BYU
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Posts: 13334




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« Reply #3 on: March 11, 2013, 03:21:37 PM »

You have, in theory, current flowing on the radiator (with maximum at the
top.)  The question, however, is where the rest of the current flows that
is connected to the shield:  it would have to  flow down the outside of
the coax.  The current in the upper 1/4 wave of coax is shielded by the
radiator tube, and if the next 1/2 wave of coax shield is radiating it would
probably be in phase.  But if the coax is longer than that, you'll get out-
of-phase currents.

So if you can choke the common mode current off at the right point
(which isn't always as easy as some designs make it appear) and you
have half a wave of coax hanging down from the bottom of the tube,
it may provide a slight bit of gain over a more conventional antenna
due to radiation from the coax.  But in other situations it could be
worse.

The article also appears to make the mistaken assumption that, if you
can match an antenna with a tuner, it "works".  I suppose that might
be true for some value of "works" that includes having very high coax
losses.
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N4JTE
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Posts: 1157




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« Reply #4 on: March 11, 2013, 05:09:39 PM »

Yeah Ham Universe is a wasteland of rediculous antenna crap, that being said I googled my call and found 4 of my antenna designs on there without my permission, go figure. But at least they work as described, hi.
Bob
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WB0KSL
Member

Posts: 94




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« Reply #5 on: March 11, 2013, 05:41:31 PM »

Thanks for the replies.  I could see absolutely no way it could get by without significant common mode issues.  Hadn't spent much time on the hamuniverse site.  I'll add this one to the lightbulb on a stick idea.

73 de wb0ksl, John
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