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[TowerTalk] 80m dipole with open-sleeve parasitic

Subject: [TowerTalk] 80m dipole with open-sleeve parasitic
From: W3OA <>
Date: Thu, 20 Jan 2011 06:33:16 -0500
List-post: <">>
  Hello Kevin -

I would like to know how you matched the N6LF folded dipole to 50 ohms.  
Did you use a 9:1 balun?  Would it handle 1500 watts?  If so, could you 
give me some details on the balun?

73 - Dick, W3OA
> Message: 6
> Date: Tue, 18 Jan 2011 22:13:08 -0800
> From: Kevin Normoyle<>
> Subject: [TowerTalk] 80m dipole with open-sleeve parasitic
> To: Tower Talk List<>
> Message-ID:<>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> Here's a 80m dipole I'm going to put up. Thought I'd see if I overlooked
> something someone might comment on.
> It uses a close-spaced open-sleeve parasitic to broaden the bandwidth.  Sims 
> say
> I've got 1.5:1 SWR from 3.5 to 3.975Mhz. (at the end of the ideal 20 meter 
> match
> section)
> (2:1 from 3.45 to 4)
> I want to do 1500W across the full band without tuning or relays or other
> adjustment.
> The background that got me thinking about this: I had put up a folded dipole
> with a open-sleeve parasitic, with a target impedance of 450 ohms, per N6LF
> (google) and that worked well.
> This new one will be at a right angle to that one.
> However, it seemed easy to get the same effect with two wires instead of the 3
> N6LF used. Yeah I know about wide cage dipoles, and cutting two dipoles and
> keeping them spread to avoid interaction.
> This  same-band open-sleeve design wants interaction. Spacing is kept at
> constant 6" with ladder-line like spacers.
> The use of parasitic open-sleeve is talked about everywhere (since 1946?) to
> broaden bandwidth. But most of the cases I find on the web are about adding
> different bands (like Force12 20m-15m-10m open-sleeve feeds)
> A variety of journal articles make the same-band case seem obvious. But I had 
> to
> play around to find the best lengths. What I found, is that it doesn't work 
> well
> to try to aim for 50 or 75 ohm impedance at the antenna.
> The antenna impedance sweet spot I ended on was about 125 ohms. I then use 
> about
> 1/4 wavelength of 75 ohm to get a feed match to 50 ohms.
> In the nec file, I have an ideal 20M length of 75ohm transmission line for 
> this,
> so it can be simulated with 50 ohm drive.  The target height is 100 feet.  The
> nec is setup so the parallel wires have the same number of segments, for nec-2
> accuracy.
> I know the N6LF antenna works as advertised. And I see that people put up wire
> pairs for the same reason (to get full 80M band). I guess I'm throwing this 
> out
> for comment in case I've missed something. I think it's going to work well. 
> It's
> no problem keeping the wires spaced at 6" with spacers every 8' or so.
> Anyone curious enough to replicate my sim: I put the nec and some graphs up at
> the url above. I run with 4nec2.. the default 50 ohm drive impedance works
> because it drives at the end of the matching line.
> The swr on the 75 ohm line is less than 2<1 so it doesn't look like there's 
> much
> loss there.
> I had simmed it without the match line, but if you remove it and drive at the
> antenna, you want to use a 125 ohm impedance.
> -kevin
> ad6z


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