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<knormoyle@surfnetusa.com>
> Subject: [TowerTalk] 80m dipole with open-sleeve parasitic
> To: Tower Talk List<towertalk@contesting.com>
> Message-ID:<4D3680F4.5080109@surfnetusa.com>
> 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.https://sites.google.com/site/knormoyle/
>
> 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.
> http://rudys.typepad.com/ant/files/antenna_broadband_dipole.pdf
>
> 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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