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[TowerTalk] 80m dipole with open-sleeve parasitic

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Subject: [TowerTalk] 80m dipole with open-sleeve parasitic
From: Kevin Normoyle <>
Date: Tue, 18 Jan 2011 22:13:08 -0800
List-post: <">>
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 
I've got 1.5:1 SWR from 3.5 to 3.975Mhz. (at the end of the ideal 20 meter 

(2:1 from 3.45 to 4)

I want to do 1500W across the full band without tuning or relays or other 

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 
play around to find the best lengths. What I found, is that it doesn't work 
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 
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 
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 

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. 
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 
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.


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